For thunderstorms to form, we need three key ingredients: moisture, instability and lift. Moisture is supplied from the Gulf of Mexico, and the higher the dew points rise, the more moisture we can use as fuel for showers and storms. Instability comes from something as simple as daytime heating as the sun heats the ground, which causes air parcels to warm and rise into the atmosphere. Those air parcels need just a little extra lift to force air upward so it can condense and clouds can form.
Stronger forms of lift come in cold fronts or drylines. Collapsing storms can produce outflow boundaries as cold air rushes to the surface from the dying thunderstorm. These boundaries can force warm air upward to spark new thunderstorm development.
For storms to continue to strengthen, there needs to be energy available for thunderstorms to tap into in the atmosphere.
This energy is referred to as Convective Available Potential Energy, or most commonly known as CAPE. It is another way we can measure the instability in the atmosphere. Thunderstorms thrive under an unstable environment, but they can only continue to strengthen if the updraft of the storm can be maintained by tapping into some of the energy.
CAPE is measured in Joules per kilogram. The more CAPE there is to work with, the more powerful storms could be if several other severe weather parameters are met. Generally speaking, you only need about 1,000 Joules per kilogram of CAPE for storms to have enough energy to be strong to severe.
During the winter and early spring months, it can be hard to find more than 500 Joules per kilogram of CAPE in our atmosphere for storms to work with, given the greater draw of colder air from the north and moisture remaining out over the Gulf of Mexico. However, this can often be just enough instability during the colder months to generate stronger showers and storms. During the spring months, this is not as much the case, and higher CAPE values would be necessary for stronger storms to develop.
Meteorologists send up weather balloons that collect various parameters necessary to diagnose the atmosphere either for real-time use as a weather event is unfolding or for input into model data. We are able to measure the energy available in the atmosphere and forecast for future events how much energy may be present at a future point in time when forecasting whether or not strong to severe storms may be possible.
CAPE is just one of many parameters meteorologists use when forecasting severe weather. It is not solely what must be relied on for severe weather to be possible. It can be a good tool for measuring how much energy is potentially available for thunderstorm development and sustainment if accessed by thunderstorm updrafts.
— Meteorologist Erika Paige