STATE UNIVERSITY, NORTHRIDGE

FORECASTING CALIFORNIA

A thesis submitted in partial fulfillment of the requirements

For the degree of Master of Arts

in Geography

By

Ilya Neyman

May 2013

The thesis of Ilya Neyman is approved:

______Dr. Steve LaDochy Date

______Dr. Ron Davidson Date

______Dr. James Hayes, Chair Date

California State University, Northridge

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TABLE OF CONTENTS

SIGNATURE PAGE ii

ABSTRACT iv

INTRODUCTION 1

THESIS STATEMENT 12

IMPORTANT TERMS AND DEFINITIONS 13

LITERATURE REVIEW 17

APPROACH AND METHODOLOGY 24

TRADITIONALLY RECOGNIZED TORNADIC PARAMETERS 28

CASE STUDY 1: SEPTEMBER 10, 2011 33

CASE STUDY 2: JULY 29, 2003 48

CASE STUDY 3: JANUARY 19, 2010 62

CASE STUDY 4: MAY 22, 2008 91

CONCLUSIONS 111

REFERENCES 116

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ABSTRACT

FORECASTING CALIFORNIA THUNDERSTORMS

By

Ilya Neyman

Master of Arts in Geography

Thunderstorms are a significant forecasting concern for . Even though across this region is less frequent than in many other parts of the country significant events and occasional does occur. It has been found that a further challenge in convective forecasting across southern California is due to the variety of sub-regions that exist including coastal plains, inland valleys, mountains and deserts, each of which is associated with different weather conditions and sometimes drastically different convective parameters. In this paper four recent thunderstorm case studies were conducted, with each one representative of a different category of seasonal and synoptic patterns that are known to affect southern California. In addition to supporting points made in prior literature there were numerous new and unique findings that were discovered during the scope of this research and these are discussed as they are investigated in their respective case study as applicable. The findings here are hoped to add to the growing interest and knowledge base of California thunderstorm and severe weather research and forecasting.

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INTRODUCTION

Thunderstorms, besides being a fascinating and many times less predictable meteorological phenomenon are less common across sections of California compared to much of the contiguous US to the east. In certain cases the mechanisms and atmospheric processes leading up to convective events in this region may be more synoptically ambiguous compared to their counterparts in other parts of the country. The purpose of this paper is to examine California thunderstorms and their formation environments. An investigation and familiarization with various weather and seasonal patterns associated with thunderstorm occurrences across California will be highlighted. Specifically, the southern California region will be focused on and for this reason four recent and original case studies were undertaken. Finally, severe thunderstorms and tornadoes are known to occur in this region as well, oftentimes presenting a significant forecasting challenge to the operational meteorologist and these issues will be addressed and focused on.

Thunderstorms in California on occasion become severe and/or tornadic.

Compared to the areas east of the Rocky Mountains, however, tornadoes are much less frequent over the (Mathews, 2009). Furthermore, in the occasional severe thunderstorm and tornadic development episodes that do occur in California marginal instability and shear parameters (compared to the central and eastern US geographic regions counterparts) are noted to be sufficient in sparking off local severe weather. This combined with recent findings that local zones have been climatologically assessed to be much more prone to occurrences in California requires further awareness (Mathews, 2009). Tornadic thunderstorms in California have only recently

1 been studied systematically, and preconceived notions that tornadic are not forecasting problems normally experienced in California are prevalent (Monteverdi et al,

2003). We will attempt to investigate this issue, especially as it relates to operational , in the hopes that an introductory awareness will yield increased familiarity with forecasting thunderstorm and tornadic environments in California.

Some questions that this research hopes to answer include what types of synoptic patterns are the most favorable for thunderstorm development across California? What seasonal variations are there for these convective events? Also included is an important look into the difference between surface-based and elevated convection and the significance of differentiating between these two modes for forecasting California convection. Finally, the topic of severe weather will be addressed, including tornado events in California and any similarities or differences in forecasting them compared with

Midwestern/eastern US regions.

Forecasting convective weather (thunderstorms) in California is important and at the same time challenging. While the frequency of thunderstorms in this region is substantially less than found across other areas of the contiguous United States periodic thunderstorms, including strong to severe thunderstorms and tornadoes do occur across

California. Oftentimes, these convective events present a significant forecasting challenge to the operational meteorologist. The aim of this paper is more practical and operational versus theoretical. The goal of the research is for the questions, findings and conclusions in this work to hopefully become an asset in California convective forecasting and to be a source of information that an operational meteorologist can incorporate in an effort to better forecast and understand thunderstorm events in this

2 region. Equipped with this knowledge weather forecasters in California can become better aware of the weather patterns associated with thunderstorms in this geographic location as they can be quite different from much of the rest of the contiguous US.

Improved convective forecasts and accuracy are tremendous assets to the mission statement of a meteorologist to do the utmost in the protection of life and property.

I. Thunderstorm formation Basics

At the basic level there are three main parameters that forecasters evaluate when attempting to predict warm season thunderstorm potential – sufficient moisture and instability, a way to lift the air parcels to their LFC () and the use of instability parameters such as CAPE (Convective Available Potential Energy) and LI

(lifted index) (Tardy, 2002).

II. Convective Types

There are different “types” of thunderstorms that are oftentimes associated with a unique synoptic and/or mesoscale environment conducive to a particular formation.

“Single-cell” storms are the most basic and shortest in duration. “Multicell – cluster storms” are the most common type of thunderstorm, consisting of a group of cells that move along as a connected unit. Spatially these storms are of greater coverage than single-cell storms. The “multicell line storm” commonly referred to by the term “squall line” is a linear line of thunderstorms, oftentimes consisting of a well-developed gust front located at the leading forward edge of the storms. Finally, the rarest and typically most intense form of thunderstorm is known as the “”. This type of storm is much more common across the central US in a region known as “Tornado Alley” and

3 only occasionally seen in California. A highly organized and long-lasting thunderstorm the supercell is the storm mode most associated with organized severe weather, large and damaging hail, and tornadoes (NSSL). A supercell thunderstorm features a persistent in the low-mid-levels of the storm structure and can also have disparate storm motions (Mathews, 2009).

III. Surface Based Convection

Most thunderstorms are rooted in the boundary layer (Tardy, 2002) meaning the lowest several thousand feet of the atmosphere at and just above the earth’s surface. As a result, the convective processes are highly dependent on surface or near-surface conditions. Note, tornadic storms are found to occur with convection rooted in the boundary layer (surface-based convection). These conditions are almost exclusive to and months across northern and central California (Monteverdi et al, 2003).

Meteorological studies show that elevated convection, which is the opposite of what is termed surface-based, tends to be associated with a reduced likelihood of producing significant severe and tornadoes (Corfidi et al, 2008).

IV. Elevated Convection

In attempting to forecast a potentially favorable environment for thunderstorm development surface conditions prove to be very important most of the time in the mesoscale assessment of factors such as temperature, moisture and instability. However, this rule comes with a very significant exception, namely the phenomenon known as

“elevated convection”. As stated above regarding surface-based convection most thunderstorms are rooted in the boundary layer (Tardy, 2002). As a result the convective

4 processes are highly dependent on surface or near-surface conditions. This is not the case with elevated convection which takes place largely above this boundary layer and as a result the initial conditions at the ground are of little significance to processes taking place some 10,000 to 20,000 feet above the surface. A percentage of California thunderstorms, including several notable impressive thunderstorm episodes, are of the elevated convection category and identifying the differences in patterns between surface and elevated thunderstorm setups is crucial to an accurate convective forecast and pattern recognition.

The above highlights the importance of recognizing mid-level convective processes and their relevance to forecasting elevated thunderstorms. A sufficient amount of subtropical moisture when combined with atmospheric lift and instability may yield deep moist convection that would otherwise not be expected if one’s point of reference was only the stable boundary layer. Forecasting these types of events is very important across California due to the impact that such thunderstorms can have on human life, property and resources (Tardy 2001). Finally, these events known as “elevated” thunderstorm episodes are not to be treated as “rare” occurrences in California and forecasters should be up to date on the relevant knowledge of forecasting in environments with stable boundary layers but moist and unstable mid-levels which correspond to favorable elevated convective setups.

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V. Seasonal Thunderstorm Patterns

A. Monsoonal Convection

Thunderstorm occurrences in California vary by season. The cool-season Pacific storm systems bring in the atmospheric conditions favorable for convection typically during the fall, winter, and spring months as a result of mid-latitude storm systems with associated unstable airmasses and moisture. However, the thunderstorms common to the months are a result of an entirely different weather pattern known as the southwestern US . The summer monsoon is characterized by a northwestward expansion of a high into the 4 corners region of the southwestern intermountain region. Deep tropical moisture from the eastern Pacific, Gulf of California and Gulf of Mexico is transported northwestward across Mexico and into the southwest states during the months of July, August and September. Westward shifts of this high pressure circulation bring increasing amounts of this humid airmass into southern

California and thunderstorms very common in the mountain and desert regions occur.

Weather hazards associated with the summer monsoon storms include -started wildfires, flash floods, damaging microburst winds, and the occasional tornado (Tubbs,

1972). As a matter of fact, contrasting geographic regions of California indicate different seasons for such severe weather occurrences as tornadoes, with the southeast desert region of California (an area that experiences the brunt of the monsoonal weather patterns and associated thunderstorms) experiencing the majority of recorded tornadoes in the warm season (Blier and Batten, 1994). This contrasts the majority of the state where the opposite holds true, namely tornadoes during the cool season.

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The System is described as the large-scale system which drives the impressive rainfall increase characteristic of the desert southwest region of the United States. Heavy summer thunderstorms are associated with flash floods among other severe weather threats and accurately predicting monsoonal thunderstorms is very important for local communities (Grantz et al, 2007).

The datasets and methodology used in the Seasonal Shifts in the North American

Monsoon study include division data, NWS Coop data, NCEP-NCAR reanalysis data and methodology. This particular study group attempted to investigate the driving factors behind the variability of the North American Monsoon from year to year. The pre- monsoon land- gradient was addressed. A very interesting hypothesis was formulated – the more moisture-laden the soil is across the southwestern United States, the longer summer heating needs to occur to trigger the appropriate land-ocean temperature gradient for the monsoon circulation to begin. The resulting links to the land, ocean and atmospheric conditions may offer long-lead forecasts of the summer monsoon

(Grantz et al, 2007).

As stated above, an in-depth study of tornado occurrences in California found that while most regions of California experience the majority of tornadoes during the cool season, the deserts of southern California are the exception, with the summer monsoon season or the warm season registering most tornadoes for this area. This is also in line with the southeastern deserts of California receiving a significant fraction of their annual during the summer months (Blier and Batten, 1994). Needles, CA for example experiences August as the month with the highest average precipitation, with

7 summer precipitation being much more convective in nature, suggesting a greater incidence of thunderstorms associated with the monsoonal circulation.

B. Winter Season Convection

The cold (winter) season features thunderstorm episodes associated with the migration of cold and deep upper level lows originating in the Gulf of Alaska and moving down the California coast (Brown and LaDochy 2001) . The majority of recorded

California tornadoes occur during the cool season (Hanstrum et al 2002). This period extends from late fall to early spring. Research conducted by Blier and Batten found that more than 80% of the tornadoes in the sample size occurred during the months of

November to April (Blier and Batten, 1994). The result is that California actually experiences its maximum tornadic activity during this cool season. This is very different from the annual distribution of tornadoes across the remainder of the contiguous United

States, where more than 67% of tornadoes occurred during the months of April to July.

There are numerous studies on severe thunderstorms and tornadoes across California that develop in the “cold sector” environment (Monteverdi and Johnson, 1996) of such cool- season storm systems. Some of these thunderstorms that form in such situations are known to be “low-topped” having the cumulonimbus thunderstorm cloud grow upwards until roughly 8,000 meters or around 26,000 feet above .

