Operational Techniques for Forecasting Thunderstorms

Module 4.2D2 Operational Techniques for Forecasting Thunderstorms

Contents

Introduction i

Diagnosis and Forecasting of Thunderstorms iv 250mb (Purple)...... v 500mb (Blue)...... vi 700mb (Brown)...... viii 850mb (Red)...... xi Surface (Black)...... xiv

Convective Parameter Review xvii Stability / Moisture Parameters...... xvii Dynamic or Wind Shear Parameters...... xx Combined Stability / Dynamic Parameters...... xxi

Convective Assessment Example xxiii

Summary xxvii

Introduction

In very general terms there are three requirements for the development of deep (surface based) convection (referring here to TCu and garden variety Cb), these are instability, boundary layer moisture (due to latent energy considerations) and some mechanism to produce vertical motion (i.e., a trigger) allowing a parcel to attain free convection. Assuming these three conditions are met we may expect deep convection to develop with its intensity dependant primarily on the degree of instability and availability of boundary layer moisture. Taking an ingredients-based approach to forecasting convection simplifies the process to one of diagnosing basic requirements to be met for thunderstorm development.

Figure 1: Ingredients necessary for thunderstorm development.

In the above figure we can imagine that the square represents all possible states of the atmosphere. Each circle represents possible states of the atmosphere exhibiting the indicated requirement for thunderstorms. Where the requirements for instability, moisture and some trigger mechanism are met is the most likely location for thunderstorms to develop. The difficulty for forecasters is that there are often many ways to determine if, when, and where these requirements can be met. There can be a tendency to seek a particular parameter or index that will give the final answer instead of simply looking at what physical requirements must be met for convection to develop. Using the shortcut approach can introduce even more confusion as many parameters will often suggest the same thing in different ways. Convective parameters are extremely useful tools but only if they are used correctly.

The three requirements listed above are fundamental to the development of all thunderstorms from short-lived single cells to destructive supercells. What separates the environments of varying storm types from one another is the degree of vertical wind shear within the pre-storm environment. Buoyancy primarily determines updraft strength but it is wind shear that determines the resulting storm evolution and morphology. In general, a good rule of thumb is that the stronger the wind shear, the more organized the resulting convection will become.

Figure 2: Ingredients necessary for long-lived thunderstorms.

Similar to figure 1, the figure above illustrates the requirements for persistent thunderstorms. The addition of vertical wind shear can be the difference between short-lived cells and long- lived severe thunderstorms. Note that no thunderstorms are expected when one of the original three requirements is missing, even if wind shear is present. Forecasting persistent thunderstorms (and severe thunderstorms) can again be simplified by using this ingredients based approach.

At this point it is useful to remind ourselves of the distinction between wind speed and wind shear. Wind speed of course refers to the vector magnitude of the wind at some level in the atmosphere, wind shear on the other hand refers to the change in wind speed between some level(s). This means that we can have an environment with strong winds aloft but little shear.

The role of wind shear in thunderstorm development includes the following: ƒ interaction of low-level wind shear and thunderstorm outflow boundaries to generate new thunderstorm cells (this is especially important for multicell thunderstorms) ƒ tilting of thunderstorm updrafts in deeply sheared environments ƒ generation of low-level horizontal vorticity in sheared flow (this is crucial for the development of mid-level rotation in supercell thunderstorms) ƒ interactions between updrafts and environmental wind shear resulting in storm splitting ƒ generation of vertical perturbation pressure gradients in supercell thunderstorms (this is significant in the production of strong updrafts in supercells and can have implications for tornadogenesis) These processes are all discussed in earlier sections of module 6.4.

Diagnosis and Forecasting of Thunderstorms

Now that we have reviewed the concepts important for understanding how thunderstorms develop we will look at how to diagnose the potential for thunderstorm development from an operational perspective. It is useful to always keep in mind the four requirements we have outlined for the development of persistent thunderstorms, namely: ƒ instability ƒ moisture ƒ vertical motion (trigger) ƒ wind shear As the forecaster goes through the analysis / diagnosis process they should always be on the lookout for these four things. More importantly, regardless of methods used or how they are characterized (e.g., tephigrams, upper air / SFC charts, GRIB viewers and various convective parameters) it is always these four basic requirements that must be diagnosed when forecasting convection. Therefore, even just reciting them in your mind during your work-up will help simplify the forecasting process. Sometimes, just one missing ingredient can be the difference between no convection and long-lived damaging supercells or multicell storm complexes.

In the Module 2.5D we reviewed the assessment of stability of a column using tephigrams. Now we turn our attention to diagnosis in spatial terms. We will focus on the use of SFC and upper air charts, hodographs to assess wind shear, and the use of GRIB viewers and convective parameters.

