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CALIFORNIA STATE UNIVERSITY, NORTHRIDGE FORECASTING CALIFORNIA THUNDERSTORMS 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 ii 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 iii ABSTRACT FORECASTING CALIFORNIA THUNDERSTORMS By Ilya Neyman Master of Arts in Geography Thunderstorms are a significant forecasting concern for southern California. Even though convection across this region is less frequent than in many other parts of the country significant thunderstorm events and occasional severe weather 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. iv 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 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 recent 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 1 been studied systematically, and preconceived notions that tornadic storms 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 weather forecasting, 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 storm 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 (level of free convection) 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 “supercell”. 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 mesocyclone 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 winter and spring 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 winds 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
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