It has been pointed out that weather forecasters are accustomed to anticipating severe thunderstorm development in synoptic patterns that are characteristic of Great

Plains “warm sector” environments (Monteverdi and Johnson, 1996) which means that the boundary layer has a high equivalent potential temperature. Forecasters without the

8 awareness of thunderstorm environments known to occur in California cold-sector setups would have difficulty envisioning supercell development in such cases. It is mentioned that until fairly recently such severe thunderstorms that are featured in a low-buoyancy or low instability environment were thought to lack . Recent evidence points to the fact that in many cases such severe thunderstorms and tornadoes are supercellelular. One of the earlier documented examples of cold-sector in

California occurred on March 5, 1994. A well-defined hook echo was observed as the intense thunderstorm passed through the Fresno area of the

(Monteverdi and Johnson, 1996). This case study represented only the second radar- verified mesocyclone in California. Afterward, however, more radars went operational and many more examples of mesocyclones with rotation have been witnessed in ensuing years.

Such “cold sector” convection typically forms north or northwest of the surface advancing from the Pacific Ocean. The common order of such a sequence is a moderate-strong disturbance moves across northern and central California, which is typically in tandem to a middle-upper tropospheric short wave trough that moves southeast along the upstream side of a long-wave trough. Such a shortwave is often found to be negatively tilted and almost always is associated with moderate to strong mid- tropospheric vorticity or (CVA), cyclonic isothermal vorticity advection (CIVA) and also strong mid-tropospheric cold air advection (Monteverdi and

Johnson, 1996). A study by McCaul has indicated that tropical storm environments known to spawn supercells are also associated with low buoyancy but strong low-level

9 shear. Buoyancy values used in the modeling studies were comparable to California wintertime cold sector tornadoes (Monteverdi and Quadros, 1994).

C. Cut-off or Unseasonal Closed Lows

As stated previously, the winter season is the more common period for thunderstorms and associated severe weather for sections of California outside the mountain and desert regions (where the summer monsoon season is generally the most active). This is due in part to the positioning of the which is an important factor in synoptic-scale storm trajectory. As winter turns to spring and summer conditions become much drier with more occasional periods of thunderstorms usually associated with monsoonal moisture surges from Mexico or northward moving moisture from decaying tropical systems. The exceptions to this rule are cut-off or unseasonal closed lows. Just one example of a late-season closed low that triggered a well-documented severe thunderstorm in southern California occurred on May 24, 1996 (Garza and Atkin,

1996). During this event severe thunderstorms affected the northwest portion of San

Diego County, producing heavy and hail and affected hundreds of thousands of residents in the communities of Fallbrook, Oceanside and Escondido. Precipitation totals of almost 2 inches in a 3-4 hour period were observed over normally dry mountainous terrain and areas of flooding were noted. Also, as previously mentioned closed upper- level-low circulations are sometimes associated with extensive thunderstorm development in California because of enhanced dynamics and more favorable moisture transport (Tardy, 2001).

D. Tropical Systems

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Occasionally a dissipating tropical system across the eastern Pacific contributes to an influx of moisture and when interacting either with California topography or an upper level trough in the vicinity sets off a round of thunderstorm activity. This typically occurs near the time of the monsoon season and into early autumn as opposed to winter when tropical system activity diminishes (Tubbs, 1972). Tropical that form in the eastern North Pacific Ocean normally weaken below tropical storm strength upon making landfall over Mexico, or after moving over colder waters associated with the California

Current (Chenoweth and Landsea, 2004). This is the reason why tropical storms are extremely rare occurrences along the California coast. That having been said, four tropical cyclones have brought tropical-storm force winds to the southwestern United

States during the twentieth century: the tropical storm of September 25, 1939 in

California, Hurricane Joanne on October 6, 1972 in , of

September 10 1976 in California and Arizona and during September

1997 in Arizona (Chenoweth and Landsea, 2004).

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THESIS STATEMENT

Forecasting thunderstorms in California can be challenging due to a number of meteorological and geographical factors. Operational weather forecasters should familiarize themselves with California’s unique convective forecasting challenges to maximize forecasting accuracy in this region.

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IMPORTANT TERMS AND DEFINITIONS

CAPE - Convective Available Potential Energy. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather.

Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg. However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent. CAPE is represented on an upper air sounding by the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is warmer than the former. (This area often is called positive area.)

Downburst - A strong downdraft current of air from a , often associated with intense thunderstorms. Downdrafts may produce damaging winds at the surface.

EF-Scale - Enhanced Fujita Scale - A scale of tornado intensity in which speeds are inferred from an analysis of wind damage.

Exit Region - The region downstream from a wind speed maximum in a jet stream (jet max), in which air is moving away from the region of maximum winds, and therefore is decelerating. This deceleration results in divergence in the upper-level winds in the left half of the exit region (as would be viewed looking along the direction of flow).

This divergence results in upward motion of air in the left front quadrant (or left exit region) of the jet max. Severe weather potential sometimes increases in this area as a result.

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Level of Free Convection – (LFC) the level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely. This occurs most readily in a conditionally unstable atmosphere.

Shear – variation in wind speed (speed shear) and/or direction (directional shear) over a short distance within the atmosphere. Shear usually refers to vertical , i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.

Instability - The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted.

Instability is a prerequisite for severe weather - the greater the instability, the greater the potential for severe thunderstorms.

Jet Streak - a point or area ("streak") of relative maximum wind speeds within a jet stream.

LCL - Lifting Condensation Level - the level at which a parcel of moist air becomes saturated when it is lifted dry adiabatically

Lifted Index - (abbrev. LI) - A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms. However there are no "magic numbers" or threshold LI values below which severe weather becomes imminent.

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Mesoscale - Size scale referring to weather systems smaller than synoptic-scale systems but larger than storm-scale systems. Horizontal dimensions generally range from around

50 miles to several hundred miles. Squall lines, MCCs, and MCSs are examples of mesoscale weather systems.

Mesoscale Convective Complex - A large MCS, generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs:

 Size: Area of cloud top -32 degrees C or less: 100,000 square kilometers

or more (slightly smaller than the state of Ohio), and area of cloud top -52

degrees C or less: 50,000 square kilometers or more

 Duration: Size criteria must be met for at least 6 hours

 Eccentricity: Minor/major axis at least 0.7

MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which time the potential for severe weather is greatest. During peak intensity, the primary threat shifts toward heavy rain and flooding.

Mesoscale Convective System - A complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more. MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and Mesoscale Convective Complexes (MCCs) (among others). MCS often is used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an Mesoscale Convective Complex

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Microburst - A convective downdraft with an affected outflow area of less than 2.5 miles wide and peak winds lasting less than 5 minutes. Microbursts may induce dangerous horizontal/vertical wind shears, which can adversely affect aircraft performance and cause property damage.

Severe Thunderstorm - A thunderstorm that produces a tornado, winds of at least 58 mph (50knots), and/or hail at least 1" in diameter. Structural wind damage may imply the occurrence of a severe thunderstorm. A thunderstorm wind equalto or greater than 40 mph (35 knots) and/or hail of at least 1" isdefined as approaching severe.

Supercell - Short reference to Supercell Thunderstorm; potentially the most dangerous of the convective storm types. Storms possessing this structure have been observed to generate the vast majority of long-lived strong and violent (F2-F5) tornadoes, as well as damage and large hail. It is defined as a thunderstorm consisting of one quasi- steady to rotating updraft which may exist for several hours.

Synoptic Scale - The spatial scale of the migratory high and low pressure systems of the lower , with wavelengths of 1000 to 2500 km.

Triple Point - The intersection point between two boundaries (dry line, , cold front, etc.), often a focus for thunderstorm development. Triple point also may refer to a point on the gust front of a supercell, where the warm moist inflow, the rain-cooled outflow from the forward flank downdraft, and the rear flank downdraft all intersect; this point is a favored location for tornado development (or redevelopment).

(Glossary courtesy of the weather.gov)

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LITERATURE REVIEW

I. General Introduction

Thunderstorms are less frequent in general across sections of California compared to much of the contiguous US to the east. Compared to the areas east of the Rocky

Mountains, tornadoes are much less frequent over the western United States (Mathews,

2009). Furthermore, in the occasional severe thunderstorm and tornadic development episodes that do occur in California marginal instability and shear parameters (compared to the central and eastern US geographic regions counterparts) are noted to be sufficient in sparking off local severe weather. This combined with findings that local zones have been climatologically assessed to be much more prone to tornado occurrences in

California requires further awareness (Mathews, 2009). Tornadic thunderstorms in

California have only recently been studied systematically, thunderstorm environments in

California.

II. Severe Weather Forecasting Challenges As They Pertain To California In

Particular

Prior to even beginning a discussion of tornadic storms in California the once common myth that such events are either non-occurring or “freak” must be dispelled.

Indeed, increasing research into severe thunderstorms and tornadoes in California along with their environments has vastly contributed to the increasing awareness in recent years that such significant storms can and do occur in this state. Forecasting severe thunderstorm and tornado development is a challenge in any geographic region, however, this is even more obvious in California. In addition to drastic terrain differences and

17 multiple climate zones within the state observed severe thunderstorm and tornado events where found to occur with certain traditional severe weather parameters marginal at best.

A study of 39 non-tornadic and 30 tornadic thunderstorms occurring in northern and central California from 1990-1994 suggests that forecasters can use wind shear profiles to evaluate the potential of moderate and significant tornadoes in California (Monteverdi et al, 2003). It has been found that 0-1km low-level shear in California tornadic environments are comparable to values observed with tornadic environments.

Buoyancy, which is the surface-based convective available potential energy

(SBCAPE) was found to be weak compared to cases elsewhere across the central and eastern US and was found to be of little to no value when used as the only indicator to predict whether a thunderstorm would become tornadic in California. For the above mentioned reason of low buoyancy values the thunderstorms that do develop are typically low-topped with equilibrium levels of 15,000 to 35,000 feet above ground level. As mentioned above it has been pointed out that California low buoyancy environments but strong low-level shear are similar to that found during modeling (Monteverdi and

Quadros, 1994).

The synoptic patterns and characteristics of in California have been found in many cases to differ from those observed in the Great Plains severe weather environments. One significant difference is that these storms in California oftentimes form behind as opposed to along or ahead of an advancing cold front. This is further substantiated by a study of synoptic environments and convective modes associated with significant tornadoes in the contiguous United States. In that paper the

18 authors note that a majority of tornado cases, which took place east of the Rocky

Mountains, they researched, occurred in the open warm sector of synoptic cyclones

(Thompson et al). Note, that paper limited the research pool to tornadoes of at least F2 strength and most California tornadoes, however, are arguably weaker. Notwithstanding, the consequence of such an environment as that typically found in California cool-season tornadoes is much less potential instability when compared to cases east of the Rockies

(Monteverdi et al, 1988). This is important for a California forecaster to know.

The median CAPE for California tornadoes is less than 500 J/kg (Hanstrum et al,

2002). The knowledge of California’s preponderance of low buoyancy and high low-level wind shear environments and associated tornadic development can help alert forecasters to the possibility of tornadic thunderstorm formation when similar mesoscale patterns come into play. (Hanstrum et al, 2002).

A documented F-2 tornado, which is characterized as a “significant tornado”

by the Fujita scale and several other reports of tornadoes occurred in the Central Valley of California on September 24, 1986. This event is noted to be characteristic of a synoptic environment long-recognized by California forecasters as being associated with severe weather in the region (Braun and Monteverdi, 1990). The particular thunderstorm in focus had supercellular characterstics and the tornado was mesocyclone induced. A semi- stationary leeside trough combined with local channeling effects helped relatively moist air to move north into the northern portions of the Central Valley. In addition to the moisture influx, the surface flow enhanced low level wind shear to be much stronger than evident in the Oakland hodograph. This sounding then became comparable with supercell soundings elsewhere in the country. The thunderstorm first formed when the vertical

19 motion field associated with a sub-synoptic scale cloud band interacted with the more humid and increasingly unstable air moving north across the Valley. Furthermore, photographic evidence suggests that this tornado had multiple vortices and came from a wall-cloud on the south portion of the right moving supercell. Additionally, there was a rear flank downdraft.