In this section we will examine the use of meteorological charts (both SFC and upper level) to look for significant features and associated processes that are conducive to convective initiation. We will look at each of the standard isobaric levels in turn, highlighting important features to look for and the conventional symbols used to represent them in the composite chart. For each level we have included an example highlighting the location of some relevant features. The charts used are all analysis charts from 12Z 2 July 2000, a day when widespread convection occurred in both AB and SK.

The information given below borrows heavily from both original work by Miller (1972) and the Summer Severe Weather Distance Learning Course (Training and Education Services Branch, Environment Canada 1995), hereafter referred to as SSWDLC (1995). Some modifications have been incorporated following an internal technical note (Whittle 1999) and discussions with operational forecasters. The list of features below is not an exhaustive one and does not include all of the features discussed by Miller (1972). We have instead included some of the more common parameters used operationally. Associated with each feature are methods of diagnosis, which of the necessary conditions for sustained convection (instability,

moisture, vertical motion, or vertical wind shear) that it addresses, and comments regarding the physical process that it represents.

It is important to note that while we may look for features using specific isobaric levels, the features themselves are not constrained to those levels. For example, the high level jet is not always at 250mb and generally exhibits vertical variability. Similarly, low-level features often associated with the 850mb level may actually exist above or below that level. As with any forecast, it is intended that the forecaster utilize all available information during the analysis / diagnosis process. This section is intended as a starting point for forecasters to develop their own personal convective assessment routine. Through experience, features not discussed here may be included and the forecaster will likely develop some of their own symbols.

250mb (Purple)

250mb Jet

J 120

Diagnosis / Comments: isotach analysis and 250mb heights. Also use CMC high-level max. wind chart. The J is used to denote the jet core with its strength in knots noted below.

Associated Process / Significance: VERTICAL MOTION

The high level jet is significant for enhancing vertical motions in the troposphere due to the ageostrophic circulations that exist around its core (refer to MOIP notes Section 6.2 H - number 8). Qualitatively and through visual inspection, for straight jet streaks, the right entrance / left exit of the jet core are preferred regions for upward vertical motion. In the case of cyclonically curved jets we look for the left exit region and for anti-cyclonically curved jets the right entrance. The resulting vertical motions can aid in destabilization, erosion of the capping lid through ascent, and enhancement of updraft strength. To assess these effects quantitatively look for 250mb positive vorticity advection and the threat area being on the cyclonic shear side of the jet. It is important to recognize that looking at the left exit of a jet core, evaluating PVA at 250mb and contouring divergence at 250mb are all essentially the same thing. The effects are not additive, they are the same so that PVA, DIV and a left exit at 250mb do not combine to further enhance vertical motion.

Figure 3: Sample 250mb chart from 12Z 2 July 2000.

500mb (Blue)

At 500mb we become concerned with both dynamic and instability processes through the presence of jets, significant cooling, and vorticity advection.

500mb Jet

J 80

Diagnosis / Comments: same as for 250mb jet. In addition, channel jets on vorticity charts are usually found just north of the 500mb jet.

Associated Process / Significance: VERTICAL MOTION / WIND SHEAR

The jet stream does not exist at a single isobaric level but fluctuates in the vertical as well. Often the jet core may subside to the 500mb level (or lower) so that in the composite plot the 500mb jet will often appear directly below at least a portion of the 250mb jet. This plays a role in vertical motions as described for the 250mb jet. A 500mb jet through a threat area is also indicative of vertical wind shear necessary to tilt the updraft and advect precipitation particles down wind, reducing water loading effects in the updraft region of the thunderstorm. This promotes persistence of the thunderstorm updraft thus increasing the probability for sustained deep convection. For severe thunderstorms (expecially supercells) look for wind speeds in excess of 30kt.

Cold Troughs (axis of coldest air)

Diagnosis / Comments: 500mb isotherm analysis at 2°C intervals. Outline / label cold pools and areas of cold advection. Alternatively, use GRIB viewer with 500mb height contours and wind barbs to identify areas of cold advection. Also examine temperature advection or progged 500mb temperature change over time. Note: Preliminary studies over Saskatchewan have found 500mb temperatures colder than -19°C to be associated with cold core funnels.

Associated Process / Significance: INSTABILITY

Cold air aloft acts to destabilize the column (most readily visualized on a tephigram). While cold air aloft promotes instability at an instant in time, cold advection into the threat area will decrease stability over time. Identifying the axis of coldest air highlights areas of the strongest cold air advection. (Recall, the Lifted and Showalter indices are based on 500mb temperatures). Cold air advection becomes especially evident when dealing with advancing cold lows.

Thermal Ridge

Diagnosis / Comments: 500mb isotherm analysis at 2°C intervals.

Associated Process / Significance: INSTABILITY

Leading edge of cold advection (i.e., cold advection upstream and warm advection downstream).

