Thunderstorm Forecasting - Silver Bullet?

BY Jeremy Wesely

For years balloonists have been looking for that single "number" in a weather briefing that would confirm the official forecast of convective activity and thunderstorms. We have tried indicators such as Total-Totals, the K Index, LI (Lifted Index), and other tools used and championed by glider pilots. Well, they are still acceptable if you can understand them and get them for your location. However, there is a new "number" (or actually two "numbers") that meteorologists are using today that you can get. These are CAPE and CIN.

Meteorologists use numerical computer model data to provide reliable outlooks, watches, and the TAF used when you get your briefing from Flight Service. They use computer output, often in the form of indices that with given rules of thumb can be used to accurately forecast the garden-variety or the most destructive tornado-producing super cell thunderstorms. Today's technologically advanced age of high speed internet access now allows anyone with an interest in weather to have quick and easy access to a wealth of computer model output. We can now see and analyze the same output that was used to create the forecast for thunderstorm in the TAF, Airmet, or Sigmet.

The Storm Prediction Center

The Storm Prediction Center (SPC), which is part of the National Weather Service (NWS), is one of the most well known and highly respected forecasting agencies in the United States. The SPC, with roots back to the early 1950's and located on the web at http://www.spc.noaa.gov, issues convective outlooks, mesoscale discussions, and severe weather watches across the continental United States. The convective outlooks highlight regions where thunderstorms and severe thunderstorms are possible while also identifying the risk level as slight, moderate, or high. In addition, the outlooks give the risk in terms of percentages of experiencing a tornado, hail storm, or damaging wind event within 25 square miles of your location (http://www.spc.noaa.gov/products/out-look/daylotlk.html). The mesoscale discussions are more specific than convective outlooks and are typically issued just prior to actual weather watches giving users a heads up on precisely where severe weather has become more favorable. Severe weather watches then indicate that weather conditions are more than just increasingly favorable and are in fact now favorable for severe storms. Within a favorable environment the storms begin to grow and local NWS offices issue storm warnings once severe limits have been reached.

CAPE
The SPC relies heavily upon the guidance of model derived severe weather indices and incorporates this information within the products that it issues. Convective Available Potential Energy (CAPE) is one of the latest weather indices that measure atmospheric instability. Therefore, CAPE indicates parcel buoyancy highlighting how rapidly an air parcel will rise in the atmosphere. Rising air then cools and condenses forming clouds and thunderstorms. CAPE values less than 1500)/Kg generally indicate weak storms, 1500-2500J/Kg moderate storms, and strong storms typically occur in environments with values greater than 2500)/Kg where stronger updrafts are possible. While these values focus on thunderstorms, the tip off for us is knowing that there will be convective activity (or at least instability) when CAPE is a high positive number.

CIN

Forecasters then use convective inhibition (CIN) to determine if the potential energy or lift will be realized. For example, a warm layer in the mid levels can inhibit the rising of air and must be overcome before an air parcel can rapidly ascend, forming the thunderstorm. This warm layer is often referred to as the "cap." In order for a parcel or box of air to rise it must be warmer than its surrounding environment.

Therefore, a warm surface parcel will begin to rise and cool as it does so. If this warm surface parcel is cooler than the capping temperatures in the mid levels then the air will sink and no storms will form. However, if the warm air that originated at the surface undergoing cooling as it rose is still warmer than temperatures at the capping level, then the air will continue to rise, condense, and form thunderstorms. Therefore, CIN helps forecasters determine if the warm rising air can penetrate the cap resulting in thunderstorms. Thunderstorms usually form in regions where CIN ranges from OJ/Kg to -50J/Kg. Thunderstorms that form under conditions where CIN values are more negative, such as -50 to - 75J/ICg are typically more isolated but also more severe. It is rare to experience thunderstorms when CIN values are more negative than -100J/Kg.

There are many other indices used to forecast thunderstorms of which CAPE and CIN are among the most basic and standard of all the indices. Forecasted CAPE and CIN values can be found at http://www.rap.ucar.edu/weather/model/ and http://www.hprcc.unl.edu/nevit/eta.htm. Real time analysis values are available at http://tornado.caps.ou.edu/wx/p/r/us/adas and http://www.hprcc.unl.edu/nevit/surface.htm. Are these indices the simple silver bullet we are looking for? Well, not yet, but we are getting closer and you can continue to educate yourself when it comes to forecasting thunderstorms. But remember, get the official TAF from Flight Service.

Title Thunderstorm Forecasting- Silver Bullet? Author Wesely, J. Journal Ballooning (Indianola, Iowa) Publisher Balloon Federation of America Date May/June 2003 Vol Issue 36(3) SIRC Article # S-883624 This material has been copied under license from the Publisher. Any resale for profit or further copying is strictly prohibited.

 

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