Although as mentioned above the cold front has typically passed through the region, a surface low is oftentimes situated over the Pacific Northwest and the trough axis is still offshore. The result is a synoptic-scale pressure gradient that allows for southerly low-level flow over the Central Valley. An additional factor that plays a profound role in low-level wind shear enhancement is the terrain and topography of the state. Mountain ranges act as barriers that help focus surface winds, oftentimes allowing them to become more backed, thus increasing veering profiles with height and drastically increasing shear values (Monteverdi et al, 2003). The paper when referencing a finding of Hales (1985) noted that this topographically-induced wind backing occurs prominently in southern

California around the basin. It has even been stated that a “tornado maximum” has been found) in the Los Angeles basin due in part to synoptic pattern interactions with the local topography. Topographic channeling is a big contributor to

California tornadic environments as surface southeasterly flow is created east of the trough axis, this also separates northward moving air from an air mass that is subsiding along the Coastal Range’s east slopes (Monteverdi et al, 2003). This topographically- induced backed flow of southeasterly winds in the Central Valley greatly contributes to buoyancy and shear at this location. Warm air advection results from the southeast winds and this acts to destabilize the environment north of the cold front. Finally, southeasterly

20 surface flow enhances the creation of a veering wind profile through the mid-troposphere with the result being enlarged low-level shear being over-layed by the strongest cross- mountain flow and deep-layer (0-6 km) shear. Further backing of surface winds occurs around topographic features such as the Sutter Buttes near Chico, CA and this only increases low level shear favorable for storm rotation and the attendant tornadic potential.

It is ironic to note that previous assumptions held that California’s rugged terrain precluded severe convective storms (Brown and LaDochy, 2001). From increasing studies on California tornadoes, the very topography in certain regions of the state actually augments such potential, with topography and synoptic scale cyclonic flow combining to produce favorable wind shear environments for convective storms. Of significance is where these topographical features are located in relation to tornado maximums and population centers. One such example of a local tornado maximum in southern California is the Los Angeles Basin. The Peninsular Ranges in southern

California provide convergence of low level moisture associated with southerly winds

(Brown and LaDochy, 2001). Several case studies are investigated by Brown and

LaDochy in the different geographic regions of California. A Central Valley tornado occurring in Madera County was detailed. In this particular scenario southerly surface winds developed behind the front and parallel to the Sierra mountain range. It was deduced that topography had an effect on wind directions and an associated convergence zone. Aside from the upper level synoptic features of a jet max approaching the coast, and very weak convective instability per upper air soundings from 12z on

March 24th 1998, low level convergence, aided by topographical effects seemed to play a large effect in the development of an F0 tornado.

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A second case study featured an F0 tornado touching down in Brentwood of

Contra Costa County. While on this day southerly flow ahead of a warm front was noted on early morning surface observations, a cold frontal passage occurred during the day.

Topographical influences appeared significant as Central Valley winds are parallel to surrounding terrain. Winds became westerly along the coast, with inland winds remaining south or southeast across the Central Valley. Valley winds from the east converged with coastal in the locality of the tornado. Finally, a case study of a southern

California tornadic event shows the importance of local topography. On March 13, 1998 two off the coast of Huntington Beach were reported. Cyclonic flow associated with a storm system produced west northwest winds off shore, which became southerly with increased friction of the Peninsula Range. In addition, offshore channel islands appear to create a vortex and convergence area at the coast. These findings when combined with those of Blier and Batten find that southern California’s tornado maximums correlate to a very high population density with the greatest incidence of tornadoes occurring almost on top of the area of highest population (Blier and Batten,

1994). The authors go on to dispel earlier assumptions of weak tornadoes dissipating in urban areas. A remarkable finding of tornado incidence for the relatively compact area around the south coastal California Los Angeles Basin location suggests a tornadic incidence similar to that found for the state of Oklahoma, which is known for tornadoes.

As alarming as the finding sounds, it is important to remember a typical tornado over

Oklahoma may be much stronger and longer lasting than its south coastal California counterpart. Still, such findings point to much more frequent tornadic occurrences in southern California than otherwise assumed.

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It is also noted that mesoscale variation is apparent in the distribution of tornadoes. One cluster of observed tornadoes extends along a line from the Palos Verdes

Peninsula north. Another segment of tornado reports extends northeast from the southeastern end of this peninsula. A third group of tornadoes was found to the west of the northwestern edge of the Santa Ana Mountains. Many tornadoes that hit the region under examination actually began as waterspouts before coming ashore (Blier and Batten,

1994). Note, it is pointed out that the actual tornado occurrences may be higher than that which has been recorded due to areas of sparse population and other factors. Other recent works have also taken a key interest in southern California severe weather associated with synoptic and topographic variables (Brown 2009).

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APPROACH AND METHODOLOGY

A thorough literature review proved essential as part of the initial familiarization with convective forecasting in California. Up-to-date findings and challenges as they pertain to thunderstorm forecasting in the state are included as the foundation for the continued research undertaken in this paper. Ultimately, four recent and original case studies were formulated for the southern California region. Each case study featured extensive use of archived atmospheric meteorological observations databases, radiosonde data, satellite maps, radars, storm reports and storm damage surveys where applicable.

The establishment of pattern recognition between various synoptic and mesoscale setups and documented events pertaining to California convection was included. Additionally, meteorological variables and parameters in California as compared to other regions of the contiguous United States were compared and contrasted.

In an effort to get a broad but general sample inclusive of various convective modes and patterns particularly applicable to southern California four case studies reflecting various convective events, parameters and seasons were selected. In addition, multiple past events and scenarios connected with various modes of convective occurrences were reviewed. As an introduction to this idea it is important to be familiar with the basics of thunderstorm formation which are already included in the thesis. A quick review of these parameters includes sufficient moisture and instability, a way to lift the air parcels to their LFC (level of free convection) and the use of instability parameters such as CAPE (Convective Available Potential Energy) and LI (Lifted Index) (Tardy,

2002). The basic ingredients necessary for a thunderstorm are moisture, rising unstable air and a lifting mechanism (NSSL). Note: the lifting mechanism can be either

24 atmospheric or orographic in the form of a low pressure system, a frontal boundary or a mountain range, with all three being common to thunderstorm processes that occur in

California. Familiarization with these parameters was important as a basis for the case study investigations.

An awareness and differentiation between different convective modes and their observable effects on the resulting convective severity was applied in the analysis of the case studies. As covered in-depth in the literature review the various types of thunderstorms that were found in the case studies can be grouped into single-cell, multicell and supercell thunderstorms, each reflective of a particular set of synoptic and mesoscale circumstances and having a unique impact on the outcome of the associated weather conditions. The supercell thunderstorms, although the least common storm type in California, were found in at least one case study. The aim of this research was to investigate what atmospheric conditions are the most favorable for the various types of storms investigated and to judge the uniqueness of the storm environment as it applies to the southern California region.

The research method included a strong focus on the categories of convective initiation in general and these can be divided into “surface-based” or “elevated” thunderstorms. This was found to be very important in California thunderstorm forecasting and played a very large role in general convective and severe thunderstorm/tornado potential for each of the case studies. Factors such as temperature profiles, stability indices and moisture levels at various levels of the atmosphere have been found to be associated with whether a particular convective occurrence is characterized as “surface-based” or “elevated”. Additionally, the types of severe weather

25 a thunderstorm is capable of can be largely affected by whether it is surface-based or elevated convection. For the purpose of our study California has a sizeable amount of data and case studies on both surface-based and elevated convective events that were referenced during the formulation of the four case studies in this thesis. This is a particularly interesting topic that is related to California thunderstorm forecasting and is of primary interest to this region and one of the purposes of this study.

The methodology continued with a sampling of convective patterns associated with seasonal variations and distinct changes in thunderstorm distribution characterized by either winter-time or summer monsoon convection, with the case studies representative of both. Several other atmospheric triggers for convection that can also be seasonally-related are cut-off lows and tropical systems, with at least one case representing the former. Finally, severe weather forecasting challenges are repeatedly addressed with this being a significant focus of the research.

The data available and incorporated in the formulation of the case studies included heavy usage of the Plymouth State Weather Center’s archived data, Storm

Prediction Center severe thunderstorm database and previous case studies which have been expanded tremendously in the last several decades due to technological advances and increased interest in convective forecasting and California thunderstorm events. In particular, some of the most important archived data analysis tools included upper-air charts, satellite loops, radar imagery from the NCDC database, surface observations, radiosonde Skew-T diagrams, National Weather Service weather warnings and storm surveys. As such these became the primary sources of information during the data- gathering, research and analysis phase. The hope of this research is to ultimately provide

26 additional knowledge to the growing field and interest in California thunderstorm forecasting and be helpful to the operational forecaster when forecasting potential convection and thunderstorms in California.

The four cases studied in this paper were found to be representative of the different kinds of synoptic weather patterns that are most often associated with southern

California convective weather, namely winter season Pacific storms, summer monsoon moisture circulation and closed or cut-off upper level low pressure systems. Additionally, both surface and elevated convective events are represented in these case studies as both types of thunderstorm developments are seen in southern California.

Some potential limitations, both from an analysis and forecasting perspective, include the use of raw data versus unanalyzed charts for September 10, 2011 and July 29,

2003 cases versus analyzed and more detailed diagrams for January 19, 2010 and May

22, 2008 which were available from the SPC severe weather events page as both of the latter events were considered noteworthy enough for inclusion in a customized severe weather archive database. Furthermore, upper air soundings and the resultant charts are only available in 12 hour intervals and storm events often occur between successive maps. Additionally, the widely spaced coverage of radiosonde stations doesn’t take into account variables that may be occurring between radionsonde samples, which may be distant from an ambient thunderstorm environment.

27

Traditionally-Recognized Tornadic Parameters

To make the case that California’s tornadic environments at times differ and at other times are similar to those found across the majority of the US to the east of the

Rockies (where tornado frequency and research is much more prevalent) it is important to outline the “traditionally-recognized” tornadic parameters that most severe weather forecasters are familiar with and base their prediction methods on. The knowledge behind severe thunderstorm and tornado development has evolved substantially over the years.

An “ingredients-based approach” (Johns and Doswell 1992) has been used operationally in forecasting supercell thunderstorms and their associated tornado potential. Of these, the most widely accepted ingredients for supercells and tornadoes include:

1.) sufficient buoyancy, LI, K-index, TT (Total Totals)

2.) vertical wind shear through a substantial depth of the troposphere

3.) measures of low-level vertical wind shear, i.e. SRH (storm-relative helicity)

4.) measures of low-level moisture and PW (Precipitable Water) for the whole column

5.) discrete convective mode can enhance the potential for significant tornadoes

(Thompson and Mead 2006).

These findings are primarily based on Central and Southern Plains data, as such they may be different in other regions of the country and during other seasons of the year and as our case studies demonstrate all of them do not necessarily always apply to southern

California.

Operational tornado forecasting has been described as employing three general approaches including synoptic pattern recognition, meteorological parameter assessment or checklists and (Doswell et al., 1993). Furthermore, synoptic pattern-

28 specific or geographically-specific situations have been used to develop specialized forecasting techniques (Doswell et al., 1993). Diagnostic evaluation of surface and upper-air data becomes dominant over model prognosis during diminishing forecasting lead times (Doswell et al., 1993). Additionally Doswell et al., identify synoptic and mesoscale upward motion, sufficient moisture and lapse rate for a parcel’s buoyancy and the vertical wind shear structure as important ingredients for tornadic potential.

It has been consistently found that high values of low-level, or near-ground, is very important in enhancing the threat of tornadoes. This is because high surface moisture values relative to the air temperature lead to low temperature- depressions and this in turn contributes to low LCL (lifting condensation level) heights and subsequent low cloud bases (Rasmussen and Blanchard, 1998).