Positive Vorticity Advection (PVA - Yellow)

Diagnosis / Comments: from 500mb vorticity charts or using a GRIB viewer, plot vorticity advection or vorticity, heights, and wind barbs as for shortwave troughs. Orange may be a better colour to use when developing composite plot.

Associated Process / Significance: VERTICAL MOTION

From synoptic meteorology we know that increasing PVA with height is generally associated with divergence aloft (in conjunction with thermal advections -- recall the QG Omega Equation) and from the continuity equation this implies upward vertical motion below 500mb. PVA and the associated large-scale lift can be an adequate triggering mechanism to initiate convection or intensify existing convection, and can (through lift) erode the capping lid. PVA tends to be responsible for lifting the air mass, which in turn leads to a destabilization in the mid levels, assuming the air mass is potentially unstable (see module 2.5). If an area of NVA

is over an otherwise unstable threat area, convection is likely to begin after the short-wave upper ridge passes and PVA begins.

Figure 4: Sample 500mb chart from 12Z 2 July 2000.

700mb (Brown)

The 700mb level is useful for assessing instability and wind shear effects through identifying locations of significant warming / cooling, jets, and veering of winds from the SFC to 700mb (helicity). Mid-level moisture also impacts convective initiation (often difficult if under cloud cover) and updraft / downdraft strength (mid-level dry air is especially important for strong downdrafts / microbursts).

Maximum wind (jet)

Diagnosis / Comments: 700mb winds and 700mb heights.

Associated Process / Significance: WIND SHEAR

In addition to being a good indicator of mid-cloud motion, the 700mb winds supply mid-level wind shear required for long-lived convection, and especially, supercell development. To achieve high helicity values favorable for supercells, the winds should veer through the storm inflow depth (usually the lowest ~3km or 700mb) so if the winds veer from the surface to the 700mb maximum wind axis, higher helicity values may be expected. It is often useful to plot wind barbs for each level at various points within your threat area (using the appropriate colour) to give a sense of the wind profile.

Thermal Ridge

Diagnosis / Comments: 700mb temperatures or θw (using GRIB viewer)

Associated Process / Significance: INSTABILITY

Cold advection behind the 700mb thermal ridge indicates destabilization at mid-levels. Sometimes convection is capped up to 700mb and passage of the thermal ridge coincides with convective initiation (assuming the other ingredients are already there). For elevated convection, warm advection ahead of the thermal ridge may increase mid-level instability. This process is analogous to heating by insolation at the surface except that in this case the instability due to low-level warming begins near 700mb (often ahead of warm fronts). In Miller’s original work he discusses the 700mb temperature no-change line but the thermal ridge seems to be more intuitive for marking the leading edge of cold advection.

Thermal Trough

Diagnosis / Comments: 700mb temperatures

Associated Process / Significance: INSTABILITY

The thermal trough marks the trailing edge of cold advection, i.e., the end of destabilization and the beginning of stabilization (for surface based convection). The area of cold advection between the thermal trough and ridge is useful to outline either instead of using the trough / ridge couplet or as well. Some forecasters will lightly shade areas of cold advection blue and warm advection red to distinguish potentially capped areas from areas of destabilization. While areas of warm advection at 700mb do not guarantee that surface-based convection will not occur, this often seems to be the case.

700mb Moisture (i.e., areas of mid-cloud)

MOIST

Diagnosis / Comments: 700mb T-Td spread < 6°C or 700mb RH 75%

Associated Process / Significance: INSTABILITY

Significant mid-cloud amounts forecast to be present or remain over the threat area hampers the ability of insolation to bring surface temperatures to the convective temperature. Significant amounts of mid-cloud may inhibit surface based convection from occurring in the absence of some other triggering mechanism. Mid-level instability may still be important however for Acc showers or embedded elevated Cb activity.

Dry-Prod or Dry Intrusion

DRY

Diagnosis / Comments: 700mb T-Td spread >> 6°C or 700mb RH << 75%. Analysed as the leading edge of dry air advection, especially when an axis of maximum wind crosses from a dry area into a moist area.

Associated Process / Significance: INSTABILITY

The dry intrusion is important for convective processes for a number of reasons. First, dry air at mid-levels implies that insolation may become important and the convective temperature may be achieved (obviously not a guarantee of this). Second, we know that dry air is less buoyant than moist air so parcels ascending from a moist boundary layer and encountering dry air aloft should experience greater vertical accelerations than in moist surroundings. Third, we also know that dry air at mid-levels over a warm moist boundary layer is an environment that can be conducive to generation of strong downburst winds. And lastly, evaporative cooling of moist air descending through the entrained dry air at mid-levels enhances thunderstorm downdrafts, most notably, the rear flank downdraft (RFD) associated with supercell storms that can both sustain the thunderstorm updraft and increase the probability of tornadogenesis (e.g., Fujita 1973; Lemon and Doswell 1979; Gilmore and Wicker 1998).

