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: interaction of low-level wind shear and thunderstorm outflow boundaries to generate new thunderstorm cells (this is especially important for multicell thunderstorms) tilting of thunderstorm updrafts in deeply sheared environments generation of low-level horizontal vorticity in sheared flow (this is crucial for the development of mid-level rotation in supercell thunderstorms) interactions between updrafts and environmental wind shear resulting in storm splitting 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: instability moisture vertical motion (trigger) 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