Wind shear has been viewed as a crucial element for tornado formation. In

“Synoptic Environments and Convective Modes Associated With Significant Tornadoes

In The Contiguous United States” Thompson et al., analyzed the wind profiles associated with significant (all F2+) tornadoes during the period 2000-2007. One of the most significant findings is that the wind shear values are region-specific. They found more unidirectional and stronger kinematic profiles characterize significant tornadoes in the

Midwest and Southeast US, while generally lower wind speeds but greater veering of wind with height, i.e. directional shear, was present over the Plains (Thompson et al.,

2008). These findings demonstrate that both speed and directional shear play an important role in the Plains.

Of all significant tornadoes during the seven year period, the mean 500 mb level wind speed was about 52 knots. The values ranged as high as approximately 76 knots in

29 the Southeast region during the winter and spring. The absolute lowest 500 mb wind speeds observed during the events in that database was approximately 25 knots, occurring in the northern Plains during the summer. The Southern Plains region’s lowest 500 mb wind speed associated with a significant tornado was approximately 37 knots during the spring (Thompson et al., 2008).

The mean 850 mb level wind speed was approximately 37 knots with values ranging from as high as nearly 62 knots in the Southeast region during the winter. The absolute lowest 850 mb wind speed was approximately 14 knots once again in the

Northern Plains region during the summer. The Southern Plain’s lowest 850 mb wind speed associated with a significant tornado was approximately 19 knots during the spring

(Thompson et al. 2008).

In “Tornado Forecasting: A Review” it is emphasized that vertical wind shear structure is becoming the key factor in distinguishing tornadic from non-tornadic events

(Doswell, et al., 1993) and this is claimed to be a critical limiting factor in delineating tornado threat areas in a forecasting setting.

Synoptic boundaries have been recognized as significant factors in severe thunderstorm and tornado development as well as influencing the convective mode

(Thompson et al. 2008). However, because of the standard station spacing on the order of

50 km in some locations, precise boundary locations cannot be known, and smaller-scale boundaries may have gone undetected in this 2000-2007 sample of cases (Thompson et al., 2008). In addition, SRH (storm-relative helicity) is often significantly augmented by both synoptic-scale and mesoscale boundaries (Markowski et al., 1998) and low-level outflow boundaries may be a dominant source of narrow zones with enhanced SRH.

30

A key finding of Thompson et al., is that synoptic systems associated with significant tornado cases vary from barely perceptible to high-amplitude waves, but the degree of “forcing” for large-scale ascent in the low-mid levels, which is often referred to as “dynamics”, does not need to be large directly over the area of significant tornadoes.

Doswell et al., (1993) go so far as to argue that while some major tornado outbreaks are characterized by widespread supercell-favorable environments and what may be stated as

“synoptically evident” setups, more than 90% of the tornado days per year are not

“synoptically evident” and involve mesoscale processes as primary contributors to their tornadic potential.

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Figure A

http://bangladeshtornadoes.org/UScases/060874/06087419zsf.gif

An example of a surface map associated with a Great Plains tornado outbreak

(dry line is labeled brown and is oftentimes a thunderstorm initiation focal point; outflow

boundary is labeled pink and can be a thunderstorm initiation focal point as well as a

zone of enhanced low level wind shear)

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Case Study 1: September 10, 2011

September 10th 2011 was a very active day for thunderstorms across many parts of southern California. Thunderstorms erupted in the morning hours across the Orange

County coastal plain producing multiple reports of quarter-sized hail from Dana Point to

Anaheim. As the day progressed convection developed across Los Angeles, Ventura and

Santa Barbara Counties as well as the High Desert and portions of Central California with locally heavy rain, lightning and more reports of hail.

To try to understand the atmospheric processes that may have been at work leading up to this convective event in an effort to better recognize and forecast similar occurrences in the future it is prudent to look at the synoptic picture of the day before.

The 500 mb analysis over the US at 12Z September 9, 2011 (figure 1) featured a very evident low pressure circulation centered over Utah. The 500 mb temperature of -16°C over Salt Lake City indicated substantial cold air aloft (for September). 500 mb winds were out of the northwest at about 25 knots over southern California, placing this region in the southwestern periphery of the upper-level low to the northeast. At 300 mb (figure

2) a high amplitude trough was seen across the Western United States. North winds greater than 50 knots were present from southern British Columbia to southwest and the base of the trough was seen across southern California into the Desert Southwest with 35 to 55 knot west-northwest flow from Point Conception to San Diego respectively.

At 700mb winds were cyclonic around the greater Southwest US and at the 850 mb level offshore northeast flow was occurring from San Francisco to Point Conception to the

High Desert.

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Figure 1

34

Figure 2

12 hours later at 00Z September 10th (figure 3) 500 mb observations indicated the upper low and associated cold air aloft was migrating westward with winds around

California lighter and temperatures colder at 500 mb. Another notable change was the configuration of the 700 mb flow over southern California. At 00Z the winds were out of the southeast at 10 to 20 knots from Point Conception to San Diego respectively, whereas

12 hours earlier they were out of the northeast at Point Conception and lightly west northwest at San Diego.

35

Figure 3

The 12Z upper air charts on September 10th (figure 4) continued to show a reconfiguration of the atmosphere with a broad cyclonic flow now centered more east to west over central California to central Nevada to Utah. At this time 500 mb temperatures had fallen to -14°C at Point Conception, -12°C at San Francisco and -9°C at San Diego, indicating that the coldest air aloft was now situated from south-central California to

Nevada. At 300 mb (figure 5) the trough axis was roughly situated from northwest

Montana through Oregon south across central Nevada and into southern California. At

700 mb winds were east to east-southeast at 10 knots at Point Conception and southeast at 20 knots over San Diego. 700 mb dewpoints had substantially risen across southern

36

California over the course of the 12 hours from the previous observations with readings changing from -20°C to 7°C over San Diego, a surge of 27°C overnight, indicating a significant mid-level moisture increase from the southeast. Point Conception’s dewpoint was almost the same as its 00Z observations, with a change of only 1°C noted. The 850 mb level, however, appeared rather dry with a -9°C reading over San Diego. 925 mb dewpoints featured a substantial rise from 4°C to 8°C at Point Conception and 0°C to

11°C at San Diego when compared across 12 hours from 00Z to 12Z.

Figure 4

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Figure 5

By 00Z September 11th, corresponding to the evening of September 10th, the 500 mb cyclonic flow was oriented in more of a south southwest to north northeast fashion with the cold air aloft still evident over California and Nevada. At 700 mb the winds were east-southeast at 20 knots at Point Conception and southeast at 25 knots at San Diego.

Satellite data indicated active convection during the early morning hours of

September 9th over portions of Arizona, New Mexico and Utah around and to the south of the closed low pressure circulation. By early afternoon of September 9th convection was initiating across the range of California and over the Great Basin region of

Nevada as well as the 4 Corner’s states of Arizona, New Mexico, Colorado and Utah.

38

There was also indication of convective attempts over the Mojave Desert region of southern California during the late afternoon hours, however, this proved to be very limited until the evening hours. At 00Z September 10th convection was occurring over far southeastern San Bernardino County. At 03Z pockets of cooling cloud tops were evident over the High Desert regions in the vicinity of San Bernardino to Kern County. During the overnight hours convection began increasing in coverage over and in proximity to these areas and continued to increase and intensify in an arc from the San Bernardino

County desert in the east almost to the Central California coast in the San Luis

Obispo/Kern County area in the west by 10Z September 10th. By 12Z convection had moved into the San Luis Obispo County coast and was developing over the Pacific coastal waters off of Point Conception in a northeast to southwest arc.

During the evening of September 9th and the overnight hours transitioning into the early morning of September 10th upper air observations and satellite data suggested a secondary area of cyclonic circulation in the vicinity of central California that was gradually shifting south somewhere between the coast and the San Joaquin Valley. By

12Z September 10th this circulation was near Point Conception and was made more visible by the developing convection wrapping around it.

Between 13Z and 14Z, or 6AM and 7AM PDT, satellite and radar data indicated compact but vigorous convection initiating along and just offshore of the coastal plain of

Los Angeles and Orange counties, a region which was previously mainly cloud-free at the mid and upper levels of the atmosphere. At 14Z (7AM) quarter-sized hail was reported in Dana Point. 25 minutes later quarter-sized hail was seen in Costa Mesa.

39

Between 10AM and 10:30AM (17Z to 17:30Z) quarter-sized hail was observed in

Anaheim.

Over the ensuing hours convection increased in coverage throughout the coastal, valley, mountain and desert regions to the north and west and moved south to north and east to west. The afternoon hours featured a significant diminishment and transition of convection out of the coasts and valleys and into the mountains and deserts where intense thunderstorms continued to develop over the course of the afternoon and evening, with convection continuing into the overnight hours over San Bernardino County.

A look at the upper air soundings for the two locations most representative of the southern California area in focus are Vandenberg AFB in Santa Barbara County and

Miramar Marine Corps Air Station in San Diego. The 00Z September 10th

(corresponding to 5PM PDT September 9th) sounding at Vandenberg (figure 6) features a very strong temperature inversion typical of the cool marine influence along the Pacific coast and late summer/early fall-like temperatures less than 1km above sea level. The air was rather dry throughout the atmosphere immediately above the marine layer, however, the layer between 900 mb and 650 mb had a modest moisture profile compared to the much drier atmosphere at higher altitudes.

40

Figure 6

San Diego’s 00Z sounding (figure 7) also featured a marine-layer temperature inversion, although not quite as strong as at Vandenberg and this is primarily because surface temperatures were warmer in San Diego than at Vandenberg and the layer of air around 900 mb was somewhat cooler in San Diego compared to Vandenberg. The moisture profile was much drier in San Diego with dewpoints below 0°C from about 950 mb to 850 mb and below -10°C above this altitude. The combination of temperature and moisture at the above locations appeared to feature a very convectively stable atmosphere.

41

Figure 7

12 hours later at 12Z September 10th the soundings had a significant reconfiguration of parameters. Vandenberg data indicated very similar and very convectively stable surface conditions with a continuation of the marine layer temperature inversion (figure 8). However, substantial changes were taking place aloft.

Lapse rates at and above 700 mb had dramatically steepened which is indicative of dramatic cooling aloft. At 00Z lapse rates were dry adiabatic from about 850 mb to 750 mb and then began to resemble moist adiabatic rates just under 600 mb and aloft. At 12Z, however, lapse rates were steadily dry adiabatic from 700 mb to above 600 mb. As a result temperature profiles between 700 mb up to above 300 mb were much colder at

42

12Z. At the same time moisture profiles indicated higher dewpoint advection had taken place between 700 mb to 450 mb and the temperature line was close to intersecting the dewpoint between 550 mb and 600 mb suggesting mid-level cloud bases at that altitude of about 4.5 km. Note, at this time convection was occurring in the area and the sounding indicates this was definitely elevated convection with a very stable surface layer and instability that was obviously supportive of convective activity in the mid layers of the atmosphere.

Figure 8

43

San Diego conditions featured certain substantial changes at 12Z as well (figure

9). Mid-upper-level cooling was also evident above 700 mb, although mid-upper-level temperatures were not as cold as at Vandenberg. A much more evident change was the dramatic increase in mid-level moisture. Apparently the overnight east-southeast winds from 850 mb to 650 mb transported a significant surge of moisture into the atmosphere above this part of southern California. For instance, just above 700 mb dewpoints surged from about -27°C at 00Z to about 7°C at 12Z, and were close to 10°C near 750 mb. At this level the temperature lapse rates were not quite as steep as at Vandenberg but temperatures were almost identical to the dewpoint, particularly between 650 mb and 700 mb. Below this altitude the air was very dry between 900 mb and 800 mb. Once again the surface marine inversion indicated very stable convective parameters at the surface. The area to watch would be the mid-levels where the cooling was taking place and moisture was increasing and these areas on the sounding are consistent with elevated convective possibilities. In addition, the San Diego sounding indicates the presence of a fair amount of wind shear which is different from Vandenberg which exhibited almost unidirectional flow and light winds speeds at the level of convection in the mid-levels. San Diego’s soundings featured east-southeast winds from about 900 mb to 650 mb and then veering winds were evident to the south and southwest from 600 mb to above 400 mb.