700mb Vertical Velocity

+W

Diagnosis / Comments: GRIB viewer or CMC chart. Regions of ascent may be labeled +W or -ω or +VV according to forecaster preference.

Associated Process / Significance: VERTICAL MOTION

Vertical velocity at 700mb is generally due to processes occurring either above or below this level (e.g., divergence aloft, PVA, low-level convergence). While this field is due to

processes at other levels it gives a quick indication of regions of mid-level ascent or descent as the former is conducive to convective development while the latter is not. Vertical velocities on the synoptic scale are on the order of cms-1 while on the mesoscale they are on the order of ms-1. It is not clear that weak subsidence on the synoptic scale can entirely suppress convection but regions of synoptic ascent are clearly more favorable for thunderstorm development.

Additional items of note: ƒ 700mb is a useful level for finding mid-level instability (e.g., Acc or persistent nocturnal convection) especially if lapse rates of temperature and wet-bulb potential temperature between 700mb and 500mb are used

Figure 5: Sample 700mb chart from 12Z 2 July 2000.

850mb (Red)

At 850mb we are concerned mainly with low-level wind shear, convergence, and moisture availability.

Low-Level Jet (LLJ)

Diagnosis / Comments: 850mb heights and isotachs, on GRIB viewer plot with wind barbs (also check 925mb). Use CMC low level max. wind chart as often the LLJ is found above or below 850mb

Associated Process / Significance: INSTABILITY / WIND SHEAR

Thermodynamically, the LLJ is often responsible for advection of both sensible heat and latent energy (i.e., moisture) into regions where severe thunderstorms are generated. Warm advection at low-levels destabilizes the thermodynamic profile increasing lapse rates while moist advection (especially in a deep layer) lowers the LFC, effectively increasing CAPE and updraft strength. The LLJ will often be nearly coincident with the 850mb thermal ridge and moist tongue (see below) indicating a deep layer of boundary-layer moisture to fuel

thunderstorms. Dynamically, the LLJ will increase storm-relative helicity in situations where the winds veer with height below 700mb, thus favoring supercell development. Convergence ahead, and to the left of the jet core can also play a role in promoting ascent to parcels in the boundary layer, thus facilitating free convection above the LFC. The LLJ is also found to be important for development and persistence of MCCs (Maddox 1980).

Axis of Maximum Moisture (Green)

Diagnosis / Comments: 850mb dewpoints contoured in 2°C increments.

Associated Process / Significance: MOISTURE

An 850mb moist axis superimposed over a surface moist axis indicates a deep layer of moisture. Tephigram analysis will often show the moisture lapse rate to fall off sharply just above the surface. As the boundary layer mixes throughout the afternoon, surface dewpoints tend to fall as the boundary layer becomes mixed. If surface dew points do not fall through the afternoon then this may be a sign that the moisture is deeper in the low levels (or that evapotranspiration or advections are occurring). The resulting energy available to thunderstorm updrafts (CAPE) is less than that represented by morning surface dewpoints if drier air does get mixed to the surface. Deeper moisture means that after mixing, air parcels can still have sufficient latent energy to fuel the thunderstorm updraft, the LFC will be lowered and CAPE can be increased significantly.

Moist Tongue (Green)

Diagnosis / Comments: 850mb dewpoints, outline areas of dewpoint > 10ºC including the moist axis.

Associated Process / Significance: MOISTURE

As for 850mb moist axis, represents an area of moist advection.

Thermal Ridge

Diagnosis / Comments: 850mb temperatures

Associated Process / Significance: INSTABILITY

As for the 700mb thermal ridge, the 850mb thermal ridge defines the boundary between warm and cold air advection. An 850mb thermal ridge can sometimes help identify the location of the strongest capping lid so that after the thermal ridge passes, cold air advection may erode the “nose” on the sounding leading to free convection from the surface. The position of the thermal ridge with respect to the moist axis is important. Having the thermal ridge upstream of the moist axis may allow latent energy to build in the boundary layer while convection is inhibited by the cap. Should the thermal ridge then advect across the deep layer of moisture, explosive development may occur in its wake as the built up latent energy is released (assuming of course the other ingredients are in place). Miller (1972) suggests that such an orientation of the thermal ridge upstream of the moist axis is characteristic of certain tornadic synoptic situations.

Low-level convergence

Diagnosis / Comments: 850mb troughs, troughs on low-level max. wind chart. Using GRIB viewer plot 850mb heights, convergence and wind barbs.

