Stability Page 1
Severe Weather Forecasting and the
Use of Atmospheric Stability Indices
Brian hit the spot when he said that storm chasing is not like "Twister". Catching the big one involves a lot of time and energy and the help of many people, not just one. If it was this simple, why are our bags packed for next years chase 2 days after we get home? If you are like me it's probably because you can't wait to get back to Tornado Alley.
Severe weather forecasting, in reality, is not going to be learned from a textbook. Yes, it helps, but knowledge and experience is the key to any successful storm chase. I have been studying severe thunderstorms and tornadoes for many years, but knowing the conditions that lead up to and beyond the initial outbreak is key. Forecasting is like a fingerprint. No two synoptic and upper level conditions are going to be the same. They may have similar characteristics, but this is where your knowledge and experience comes into play. It is your job as a forecaster to include as many "what could happens" into your forecast as possible.
There are many "forecasting tools" that can be utilized to your benefit. All of these are important, and none should be overlooked. You should try to get as much information as possible before you attempt to make an accurate forecast. One of the most useful tools that a forecaster can use is called the Skew-T diagram. This diagram shows the atmosphere in its vertical, and tells us such things as temperature, dewpoint, and winds at different levels. With a Skew-T, you can actually trace an air parcel from the surface all the way up to well and beyond the 100mb level, so you can see how the forces of nature will act upon it.
What do I mean by "forces of nature"? We all know that for the most part the lapse rate (temperature change with height) is negative as we ascend through the troposphere. When a parcel of air is forced upward, either by frontal activity or other forces, it cools. If this parcel of air is forced high enough, it will begin to condense, and this is when we will see the little white puffy cauliflower-shaped cumulus clouds. The air parcel has reached its Convective Condensation Level (CCL). Further lifting of the air parcel beyond its condensation level will allow it to become lighter than the environmental temperature, which is the Level of Free Convection (LFC), thus causing it to rise freely until the temperature of the parcel is the same as the environmental temperature. We call this the parcel's Equilibrium Level (EQ). It is not uncommon to see supercells with cloud heights of 12 miles. The two processes responsible for this are evaporation and condensation. When air evaporates it cools, and when it condenses it warms.