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Figure 9

Case Study 1 Concluding Thoughts

The southern California widespread thunderstorm event of September 10, 2011 is a classic example of intense elevated convection that is rooted above a very convectively stable boundary layer. If a forecaster was just paying attention to surface conditions as a determinant of convective possibilities they would have erroneously concluded no risk of thunderstorms and would have entirely misdiagnosed the thunderstorm potential and would have missed the forecast. Indeed, the surface and near-surface conditions which featured relatively little moisture and cool temperatures underneath a very strong temperature inversion proved that surface-based convection was just about out of the

45 realm of possibilities across coastal southern California, especially in the morning hours.

The key to an accurate forecast, however, was properly assessing the steep lapse rates and high moisture advection at around the 700 mb level, as well as the cold air aloft at even higher altitudes. Of course, these characteristics were symptomatic of the synoptic pattern at the time which featured an upper-level trough and an associated area of low pressure aloft.

Also of note is the apparent upper-level divergence occurring during the most convectively active period and coinciding with the axis of persistent thunderstorm development over southern California. The 300 mb chart at 12Z September 10th featured a great example of this with southwest winds of 30 knots across south-central Nevada and east-northeast flow of 20 knots at Point Conception. This pocket of upper-level divergence, when combined with the cold air aloft and associated vorticity near Point

Conception, may have been the impetus to enhance rising air motions concurrent with the ongoing mid-level moisture transport from the southeast. As such, once the level of free convection was reached and air parcels would become buoyant due to the significantly cooling air aloft, vigorous thunderstorm updrafts would develop, all rooted above the stable boundary layer. The multiple reports of quarter-size hail, which is rather impressive for California standards, is indicative of such vigorous updrafts and cold air aloft.

When dealing with a potential scenario of elevated convection the forecaster must tune in to the irrelevance of surface conditions and focus instead on the mid and upper levels of the atmosphere. As a result of this the distribution of elevated convection is much wider and not limited to micro-regions such as during surface-based convection

46 which is heavily based on thermal instability developing from the surface up through the atmosphere. This is also why the first signs of convection in the vicinity of Los Angeles and Orange Counties were first noted over the coastal waters just offshore and also the most impressive convection began occurring during the early morning hours, which is well before peak surface heating and associated surface destabilization would take place.

Convection may have transitioned to more of a surface-based scenario during the afternoon in the High Desert, however, and at this time many of the coastal areas that had morning thunderstorms were clearing out.

Of course, the entire synoptic situation of September 10, 2011 revolved around a closed and semi cut-off upper level low pressure circulation. This is in agreement with previous studies that have noted that closed upper level low pressure circulations are sometimes associated with extensive thunderstorm development in California as a result of the enhanced dynamics and more favorable moisture transport associated with them

(Tardy, 2001). In regards to moisture transport this was an early September case when monsoon moisture is still readily available around or close to the region. As the mid-level flow became southeasterly in response to the upper level low moving into position over the southern California area remnant monsoon moisture that was apparently in place over parts of the Desert Southwest and northwest Mexico surged northwest and helped provide the necessary moisture for the instability to work with. This case was a good example of a closed upper level low that became the catalyst for an impressive outbreak of elevated convections across southern California.

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Case Study 2: July 29, 2003

During late July 2003 the Southwest US monsoon became quite active with multiple days of widespread thunderstorm activity from Mexico to Canada and from the

High Plains to the Pacific Ocean. Thunderstorms are quite common in the mountains and deserts of southern California during the summer months from July through September, usually associated with the Southwest US monsoon. This is due in large part to a westward shift of a high pressure circulation resulting in increased amounts of humid air and resultant thunderstorms across the above mentioned areas (Tubbs, 1972).

Microbursts, characteristic of sudden of wind, are also seen across the desert floor as heavy bursts of precipitation falling out of a thunderstorm evaporate during the multi-thousand feet descent across typically dry and hot atmosphere on the way to the surface. Dry sub-cloud air has been found to be important in generating and maintaining strong downdrafts associated with evaporative cooling (McCarthy et al. 1982).This sudden evaporation can lead to a pocket of heavy and cool air that virtually falls through the atmosphere and then spreads out upon making contact with the ground and can lead to brief gusty winds underneath and in the vicinity of a desert thunderstorm.

On July 29th southern California was in the midst of an impressive monsoonal moisture surge. During the late afternoon and early evening a severe thunderstorm moved through the Antelope Valley producing vivid lightning, heavy rain, hail but most importantly a damaging microburst. The aftermath of the brief but very strong thunderstorm-induced winds included many trees uprooted, roofs damaged, a steel billboard sign severely bent over, car windows shattered and power outages. At a

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Lancaster Walmart customers were locked inside the store for 15 minutes as a safety measure during the storm (see SPC storm reports).

2 SW LOS TREES DOWN AND MINOR ROOF 0035 UNK CA 3467 11816 DAMAGE @ SUNSET RIDGE APTMTS LANCASTER ANGELES LOCATED 17TH ST W & K2 ST. (LOX) 535 PM SMALL TREES BLOWN DOWN LOS ROOFS TORN UP, AT E20 TH ST AND AVE 0035 65 LANCASTER CA 3469 11813 J 1 FOOT OF WATER AND AT WALLMART. ANGELES FOR SAFETY PEOPLE LOCKED IN BUILDING FOR 15 MINUTES. (LOX) SPC Storm Reports

At the synoptic level on the morning of July 29th a strong ridge of high pressure was situated in an east-west fashion from western Colorado to the Pacific Ocean just off of northern California. This configuration allowed a moderately strong east flow of 15 to

25 knots to push into southern California.

49

Figure 10 This easterly flow continued at the 700 mb level (figure 11) as well indicative of a good westward push of air across the mid-levels of the atmosphere.

Figure 11

The upper air sounding most representative of the Antelope Valley is the Edwards

AFB. However, this launch data is not always available and for this particular day we need to look at the surrounding sites to get a general idea of the atmospheric profile. The two closest locations would therefore be San Diego to the south southeast and Las Vegas to northeast. Also because the winds in a large portion of the atmosphere were evidently easterly, the observations toward the east would be representative of the conditions

50 upwind from the vicinity. However, on this day the archived sounding data for Las Vegas was not readily available so San Diego was our closest alternative.

At 12Z the sounding launched from San Diego (figure 12) showed nearly unidirectional easterly flow throughout much of the atmosphere. A temperature inversion was seen from the surface to about 900 mb (roughly 3,000 feet) and a shallow moist layer near the surface underneath a moistening atmosphere from about 900 mb up to 550 mb.

Any surface based convection with the current sounding profile would stand a chance of being initiated with significantly warmer surface temperatures, something that would be fairly challenging to come by at the location of the sounding given the strong marine- layer induced temperature inversion. However, above the inversion the chances of an air parcel attaining the level of free convection would increase given adequate and increasing low to mid-level moisture values and a modestly favorable temperature profile. The mountains and deserts of southern California are almost always above this marine layer inversion, being farther away from the coast and the mountains essentially acting as a barrier preventing the marine layer from penetrating into the deserts, and as as such these areas would need to be watched for convection in response to surface heating.

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Figure 12

A comparison with the 00Z San Diego sounding (figure 13) still showed very little prospects of surface-based convective support due to the continued presence of a capping inversion above the typical marine layer, however, air parcels rising from landmasses above that inversion, which would be found across interior valleys, mountains and deserts, would attain their level of free convection and become buoyant as a result of strong surface heating and high moisture levels (PW of 1.73) at the points in the atmosphere corresponding to locations above the marine inversion.

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Figure 13

At 00Z July 30th, corresponding to the late afternoon of July 29th the easterly flow at 500 mb was even stronger than the morning observations had indicated with a 30 knot east flow at Point Conception up from 15 knots at 12Z and a 30 knot east southeast flow at San Diego up from 20 knots at 12Z (figure 14).

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Figure 14

At 700mb there was little change in winds compared with 12Z data (figure 15).

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Figure 15

At the surface temperatures exhibited a very summer-like pattern with very warm conditions over the deserts and much milder readings at the coast. The 23Z map of surface temperatures (figure 16) indicates the late afternoon conditions corresponding to

4PM PDT, which is about an hour before the severe thunderstorm hit Lancaster.

Temperatures in the mid to upper 90s are seen across the western Mojave Desert, triple digits in parts of the San Joaquin Valley and eastern desert locations while coastal readings were only in the 70s.

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Figure 16

A look at 23Z dewpoint readings (figure 17) reflected a moisture-rich airmass in place across southern California. Dewpoint temperatures were in the mid to upper 50s across the western Mojave Desert and in the mid 60s across the San Bernardino County deserts. Also mid 60°F dewpoints were common in the coastal and valley regions from

Ventura to San Diego Counties. Dewpoints up to and exceeding 70°F were evident in the lower deserts around Palm Springs and in the lower Valley.

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Figure 17

Surface winds were light with modest onshore flow, typical of summer afternoons as thermal low pressure develops over the deserts (figure 18).

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Figure 18

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Figure 19

The combination of a moisture-rich airmass and temperatures at and above the convective temperatures necessary for surface-based convective initiation set the stage for the development of thunderstorms by 21Z (2PM PDT) over the mountains and High

Desert areas. The moderately-strong easterly mid-level flow helped the convection move westward at a good pace. At 3:25PM a strong thunderstorm was located near Victorville.

At 3:40PM a new thunderstorm cell was developing near Lake Los Angeles. By 4:30PM a complex of thunderstorms was evident across the southeastern Antelope Valley centered near Lake Los Angeles and moving approximately west to west-southwest. By

5PM this area of thunderstorms was moving into eastern sections of Lancaster. Over the

59 course of the next 45 minutes or so the thunderstorm moved across Lancaster and while weakening somewhat still held together as it moved into the mountains along the southwestern Antelope Valley.

Case Study 2 Concluding Thoughts

The severe thunderstorm of July 29, 2003 is a classic example of surface-based convection very typical of the monsoon season in southern California’s desert regions.

The convection was very diurnally-driven with copious monsoonal moisture combined with strong daytime heating sufficiently destabilizing the atmosphere, resulting in mid- late afternoon thunderstorm initiation over the mountains and deserts. One thing that stands out with this event, compared to more typical monsoon setups, is the unusually fast mid-level flow across southern California. With 500 mb winds of 30 knots the thunderstorms were moving quicker than typically found with southern California summer thunderstorms that are normally slow-moving. These winds may have also aided in storm organization and sustained thunderstorm strength with the quick movement helping to maintain adequate storm-scale inflow of warm, moist and unstable air. This can be seen with the thunderstorms that moved across the Antelope Valley lasting more than two hours.

Despite the high moisture content in the lower levels of the atmosphere characterized by Antelope Valley dewpoints in the mid to upper 50s the temperatures were in the mid to upper 90s immediately before the thunderstorms moved across the area. This nearly 40 degree temperature-dewpoint spread, or difference, may have allowed a sufficient evaporative cooling and density-increasing downdraft to occur that

60 resulted in the damaging microburst in Lancaster. It is important to point out that this was not a dry, but a wet microburst, accompanied by very heavy rain and hail. Also, it may be assumed that the strong and fast mid-level flow may have contributed some momentum transfer, aiding in the strong winds surfacing across the Antelope Valley floor during the microburst.

Monsoonal pattern recognition is a necessary first step in forecasting southern

California mountain and desert summer thunderstorms. A good representation of monsoonal circulation is usually seen by looking at 500 mb charts and assessing the placement of the upper-level ridge of high pressure. On July 29th the position of the high pressure in an elongated fashion from Colorado to the Pacific Ocean just west of

California allowed for the moderately-strong east to east-southeast mid-level flow above southern California. This configuration allowed moisture-laden air to circulate into the region and also helped thunderstorms to move in a westward direction, bringing them into and across the Antelope Valley. Paying attention to the temperature/dewpoint spreads, particularly around peak afternoon heating, may suggest a large difference between the two parameters. This can contribute to evaporative cooling and microburst potential. In addition, the relatively strong 500 mb flow may have also contributed to thunderstorm organization, persistence and strong damaging wind potential.