Associated Process / Significance: VERTICAL MOTION

Convergence is one of the most effective triggering mechanisms for convective initiation. A surface trough is a favourite area for thunderstorm development. If a deep layer of convergence persists through to 850mb then the situation is even more favorable for thunderstorms. Often the low-level convergence zone will be found in a trough and to the west of the LLJ making the low-level max. wind chart produced by CMC ideal for finding its location. In situations where a low-level inversion inhibits surface based convection, 850mb convergence can often be a trigger for elevated convection (e.g., nocturnal thunderstorms).

Figure 6: Sample 850mb chart from 12Z 2 July 2000.

Surface (Black)

A detailed surface analysis is extremely important when forecasting convection, this includes nephanalysis, frontal analysis (including the identification of weak boundaries which may act as triggers), isodrosotherm analysis (dewpoints) and isotherm analysis. Significant thunderstorms tend to develop near mesoscale discontinuities due to convergence (e.g., fronts, trofs, drylines, outflow boundaries) or differential heating effects and their associated secondary circulations (e.g., terrain induced, cloud-no cloud boundaries, surface moisture discontinuities, land - sea breezes, etc.). The surface analysis is the best and most complete way to capture the subtleties of the mesoscale environment.

Convergence

OR

Diagnosis / Comments: surface trofs, wind shift lines, plot of convergence using GRIB viewer. Using arrows to show the direction of the flow may be more conventional (i.e. streamlines) but it may be more convenient to use the symbol for convergence at 850mb coloured black to represent surface convergence.

Associated Process / Significance: VERTICAL MOTION

Surface troughs are one of the most common triggering mechanisms for thunderstorms. Often there will be numerous troughs or wind shift lines apparent in the surface analysis, those evident for a number of hours pose a more significant threat than transient ones. Identifying which ones will be dominant however is sometimes difficult.

Axis of Maximum Moisture (Green)

Diagnosis / Comments: surface dewpoint contoured in 2ºC increments.

Associated Process / Significance: MOISTURE

Moisture is the energy that fuels thunderstorms. Without sufficient moisture, atmospheric instability won’t be realized in the form of moist convection (i.e., thunderstorms). Recall that moisture depth is important and if the atmosphere dries significantly just above the surface, even climatologically high surface dewpoints may not result in convective initiation. The isodrosotherms will often appear to have maxima / minima in elongated ribbons due to moisture advection. Thunderstorms initiated in drier air that move into areas of higher dewpoints will usually intensify.

Moist Tongue (Green)

Diagnosis / Comments: outline surface dewpoints > 12ºC including axis of maximum moisture.

Associated Process / Significance: MOISTURE

The location of the surface moist tongue often suggests a region in which convection is expected, especially if coincident with the 850mb moist tongue (deep layer of low-level moisture). Areas of convergence overlapping the moist tongue are a good indication of where to expect convective development. Significant gradients in surface moisture may become important due to differential heating and the secondary circulations that result.

Thermal Ridge

Diagnosis / Comments: surface temperature

Associated Process / Significance: INSTABILITY

Surface thermal ridges or warm tongues are probably less important than areas of significant moisture as the thermal ridge alone will likely not indicate where thunderstorms will form. That being said, surface temperature is important to delineate fronts and determine as to

whether or not the convective temperature will be reached.

Isallobaric Field

Diagnosis / Comments: 3 hour pressure tendency, identify significant rise / fall centers.

Associated Process / Significance: VERTICAL MOTION / MOISTURE

The isallobaric field is influenced by both vertical motions and thermal advections and as such gives real evidence of atmospheric motions above the surface. The main focus for the convective assessment is the effects of the isallobaric wind. A strong isallobaric gradient implies ageostrophic flow towards the fall center which may alter the direction of the surface winds leading to increased convergence and moisture flux convergence towards the fall center.

Drylines

Diagnosis / Comments: surface dewpoints, equivalent potential temperature (θe), mixing ratio.

Associated Process / Significance: VERTICAL MOTION

The dryline has received little attention in Canada and is primarily a west prairie and High Plains phenomenon. In a southwest upper flow over southern Alberta (700mb flow 20kts and within 30° of normal to the Rockies), dry air subsiding along the foothills can spread eastward strengthening an existing north-south moisture gradient. This can form a dryline, providing a convergence boundary (i.e., a lifting mechanism) in the vicinity of which thunderstorms may develop. The dryline often bulges eastward in southern regions sometimes reaching southwestern Saskatchewan. In extreme cases, the dryline may bulge such that storms initiated on the Alberta foothills can interact with the dryline further east altering the dynamics of the existing storm. The most prominent case of this phenomenon to date is the Pine Lake tornadic storm of 14 July 2000.

Animation available on the web: Pine Lake tornadic storm on 1.5 km Cappi radar imagery - the storm is approximately 160km south of the radar.

