Study of indices gives solid forecast

Severe weather coming offers telltale clues.

Atmospheric sounding from the morning of Jul 24, 2010 at Lincoln IL. The yellow parcel trace to the right of the white temperature line indicates a potential for convection and translates into a Cape value, listed along the right side, of 1713. Even 10 hrs in advance, this sounding reflected a moderate chance of severe convection, and by 22Z strong storms moved across the region.


Unfortunately, even armed with all of this information, which is based in dynamic meteorology, the indices are still only a guideline. Each of them has limitations that limit its prognostic ability—for example, most of them do not work over high elevation terrain or in the mountains. They are also of limited use in forecasting cold season storms.

In addition, conditions in an unstable atmosphere can change quickly and the weather balloon soundings are only available at 00Z and 12Z. So, while the indices at 12Z might indicate stable conditions, by 15Z severe weather could be brewing.

Conversely, inflow of dry air at 850 mb at 13Z might strengthen a Cap that ensures a hot and muggy, but rain-free, day, even though all of the 12Z indices pointed toward a widespread severe outbreak. These indices do serve a very useful purpose to the pilot who knows how to interpret them.

It is a great deal more information about the atmosphere than a simple weather map can provide, and the information can improve your flight planning significantly. However, there is far more to the physics behind these indices, and your best bet is to take in the information at face value, add it to the other information you have from looking at the weather maps, and then talk with your briefer.

They often have a great deal of experience working severe weather indices into their broader forecasts and understand how one index may play against another over a certain area, or under a given air mass. As a result, your briefer can provide you with a well informed analysis which, as a result of your ability to differentiate Cape from Cap, can make a lot of sense.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University. Shein holds a commercial license with instrument rating.




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