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Case Study 3: January 19, 2010

On January 19, 2010 a strong winter Pacific storm system came ashore across

California. Several reports of tornadoes and severe thunderstorm wind gusts dotted the southern California coastal plain. Of particular interest on this day was a very well- defined supercellular mesocyclone that developed over the coastal waters west of

Catalina Island and continued to exhibit very impressive circulation signatures as it moved toward the coast and then onshore across the Long Beach to Huntington Beach corridor during the early afternoon. The type of radar signature associated with this tornadic thunderstorm is indeed rather rare in Southern California.

12Z 500 mb upper air analysis on the early morning of January 19 was indicative of a cold trough approaching the West Coast with temperatures at and below -20 Celsius across almost the entire Western US region (figure 20). Cyclonic curvature was evident with 50 knot westerly winds at Point Conception, CA, 50 knot west-southwesterly winds at San Francisco and 45 knot southerly winds along the Washington coast. 500mb temperatures and wind patterns suggest an expansive “cold-core” upper-level low pressure circulation along the West Coast.

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Figure 20 Comparing winds at lower levels of the atmosphere the direction of flow gradually backs with lower altitude until surface winds are south-southeast from southern to central California (figures, 21, 22, 23). This is indicative of rather robust wind shear

(both speed and directional) across southern and central California with a strongly veering wind profile with increasing height, one important ingredient in potential tornado development (Doswell, et al., 1993).

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Figure 21

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Figure 22

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Figure 23

Meanwhile, at 300 mb a 125 knot jet stream was helping to carve the trough just southwest of southern California (figure 24). A 150 knot northwesterly jet streak was visible several hundred miles west-southwest of Point Conception, which would begin putting southern California into the favorable left front quadrant and providing upper- level divergence and uplift as it approached.

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Figure 24 300 mb Upper Air Observations, Isotachs, Streamlines, Divergence ()

Taking a look at the 12Z upper-air sounding from Vandenburg, CA (figure 25) shows a very impressive wind shear environment with a very strong clockwise (veering) curvature profile. Some warm air advection between about 850 mb and 750 mb and again above 600 mb suggests not very favorable instability parameters for the time being and for this reason the early morning CAPE value was only 45 J/kg and the LI came in at a stable 5 degrees Celsius. Of course with cold air aloft nudging into the coast through the day the instability would only be expected to increase. In such a favorably-sheared environment the combination of cooling air aloft along with strong frontal forcing for

67 ascent would potentially allow any sustained convective updrafts to pose the chance of rotation.

Figure 25

An 18Z visible satellite image (figure 26), corresponding to 10AM PST, features a well-defined frontal boundary, as evidenced by a sharp line of clearing at the western boundary of the clouds, aligned roughly north-northwest to south-southeast along the northern and central California coast nearing Point Conception. Surface observations confirm the frontal position with a marked surface wind shift line coinciding with the clouds to clearing moving into the aforementioned coastline. At 18Z winds were generally southeasterly east of the front along and east of the coastline and becoming southwesterly over the coastal waters pushing onto the northern and central California coast (figure 27). The coastal waters and coastal plain of southern California, being

68 geographically displaced southeast of this frontal boundary, was almost entirely under southeast to east-southeast flow at about 10 to 30 knots of sustained wind, the strongest readings being found across the coastal waters.

Barometric pressure readings were well correlated with the surface winds (figure

28). A 999 mb reading is evident south of Point Conception, which is the lowest reading in the southern California region. A secondary region of low pressure is also seen toward the San Francisco bay area. Substantially higher pressure readings were seen across the

Los Angeles Basin (1007.5 mb) and inland across the valleys and deserts, as well as toward the southeast coastal waters. This pressure configuration features a moderate pressure gradient over a short distance.

Figure 26 Source: Naval Research Laboratory

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Figure 27

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Figure 28

While there is the presence of convection associated with the cold-core of the upper low over the Pacific Ocean, of particular interest are the convective enhancements seen in the cloud structure along and even ahead of the surface front across the southern

California coastal waters. Satellite imagery at 18Z suggests the presence of cumulonimbus development with several areas of mature updraft and downwind anvil structures from the Point Conception vicinity south-southeast. In addition, the frontal and overall cloud distribution configuration resembles a negatively-tilted cyclone, a synoptic feature oftentimes associated with enhanced convective activity (MacDonald, 1976).

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At 20Z (12PM PST) Temperatures were generally in the mid 50s from Point

Conception to the LA Basin and the San Diego County coastal temperatures were in the upper 50s to low 60s (figure 29). Temperatures across the southern California coastal waters were a handful of degrees warmer with readings mostly in the upper 50s to 60 degrees Fahrenheit.

Figure 29

20Z dewpoints were generally in the upper 40s to low 50s along the coast, with higher values into the mid 50s over the coastal waters (figure 30).

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Figure 30

During this time the frontal boundary and associated wind-shift line were very evident on surface wind observations as it was advancing eastward and approaching the inner-coastal waters of the southern California bight (figure 31). Winds were southeast to east-southeast from the inner-coastal waters to the southern California coast with westerly winds over the outer coastal waters behind the front. Strongly backed surface winds of generally 10-15 knots were evident along the coastal plain from Santa Barbara to San Diego counties.

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Figure 31

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At 20Z surface pressure had fallen to 998 mb between Catalina Island and the

Channel Islands (figure 32). This is a 5.2 mb drop in 2 hours for that buoy and based on these observations it is apparent that the surface low pressure center was moving east southeast, now centered over the inner coastal waters, approximately 40 miles west southwest of the LA Basin. Elsewhere across the southland pressures were falling across most locations when compared to 18Z observations, with the most dramatic pressure drops east of the low pressure center.

Figure 32

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At 21Z the surface frontal boundary and associated wind shift line (figure 33) moved across Santa Barbara and Ventura counties and was moving into LA County as evidenced by the wind barb and vector maps (figure 34).

Figure 33

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Figure 34

Surface wind charts and pressure readings confirm the presence of a 997.5 mb low pressure center near Burbank (figure 35).

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Figure 35

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Figure 36

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Figure 37

As the frontal boundary was approaching the California coast on the morning of

January 19th moderate to heavy rain showers and embedded thunderstorms were moving ashore onto the central California coast. The first severe weather related reports of the day came in with winds gusting to 75 mph along the Golden Gate Bridge of San

Francisco shortly after 6AM. Two more wind reports greater than 70 mph were registered shortly thereafter in San Mateo County (73 mph) and Santa Clara County (75 mph) as this area of heavy rain and thunderstorms was moving through the Bay area (SPC storm reports).

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As the morning progressed rain showers began moving into the southern section of the central California coastline with embedded thunderstorms rolling into the San Luis

Obispo area around 10AM. Around this time a wind gust of 60 mph was reported near

Pismo Beach of San Luis Obispo County.

Around 10:30AM the day’s first tornado was spotted in Goleta in Santa Barbara

County (SPC storm reports). As the front continued pushing east radar reflectivities were intensifying over the southern California coastal waters suggesting moderate to heavy rainfall and embedded thunderstorms in progress. Infrared satellite imagery confirmed the presence of embedded convection with the frontal band with several cumulonimbus elements amidst the variety of frontal and pre-frontal cloud cover.

At around 19Z one such convective region began developing over the southern

California coastal waters as evidenced by a cooling cloud top between Catalina and San

Nicolas Island (as seen on the CIMSS satellite blog). A second and separate vigorous updraft began forming shortly thereafter just south of this convective buildup between

San Nicolas and San Clemente Island.

Radar imagery confirmed this convection with high reflectivity values associated with a well-organized thunderstorm cell due west of Catalina Island after 20Z (12PM

PST). Of note and importance was the radar signature that this particular thunderstorm exhibited. A very tightly wound area of rotation was seen in a coma-head formation as the cell moved into and across the northwestern tip of Catalina Island (figure 38).

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Figure 38 Radar (http://vortex.accuweather.com/adc2004/pub/includes/columns/community/2010/rad120a nim.gif)

At 12:36 PM a tornado warning was issued for this thunderstorm as it was 16 miles southwest of Long Beach and moving northeast at 35 mph. The radar loop showed a very obvious and impressive circulation with this thunderstorm as it continued moving northeast toward Long Beach. A classic “hook echo” was evident as the circulation neared Long Beach. At the same time the eastern flank of this storm bowed out and separated somewhat from the comma head circulation. Moments later as this eastern segment of the storm approached the coastline a new compact circulation developed in

82 this eastern flank of the thunderstorm cell that had managed to move several miles ahead

(east) of the old circulation passing south of Long Beach. To apparently account for this development a new tornado warning was issued for Orange County as this severe thunderstorm capable of producing a tornado was located 17 miles southwest of

Huntington Beach, moving northeast at 30 mph. This new circulation along the eastern flank of the thunderstorm complex continued as a hook echo as it moved onshore into

Huntington Beach.

As this complex of thunderstorms moved into the coast with the afternoon progressing several other areas of apparent circulation (per radar loops) were evident, one in the Laguna Beach region and another just southwest of the Orange/San Diego County line.

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Figure 39 300 mb Upper-Air Observations, Isotachs, Streamlines, Divergence (Storm Prediction Center)

Case Study 3 Concluding Thoughts

The storm system of January 19, 2010 proved to be a rather impressive severe weather maker for California standards. However, what stands out with this particular case study are both the resemblance to and differences from synoptic and mesoscale features associated with more classic severe weather patterns in the eastern United States.

Some notable features of this storm and associated thunderstorm/tornado development:

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1. Convection occurred along and ahead of the Pacific cold front, as opposed to

behind the cold front and associated with the cold core of a storm which is much

more typical of California winter season thunderstorms (Monteverdi and Johnson,

1996). The prevalence of the severe and tornadic thunderstorms occurring ahead

of the frontal boundary resembles severe weather setups east of the Rockies and

for this reason it can be argued that this was not a typical California cold-sector

tornado event but resembled a “warm-sector” environment.

2. As a result of the above surface winds were already synoptically-backed across

much of California to the east of the frontal boundary. This, when combined with

a strongly veering and increasing wind profile with height, served to augment the

wind shear dramatically and ultimately resulting in very impressive upper-air

soundings that strongly resembled tornado setups in the eastern United States. The

winds were backed at the surface in response to the pre-frontal environment and

synoptic configurations and not as a result of topographic channeling which has

often been pointed out as being a big contributor to California tornadic

environments (Monteverdi, et al, 2003).

3. A formation of a surface low traversing along the southern California bight. This

seems to have created a focal point for the southern California

supercell/mesocyclone ultimately becoming the tornadic Huntington Beach storm.

This local mesoscale setup actually resembles a “triple point” scenario, a

phenomenon which is much more common over Tornado Alley, and sometimes

associated with impressive tornadoes (Maddox et al. 2011 and Rose et al. 2004).

This feature, may have strongly contributed to surface convergence, which may

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have helped the thunderstorm initiate and then be sustained for an extended period

of time as it tracked across the coastal waters and then inland. In addition, the

surface wind flow immediately ahead of this surface low was very strongly

backed with winds out of the southeast to east and even east northeast in a few

locations from the LA Basin to the Inland Empire. This would have contributed to

impressive low-level shear in the lowest several kilometers of the atmosphere

across the aforementioned areas at the time of the thunderstorm’s approach. In

addition, and particularly relevant to the LA Basin to Orange County to Inland

Empire corridor the surface wind observations note a possible boundary of west

southwest to east northeast surface convergence which would be immediately

ahead of the approaching surface low and situated roughly along the I-10 corridor.

The thunderstorm may have also been riding along this particular boundary as it

moved east northeast into the LA Basin and Orange County.

4. Southern California being at the left exit-region of a 150 knot 300 mb jet streak

(reference figure 39), a region oftentimes associated with rising air motions,

strong surface low pressure centers and tornadoes (Rose et al. 2004). Also this

area looks to be under upper-level divergence which is another indicator of

dynamics for rising air motions.