Additional features of interest:

ƒ axis of moisture flux convergence: for increasing instability and convergence

ƒ θe ridge: axis of instability

Convective Parameter Review

With the operational convenience of workstation GRIB data viewers, it is straightforward to incorporate various thermal and dynamic convective parameters into the convective assessment process. These parameters should become part of the work-up routine with the forecaster determining for themselves the best way to delineate them on the composite plot. Contouring convective parameters is intended to complement a complete sounding analysis (actual and prognostic) including modifying tephigrams and lifting parcels. There are many parameters readily calculated from observational and model data. The list below summarizes some common ones and is by no means intended as an exhaustive list. No one parameter holds the solution to where and how strong convection will be. Each one tells a slightly different story and some are highly correlated to one another so using all of them every day is not always useful. The list below is subdivided into stability parameters, dynamic parameters, and parameters combining both aspects. Most of the defining equations can be found in Module 3.1H of the MOIP notes.

Stability / Moisture Parameters

CAPE (Jkg-1) (Convective Available Potential Energy)

Description: Quantifies buoyant energy available to thunderstorm updrafts based on difference between lifted parcel and ambient temperatures. Can suggest maximum updraft velocity via

wCAmax = 2 PE, useful for maximum hail size forecasting.

Comments / Threshold Values: Often calculated using surface based T and Td values (e.g., on GRIB viewers and CMC charts) but unless the boundary layer moisture is mixed (lowest 50- 100mb) will overestimate available energy and resulting thunderstorm tops. American researchers often characterize values of 1500 Jkg-1 as moderate but lower values are likely significant in Canada (may vary regionally). The vertical distribution of CAPE ("shape of the CAPE") is important as “fat” CAPE (viewed on tephigram has broad positive area) will give rise to stronger updrafts than “skinny” CAPE (e.g., Blanchard 1998).

LIFTED INDEX (LI -- °C)

Description: Index based on difference between lifted parcel (from the surface) and

environmental temperatures at 500mb.

Comments / Threshold Values: Uses surface T and Td and so has similar problems as CAPE in overestimating instability. Good for quick estimate of instability assuming mixed boundary layer and can be updated hourly.

LI Value Convective Weather

> 3 no convection

1 to 3 rain showers

0 to -2 few light thundershowers

< -2 risk of moderate - severe thunderstorms

SHOWALTER INDEX (SI -- °C)

Description: Similar to LI but parcel is lifted from 850mb.

Comments / Threshold Values: Good indication of instability above the boundary layer (approximated by 850mb). On morning soundings indicates instability above nocturnal inversion and is useful for nocturnal or elevated convection. If surface based convection expected then LI is likely a better choice.

SI Value Convective Weather

> 4 no convection

1 to 4 rain showers possible

0 to -3 light - moderate thunderstorms likely

< -3 risk of severe thunderstorms

TOTAL TOTALS INDEX (TT)

Description: Uses 850mb-500mb temperature lapse rate and 850mb Td to examine the depth of boundary layer moisture.

Comments / Threshold Values: Originally developed to determine frequency of thunderstorms, not intensity. Tends to be unrepresentative in colder air masses (SSWDLC 1995).

TT Value Convective Weather

< 40 no thunderstorms

40 to 47 SCT thundershowers - FEW moderate thunderstorms

47 to 50 SCT moderate thunderstorms - isolated severe thunderstorm

> 50 severe thunderstorms with possible tornado(es) [with hail >20mm and 50kt gusts]

K INDEX (°C)

Description: Similar to TT in that it considers the difference in 850mb and 500mb temperatures and 850mb Td but also includes the dewpoint depression at 700mb (for mid-level dry air). Suitable for warmer air masses but overestimates convective potential under colder conditions (SSWDLC 1995).

Comments / Threshold Values: Developed to quantify frequency / probability of thunderstorm development, not intensity.

K Index (C) Thunderstorm frequency Thunderstorm Probability

< 15 - 0%

15 to 20 none 20%

20 to 25 isolated 20% to 40%

25 to 30 widely scattered 40% to 60%

30 to 35 scattered 60% to 80%

35 to 40 numerous 80% to 90%

> 40 - 100%

700-500mb LAPSE RATE (°C km-1 or °C mb-1)

Description: Quantifies mid-level lapse rates, sometimes only temperature difference between 700mb and 500mb is used.

Comments / Threshold Values: Using GRIB viewers look for the leading edge of the lapse rate gradient to overrun moist low level air. No published thresholds for Canada to date although temperature differences of 20°C have been associated with severe weather operationally.

CONVECTIVE INHIBITION (CIN) (Jkg-1)

Description: Negative buoyant energy, integrated from the surface to the LFC (should mix lowest 50-100mb as for CAPE), a measure of low-level stability.

Comments / Threshold Values: Convective inhibition allows boundary layer moisture (latent energy) to “pool” without being released until the point that it is sufficient to overcome the capping lid. Without any form of cap, the energy is expended early in the day on smaller convective clouds (Cu, TCu). With sufficient capping, however, the energy may remain untapped until later in the day at which point fewer, more intense convective clouds can result. While a “strong” cap may suppress convection entirely, some CIN is required for severe thunderstorm development. Values of 15 Jkg-1 have been quoted as being sufficient to delay convection enough for fewer, stronger storms to develop while values less than 10 Jkg-1 may be too little (Djuric 1994). CIN over 50 Jkg-1 may be enough to suppress convection altogether.

PRECIPITABLE WATER (PW) (kgm-2 or mm)

Description: The mass of water vapour in a 1m2 column over the earth’s surface.

Comments / Threshold Values: Most convective indices consider the amount of available water vapour either at the surface or at discreet levels (e.g., 850mb). The intensity of convective updrafts is dependent on the amount of latent energy available for release within them. In the absence of significant mid-cloud cover, most water vapour in the atmosphere is concentrated at low levels so PW is effectively a measure of the total amount of low-level moisture available for latent heat release. In the southern U.S., values of 25mm or more have been found to support thunderstorms (Djuric 1994) but Taylor (1999) found values of 16mm to be sufficient to support central Alberta hailstorms.

Dynamic or Wind Shear Parameters

MEAN WIND SHEAR (s-1)

Description: The change in the shear vector integrated over some depth of the troposphere, usually up to mid-levels from 4 to 6km or 500mb, normalized by that depth of the troposphere.

Comments / Threshold Values: The most severe thunderstorms (i.e., supercells) develop in strongly sheared environments with large CAPE. While strong shear may initially inhibit deep convection, it later acts to separate updrafts and downdrafts within the storm and enhances perturbation pressure gradients, thus enhancing updraft strength and storm longevity (e.g., Davies-Jones 1985). From the representative hodographs for each storm type developed by Chisholm and Renick (1972), Taylor (1999) calculated surface to 6km mean wind shears (below).

Storm Type SFC to 6km Mean Shear

Air Mass 2 x10-3 s-1

Multicell 5 x10-3 s-1

Supercell 8 x10-3 s-1

STORM RELATIVE HELICITY (SRH) (m2s-2)

Description: SRH is a measure of the tendency for the formation of rotating air columns within thunderstorm updrafts leading to the development of mesocyclones. SRH is generally measured below 2km or 3km as an approximation for typical storm inflow depths.

Comments / Threshold Values: SRH is characterized primarily by clockwise hodograph curvature (in the northern hemisphere) in the lowest few km and storm motion to the right of the surface - 6km (~500mb) mean wind. For a more complete understanding of how and why SRH is important the reader is directed to the references and suggested reading list at the end of this module. Values of SRH have been correlated to tornado intensity below (Davies-Jones et al. 1990).

Tornado Intensity SRH (m2s-2)

Weak 150-299

Strong 300-449

Violent >450

Combined Stability / Dynamic Parameters

BULK RICHARDSON NUMBER (BRN, R)

Description: The bulk Richardson number quantifies the interaction between thunderstorm updrafts (via CAPE) and environmental wind shear (CAPE/shear). The wind shear used is the vector difference between the 0-6km mean wind (density weighted) and the mean 500m wind.

Comments / Threshold Values: BRN may be thought of as a ratio of vertical to horizontal kinetic energy within the storm environment. A high BRN (>350) is characteristic of an environment with insufficient wind shear to support persistent deep convection while a low BRN (<10) is characteristic of an environment with insufficient CAPE (relative to wind shear) to support deep convection. BRN has been correlated to convective storm type as follows (Weisman and Klemp 1986).

BRN Storm Type

< 10 no thunderstorm

10-45 supercells

45-350 multicells

> 350 air mass or “pulse” thunderstorms

Figure 7: Illustration of variability in bulk Richardson number.

The diagram above illustrates variability in bulk Richardson number. Horizontal arrows left of the cloud and the vertical arrow within the cloud represent varying strengths of wind shear and CAPE, respectively. In the high CAPE vs. weak shear regime (left side), precipitation particles fall back through the updraft as there is insufficient shear to transport them down wind. The associated BRN is >350. In the low CAPE vs. strong shear regime (right side), environmental wind shear overpowers the relative strength of the updraft and the cloud is sheared apart before precipitation has a chance to form. The associated BRN is < 10.

SWEAT INDEX (Severe WEAther Threat)

Description: Considers the TT index, 850mb Td, 850mb and 500mb wind speeds and direction.

Comments / Threshold Values: One of the few early storm severity indices to include dynamics as well as stability and moisture. SWEAT is intended for forecasting severe thunderstorms only as reflected by the constraints placed on keeping some terms non-zero (refer to Module 3.1H -- SWEAT ). SWEAT has been considered to have little usefulness for values <250 (Miller 1972) but as with most convective parameters, threshold values may differ in Canada.