5. The radar presentation featured a very impressive rotating supercellular

mesocyclone with classic hook echo formation around the time of the confirmed

tornadic activity in Huntington Beach. It can be speculated that there was a high

likelihood of waterspouts, potentially strong, occurring as the storm was

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traversing the coastal waters during its approach to the coast before the

confirmation of the tornado in Huntington Beach.

6. Surface observations revealed very low temperature/dewpoint spreads with

differences between the two of only several degrees Fahrenheit. This would yield

high relative humidity values and very low LCL heights which has been found to

contribute to tornado formation (Davies, 2005).

7. As the complex of thunderstorms moved into the coast during the afternoon

several other areas of discrete thunderstorm formation and apparent circulation

(per radar loops) were evident, one in the Laguna Beach region and another just

southwest of the Orange/San Diego County line. This is indicative of the highly

sheared environment and adequate relative instability along the southern

California coast.

8. Things that are different from eastern US severe weather and tornado setups, and

more in line with California cold-season thunderstorms and tornadoes include the

relatively low temperatures in the low to mid 50s in the ambient thunderstorm

environment and marginal surface moisture with dewpoints only in the upper 40s

to low 50s inland and mid 50s over the coastal waters. As a result of the above the

mean layer CAPE values were very modest of only 200-300 J/kg according to

RUC model proximity soundings.

In conclusion the January 19, 2011 southern California tornado event was essentially a hybrid of California and eastern US tornadic parameters. It can be presumed to be a pre-frontal “warm-sector” environment on the one hand, minus the typical thermodynamic instability that is usually associated with it on the other as evidenced by

87 low temperatures and dewpoints yielding low CAPE values. Also, as pointed out earlier, the winds were synoptically-backed versus topographically-backed which is another similarity to eastern US tornado environments.

It appears that the most favorable mesoscale environment supportive of the severe weather and tornadoes occurred over the coastal waters and along the immediate coast of southern California and indeed based on the storm reports all 17 confirmed severe weather reports, including 14 severe thunderstorm wind events and 3 tornado reports were seen in coastal regions of the San Francisco Bay area and southern California.

Reviewing surface charts the highest temperatures and dewpoints (which would yield highest instability values) were seen over the coastal waters and immediately onshore in the coastal regions. Winds were also favorably backed increasing low-level shear and thus highly contributing to the tornadic development. However, one of the apparent effects of the backed winds may have been dry air-entrainment from the east and this may be why the more moisture-rich air was confined to the immediate coast and offshore waters. The dewpoints were in the 30s to 40s in parts of the Central Valley and desert regions and even with precipitation falling dewpoint readings were only in the mid

40s over the Inland Empire. This may have been a big factor in limiting the severe weather to the coast and potentially precluding what may have otherwise been a more widespread severe weather outbreak if the instability would have been greater further inland.

From a forecasting perspective knowledge of California’s low instability but high wind shear environments and cool-season tornadoes is crucial and would aid the

88 forecaster during the lead-up to a storm such as the January 19, 2010 example, but is not enough as this is a rather unique case. A rather significant difference and uniqueness with this case study, as previously highlighted, is the convection occurring along and ahead (to the east) of the Pacific frontal boundary, rather than behind (to the west) of it. This is rather rare for California and more characteristic of synoptic thunderstorm environments in the eastern US.

Satellite observation is very important in determining any possible signs of convection such as building cumulus and cumulonimbus developments and cooling cloud tops near the frontal boundary. Also the dynamic nature of the storm system and the seeming negatively-tilted synoptic configuration may increase the forecaster’s awareness of the possibility of convection (MacDonald, 1976). Once radar reflectivity is able to be monitored it can become evident whether the precipitation is stratiform or convective in nature.

It is important not to get too hung up over the lack of instability or very modest instability and low CAPE values as seen on area soundings as it has been pointed out that low instability is sufficient for convection and severe weather in California (Hanstrum et al. 2002). Also, the instability parameters would most likely become substantially different and more impressive with the introduction of daytime surface heating and upper level cooling associated with the incoming cold-air aloft and oftentimes this occurs after early morning soundings are taken.

Once thunderstorms are spotted or thought likely the wind shear environment needs to be monitored for severe weather and tornadic potential. A temperature and

89 dewpoint analysis may suggest locations of highest moisture and warmest temperatures and/or lowest temperature/dewpoint spreads needs to be monitored for highest convective, severe weather chances and tornado chances. Finally, the forecaster needs to be aware of the presence of upper dynamics such as jet streaks and areas of upper-level divergence that can serve to help trigger or enhance thunderstorm formation and any surface low pressure centers and areas of surface convergence and boundaries need to be monitored as possible focal points for convective initiation and tornadic development.

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Case Study 4: May 22, 2008

During the late afternoon of May 22, 2008 a thunderstorm developed over the

Inland Empire and produced one of California’s best photographed and videotaped tornadoes. Actually, there were at least two tornadoes in progress at the same time from the parent thunderstorm cell near March Air Force Base in Riverside County, creating scenes more reminiscent of the High Plains of the US than the Inland Empire of southern

California. The National Weather Service conducted a damage survey and the tornado was officially rated as an EF-2, with estimated winds of 115 to 120 mph. This made it the first EF-2 tornado in California since the implementation of the EF scale in February of

2007. The tornado began around 4:42PM on March ARB and ended around 5:03PM 3 miles southwest of March ARB near Moreno Valley. It was found to be 100 yards wide at its widest and traversed the ground for 3 miles. As the tornado moved across I-215 it lifted a tractor-trailer and blew over 9 train cars on railroad tracks adjacent to the freeway

(NWS Public Information Statement). However, the above while very noteworthy, is not the only exceptional feature of that day. At the same time of southern California’s tornadic activity a full-scale tornado outbreak was unfolding over the Great Plains. As we are about to see both southern California’s tornadoes and those over the Great Plains were associated with the same weather system at the same time.

On May 20, 2008 a cold and late-season trough of low pressure moved into the

Pacific Northwest. By the evening of May 21st a highly amplified and unseasonably deep trough became established over the intermountain West. 500 mb temperatures were recorded as cold as -23°C over portions of and Oregon with -24°C at 00Z May 22nd at Seattle, Washington (figure 40).

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(Figure 40)

By 12Z May 22nd a closed upper-level low pressure circulation at 500 mb was evident centered over southwest Utah with cyclonic flow around it from the High Plains to the West Coast (figure 41). At 300 mb a vigorous 125 knot jet stream along the West

Coast was helping to dig the trough further south, with a 150 knot jet streak along

Vancouver Island (figure 42). Southern California was located southwest of the upper- low and underneath the nose of the northerly 300 mb jet.

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Figure 41 500 mb Upper-Air Observations, Heights, and Temperatures °C in red (Storm Prediction Center)

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Figure 42 7300 mb Upper-Air Observations, Isotachs, Streamlines, Divergence (Storm Prediction Center)

North-northwest flow continued at 700 mb (figure 43) and 850 mb (figure 44) across southern California with the region along the backside of the high amplitude trough.

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Figure 43 700 mb Upper-Air Observations, Heights, Temperatures, Dewpoints >= 4°C (Storm Prediction Center)

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Figure 44 850 mb Upper-Air Observations, Heights, Temperatures, Dewpoints >= 8°C (Storm Prediction Center)

At 12Z the upper-air sounding launched from San Diego showed very modest moisture from the surface up to around 850 mb (figure 45). Above this level the moisture dropped off and the temperatures rose, indicative of what looked like a more than 1 km high marine layer. Winds were light south southeasterly under the inversion and became strong west-northwest to northwest from 850 mb and above with very strong west- northwest flow around and above 600 mb. The temperature profile was very stable with another thick capping inversion above 700 mb.

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Figure 45

The Storm Prediction Center’s convective outlook for the day showed the anticipation of a severe weather outbreak over the Central High Plains (figure 46). An area of general convection was possible along portions of southern California.

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Figure 46

12Z Surface analysis for the morning of May 22nd featured a good reflection of the upper- level synoptic pattern with west to northwest flow and lowering dewpoints resembling a post-frontal and cold air advection pattern for southern California (figure 47).

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Figure 47

At 12Z a band of mid-level clouds was evident on satellite imagery from the

Sierra Nevada mountain range into the San Bernardino County deserts. This area of clouds then began retrograding southwest and enhancing somewhat with radar reflectivities of shower activity beginning over the San Bernardino County deserts at

14Z. The showers began appearing convective in nature and possibly included embedded thunderstorms as they proceeded to become more numerous and rotated around the broad cyclonic flow south and east into the San Bernardino County mountains and the Inland

Empire by 17Z to 19Z (10AM to 12PM PDT). As the first batch of showers shifted south a temporary area of clearing developed. Around 20:40Z (1:40PM PDT) radar imagery

99 indicated renewed convective echoes developing along the I-210 corridor between

Pasadena and Ontario and convection developing near the south slopes of the San

Bernardino mountains around Yucaipa. After initiation this convection became nearly stationary. By 22:30Z (3:30PDT) convection continued to develop along the southern slopes of the San Gabriel mountains and the convection near Yucaipa had now moved onto I-10 from Yucaipa to Beaumont. Convection was slowly drifting south. At 4PM convection was strengthening between Beaumont and Moreno Valley. Around 4:30PM this thunderstorm began building southwest into Moreno Valley and it was around and just after this time that tornadic activity commenced around the March ARB area.

Afterward convection continued to develop off of the southern slopes of the transverse mountain range and moved south into the eastern LA Basin, San Gabriel Valley and the

Inland Empire before dissipating later in the evening.

A look at surface observations leading up to and including the time of tornadic activity around the Moreno Valley doesn’t suggest any smoking gun surface ingredients that would suggest support for severe thunderstorms, much less tornadoes. At 22Z (3PM

PDT) temperatures were generally in the mid 60s across most coastal and valley locations and dewpoints were mainly in the mid 40s across these areas, with upper 40s to low 50°F dewpoints around San Diego. Dewpoints across the High Desert were even lower with

20s to 30s common across the Mojave Desert. Winds were west to west to northwest over the coastal waters, north winds were seen around the Antelope Valley and light south to southwest winds were occurring from the Los Angeles to San Diego coastal areas. March

ARB in the Inland Empire had a temperature of 66°F with a dewpoint of 43°F and a light

100 northerly wind at 5 knots. To the northwest near Ontario the temperature was 61°F with a

54°F dewpoint indicative of the light rain associated with the convection in that area.

Figure 48

At 23Z (4PM PDT) at March ARB (figure 49) the temperature fell to 62°F, the dewpoint rose to 47°F and the winds became southerly at 15 knots. Conditions were mostly cloudy.

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Figure 49 At 00Z (5PM PDT) (figure 50) the thunderstorm and tornadic activity was still ongoing around the March ARB and the temperature dropped to 59, the dewpoint rose to 49 and winds were easterly at 20 knots. The localized and rapidly updating observations for

March ARB indicated that around the time of the first observation of the tornado the temperature was around 61 and the dewpoint was 46. Afterward temperatures eventually cooled down into the low 50s and dewpoints rose to 50 after a period of rain.

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Figure 50

At 00Z May 23rd the San Diego sounding (figure 51) along with the temperature profile of the atmosphere had substantial changes compared with the 12Z early morning data. There was significant cooling of the atmosphere at virtually all levels above the surface to about 350 mb and this was reflected by a LI value of 0.1 and TT of 52. The temperature inversions were almost entirely eroded where there were very strong capping inversions in the morning sounding and an air parcel rising from the surface would readily attain its level of free convection with temperatures just slightly higher than what the surface temperatures at the sounding location were. Winds were still lightly southerly from the surface to 850 mb with modest lower level moisture in this section of the

103 atmosphere. Also, where the winds were out of the west-northwest from about 800 mb and higher they were significantly weaker than the conditions seen at 12Z.