SWEAT Convective Weather

<300 no severe thunderstorms

300-400 possible severe thunderstorms

>400 possible tornadoes

ENERGY HELICITY INDEX (EHI)

Description: Index based on the product of CAPE and SRH to indicate the potential for tornadoes with parent mesocyclones.

Comments / Threshold Values: Mostly empirical in nature, the EHI has been related to tornado strength in the U.S. with most strong tornadoes (F2-F3) appearing to be associated with EHI > 1 and violent tornadoes (F4-F5) with EHI > 2.5 (Brooks et al. 1994). To date no validation of this parameter has been published in Canada.

Animation available on the web: An example of a Mesocyclone on 1.5 km Cappi radar imagery - in the early evening, the mesocyclone is just west of the radar near Wabamum - right moving southeastward

MOISTURE FLUX CONVERGENCE (MFC) (gkg-1hr-1)

Description: Combines mass convergence and moisture advection to identify axes favorable for thunderstorm initiation or persistence.

Comments / Threshold Values: Being a combined parameter, the effects of convergence and moisture advection cannot be separated but are analyzed in their own right during the convective work-up process. Storms moving into areas of positive MFC often intensify. As with surface convergence, persistent areas of MFC are most significant.

Convective Assessment Example

An example of what a Miller composite plot may look like is shown in figure 7. The plot shown here is valid 00Z 3 July 2000 using T+12HR model data. On this day at least one severe thunderstorm developed in AB and moved into SK resulting in 70mm hail and a tornado. Not all of the features listed in section 2 have been drawn in an effort to simplify the diagram. The dominant features / processes are the following:

· moist axis at the surface and 850mb from south central SK into east central AB (deep

low-level moisture)

· low-level jet nearly coincident with moist axis (strong moisture advection, helicity)

· frontal wave analysed with the cold front extending from southwestern AB to east central AB providing convergence

· southeastern AB / southwestern SK under area of diffluence at 500mb and 250mb with left exit region over east central AB

· insolation expected behind synoptic cloud expected to destabilize the atmosphere at low-levels through heating of the boundary layer

· areas of negative LI (orange) and SH (pink) are outlined (hatched lines)

· thickness ridge and TROWAL (approximates 700mb thermal ridge) moving east into SK after 00Z (cold advection aloft behind)

· areas of SWEAT > 300, LI < -6, and SRH > 150 m2s-2 are hatched with the legend in the upper right of the figure

Figure 7: Miller composite valid 00Z 3 July 2000.

Figure 8: Severe weather outlook developed from figure 7.

Following completion of the Miller composite, the forecaster is required to use the represented processes to define boundaries for areas of convective development (the severe weather threat area). The severe weather outlook for this example is shown in figure 8. In this case, the eastern edge of the slight risk area was defined using the location of the TROWAL or 700mb thermal ridge and is nearly coincident with the forecast location of the synoptic mid-cloud boundary. The remainder of the region is defined by the superposition of the LI and SH contours. Within the slight risk region is an area of moderate to high risk of severe weather. The high risk area is defined by the moist tongue (not drawn but surrounds the moist axis) and the thickness ridge to the east. To the north we have used the TROWAL and mid- cloud shield and to the south and west the SH contour and 850mb trough. The moderate to high risk area encompasses the region of highest CAPE (~2000 Jkg-1) as well as highest SRH, SWEAT, and LI. This area is also in the vicinity of the area of diffluence and left exit of

the jet.

In the forecast then, we may expect scattered light showers or thundershowers in the slight risk area. In the moderate - high risk area the potential for severe weather is higher and as a result there would be thunderstorms with the risk of a severe thunderstorm in the forecast. The high risk area would be closely monitored and if the potential for thunderstorms persists, a severe thunderstorm watch would be issued as early as possible (as determined by regional guidelines).

Summary

Forecasting convection (especially severe convection) will pose a challenge throughout any forecasting career. Experience may teach us what to look for, but the atmosphere will always bring surprises. Sound understanding of conceptual models, learning how to analyze the situation using the appropriate tools and knowing how to apply these tools affords us the best chance of anticipating convection and issuing timely watches and warnings.

Having a strong conceptual model to rely on will help the forecaster to anticipate what will happen given different convective situations. Conceptual models, coupled with in situ and remote sensing data (observations, severe weather reports, satellite and radar imagery) are necessary to diagnose convective development.

Below are some of the major points covered in this section: ƒ upper-level charts are mainly associated with vertical motion, instability, and wind shear ƒ lower-level charts are mainly associated with vertical motion and moisture ƒ isodrosotherm analysis on SFC maps is a must ƒ tephigrams are useful to assess column stability and wind shear but GRIB viewers or charts must be used to define a convective threat area