Figure 51

00Z upper air charts indicated important changes in the configuration of the synoptic picture. At 300 mb the jet streak that was becoming evident along Vancouver Island at

12Z had rapidly surged south during the course of the day and the nose of the 150 knot northerly jet streak was approaching Point Conception at 00Z, placing southern

California in the left exit region of the jet streak (figure 52), a region known for upper- level divergence and synoptically rising air (Glossary). 300 mb winds over San Francisco

104 were clocked at 165 knots, which is very impressive for any season and especially extremely strong for late May. Furthermore, the jet stream became aligned more north to south at 00Z rather than north-northwest to south-southeast at 12Z and supported further amplification of the trough.

Figure 52 300 mb Upper Air Observations, Isotachs, Streamlines, Divergence (Storm Prediction Center)

The 500 mb reflection of this upper-air configuration was that the low pressure circulation expanded in areal coverage and actually retrograded southwest, with southern

California now within the periphery of the upper-low’s circulation and most importantly

105 now underneath the unseasonably cold air aloft with 500 mb temperatures at and below

-20°C overspreading the region (figure 53).

Figure 53 500 mb Upper Air Observations, Heights, and Temperatures °C in red (Storm Prediction Center)

Case Study 4 Concluding Thoughts

The March ARB tornado of May 22nd 2008 was notable as becoming the very first

EF-2 rated tornado in California since the implementation of the EF-scale in February

2007 and the strongest officially-rated tornado in the state since the Sunnyvale tornado of

May 5, 1998. However, perhaps even more remarkable than the official statistics was the

106 fact that the thunderstorm and subsequent tornadoes developed in a synoptic region not at all typically associated with southern California tornadoes. In addition, the surface conditions featured very marginal amounts of low-level moisture with dewpoints only in the mid 40s at the location of initial tornadic development at March ARB.

Realistically speaking, it may be very hard to nearly impossible to accurately forecast tornadic potential in an atmospheric setup such as that of May 22, 2008. It appears that very subtle storm-scale processes may have come together in precisely the right way supporting the development of the tornadoes. A look at these storm-scale processes is beyond the scope of this paper and perhaps is an idea for future research or a more in depth study. In terms of the synoptic and even mesoscale levels a very general frame of reference can be applied in terms of pattern recognition, namely that severe thunderstorms and tornadoes, although rare, can apparently occur in southern California at the southwestern quadrant of a closed upper-level low if synoptic conditions are supportive of surface-based convective development in general and storm-scale processes help trigger tornado formation in particular. Seemingly marginal surface moisture levels, characterized by dewpoints in the mid 40s are also not to be construed as a limiting factor precluding tornadogenesis, given other more complicated storm-scale factors are at work in the rare cases such as this. To have at least two tornadoes on the ground simultaneously from one thunderstorm, including one officially rated an EF-2 tornado and on the ground for more than 20 minutes with such a synoptically ambiguous weather pattern in late May is truly incredible.

As the upper-level low retrograded and southern California came under the influence of unseasonably cold temperatures aloft the instability of the atmosphere

107 increased substantially. The San Diego sounding maintained a surface temperature profile just shy of the apparent convective temperature, however, temperatures across the Inland

Empire, where thunderstorms were ongoing, were several degrees warmer than those at the sounding location (before thunderstorm-induced surface cooling took place). Also temperatures aloft may have been slightly cooler as the Inland Empire was closer to the core of the upper-low, and finally topographic effects were instrumental in thunderstorm initiation as satellite and radar imagery suggested with convection forming over and near the San Gabriel and San Bernardino Mountains during the afternoon hours before drifting south into the Inland Empire. The combination of the above was enough to allow surface- based parcels to attain their LFC and at the same time upper-level support in the form of the left exit region of a powerful jet streak placed southern California in a region of upper-level divergence and upward air motions that may have enhanced thunderstorm development.

Wind profiles as seen on upper-air charts and on the San Diego sounding indicated the presence of both directional and speed shear with a veering profile with height. Also, the mid-level winds at 00Z May 23rd while still moderate, were dramatically weaker than those seen at 12Z May 22nd and this may have allowed convective development to build up without being sheared apart if the winds were too strong for the updrafts.

The tornadic Moreno Valley thunderstorm initially began as early to mid- afternoon convection that was initially confined to the San Bernardino Mountains and the

Yucaipa area. During mid-late afternoon that complex of thunderstorms drifted south and the thunderstorm that affected Moreno Valley actually developed toward the southwest.

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With a northwesterly mean wind flow at mid-levels of the atmosphere the Moreno Valley storm began moving/developing at a right angle to the mean wind. This can be compared to a right-moving supercell and the combination of that motion along with southerly low- level winds may have increased the storm-relative inflow and shear, which may have significantly contributed to tornadogenesis at March ARB.

Finally, at the surface there may have been multiple outflow boundaries sprawled across the Inland Empire from surrounding convection. This is supported by surface charts (see figures 48,49,50) from 22Z May 22nd , 23Z May 22nd and 00Z May 23rd indicating rain-cooled outflow boundaries and wind shifts. It is known that boundaries can serve as focal points for tornadogenesis (Maddox et al. 1979).

When attempting to make the argument that the environmental setup of May 22,

2008 did not appear to be a synoptically evident tornadic pattern Doswell et al. argue that

“more than 90% of the tornado days per year are not “synoptically evident” and involve mesoscale processes as primary contributors to their tornadic potential (Doswell et al.

1993)”.

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Figure B Near March ARB, May 22, 2008 http://www.telegraph.co.uk/news/2014222/Pictures-of-the-day-23-May-2008.html

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CONCLUSIONS

Convective forecasting is an interesting and oftentimes challenging component of operational . California’s heavily urbanized and high population density makes the issue of forecasting thunderstorms in this region important. Forecasting thunderstorms in southern California requires a general knowledge of convective environments and a specific familiarity with the environments and weather patterns unique to this geographic region. In this paper four recent thunderstorm case studies were conducted, with each one representative of a different category of seasonal and synoptic patterns that are known to affect southern California. In addition to supporting points made in prior literature there were numerous new and unique findings that were discovered during the scope of this research and these were discussed as they came up in their respective case study as applicable. These cases show that severe and potentially life-threatening and property damaging weather can and does occur precisely in and near the regions of high population density and it can be assumed that in general there may be low severe weather awareness in the general public.

Some examples of important forecasting guidelines include the difference between surface-based and elevated thunderstorms and the significant differences in accurate forecasting methods as was seen in the September 10, 2011 study. For instance, whereas surface-based thunderstorms are heavily dependent on the boundary layer and its associated instability, elevated convection is rooted entirely above the surface and as a result the boundary layer conditions are just about irrelevant. As a result, storm distribution is more widespread and not limited to favored micro-regions of sufficient surface instability which vary by season and location in southern California. This is very

111 important for a forecaster to be abreast of as southern California experiences a sizeable amount of elevated thunderstorm episodes. In addition, closed upper-level low pressure systems and their associated cold air aloft and dynamics are often catalysts for southern

California thunderstorms.

The July 29, 2003 case is a classic example of an intense monsoonal thunderstorm episode that is typical in the mountains and deserts during the summer season. That particular case study featured mid-level winds that were moderately strong and as a result may have contributed to the severity of the thunderstorm that hit Lancaster. Forecasters looking at sounding data need to keep in mind that surface conditions are oftentimes drastically different around southern California’s diverse topography. This is especially true considering two of southern California’s most regularly updating soundings come from coastal locations, which are far from representative of conditions areawide. Stable marine layer inversions at the coast often give way to unstable conditions inland and this is particularly true during the summer monsoon season in the local mountains and deserts. This is a unique forecast challenge to southern California. On a given day different zones will have different convective parameters. The forecaster must be aware of this and know how to look at a sounding and modify it for different locations.

The January 19, 2010 case study presented a rare pre-frontal severe thunderstorm and tornadic event and as discussed in the relevant section it was found to be a “hybrid” of both Eastern US tornadic parameters and those find in California tornado episodes. As a result neither set of forecasting parameters could entirely describe this event alone, and this case study was found to potentially add several significant findings to the growing interest and knowledge base of California thunderstorm and severe weather research.

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Finally, the unusual case of significant and unseasonal tornadoes occurring along the southwestern quadrant of an upper-level low pressure system on May 22, 2008 was investigated. It was found that in addition to a synoptic pattern that apparently proved to be supportive of organized severe weather, despite marginal surface moisture and atypical wind configurations there may also have been surface boundaries and mesoscale processes that significantly augmented the tornadic potential on that day and resulted in a highly localized tornadic environment around the Moreno Valley.

Thunderstorms and their associated threats of severe weather, while less common in California when compared to other parts of the country are nevertheless part of the annual makeup of weather in this region. Furthermore, the incidence of severe weather and tornadoes in certain regions of the state is much higher than would commonly be expected. Because of the reasons listed above and that California tornadoes tend to form in environments that are much different from those traditionally observed in other parts of the country there is strong scientific motivation to research and attempt to more accurately understand the meteorological conditions in which they develop (Blier and

Batten, 1994). One of the most significant factors from a human perspective as to the importance of forecaster familiarity with California tornadoes and the non-traditional environments of development is perhaps the very high population density of regions of

California that experience enhanced tornadic frequency, such as Los Angeles and Orange

Counties. Different convective modes present their own series of forecasting challenges and the difference between surface-based and elevated convection was highlighted in depth. Even though thunderstorms are typically rooted in the boundary layer, or surface based (Tardy, 2002) exceptions to this rule are what are known as “elevated

113 thunderstorms” and California has a fair share of such convection including several notable elevated convective outbreaks (Tardy, 2001). During such cases the weather forecaster needs to be aware of the minimal influence surface conditions play regarding thunderstorm initiation, as the convective processes occur well above the boundary layer.

The seasonal variations of thunderstorm occurrences were reviewed with the year being generally split into cool (winter storms), warm (summer monsoon) seasons and occasional cut-off and unseasonal upper-low pressure systems and their accompanying

California pattern changes. The cold winter season features thunderstorm episodes in response to cold and deep upper level lows from the Pacific and moving down the

California coast (Brown and LaDochy, 2001) and the January 19, 2010 case study was a good example of this. Additionally, the majority of recorded California tornadoes occur during the cool season (Hanstrum et al, 2002). The warm season is often associated with westward shifts of high pressure circulation from Mexico and the Desert Southwest bringing increasing amounts of humid air into southern California and resulting in monsoon thunderstorms very common to the mountain and desert regions (Tubbs, 1972) such as was the case with the July 29, 2003 monsoonal thunderstorm event. Also, occurrences of elevated thunderstorms are found commonly during the warm season and this presents its own set of forecast challenges (Tardy 2001, 2002) with the September

10, 2011 southern California elevated thunderstorm outbreak a good example of this.

Finally, remnant tropical moisture when encountering southern California during the warm season has contributed to periods of thunderstorms and heavy (Tubbs, 1972).

An example of organized remnant tropical moisture intrusions into southern California, however, was outside the scope of this paper.

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Aside from clearly delineated winter and summer seasonal weather patterns and their accompanying thunderstorm environments unseasonal closed or “cut-off” lows add to the forecasting challenge of California thunderstorms (Garza and Atkin, 1996). These authors focused on a late-season closed low that triggered a severe thunderstorm in southern California on May 24, 1996. In this paper the case of May 22, 2008 was a good example of a very unseasonably cold and closed low that moved across the Great Basin region and proceeded to retrograde toward southern California, setting off thunderstorms and a significant tornado occurrence in the March ARB area.

Meteorologists familiar with southern California thunderstorms may be more adept at anticipating certain weather situations capable of initiating convection mainly through experience and pattern recognition. However, a forecaster unfamiliar with southern California’s unique forecasting challenges may be in for a learning curve. These and other case studies can be advantageous to geographers in learning more about southern California’s climate and severe weather.

Weather forecasters in California are encouraged to be aware of the weather patterns associated with thunderstorms in this geographic region as they are sometimes quite different from much of the rest of the contiguous US. Improved convective forecasts and accuracy are tremendous assets to the mission statement of a meteorologist to do the utmost in the protection of life and property and new research into southern

California thunderstorm events and subsequent forecasting guidelines is an essential step in this direction.

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