What Is The Cap?

5:30 PM CDT | May 10, 2018

I wanted to take a few minutes to discuss our "cap". You are probably familiar with the "cap" if you live in Texas. The cap is what shuts down our storm chances frequently in the summer. I drew up the following graphic to help explain the cap. There are three main ingredients needed for shower/storm development - moisture, instability, and lift. Lift can come in the form of surface heating, low-level convergence near a boundary, the approach of an upper-level system, etc. For storms to develop, parcels of air at the surface need to be able to lift upward and rise. When there is a layer of warm air above the surface, this prevents these parcels from rising. This is because the surrounding air is warmer than the parcels, and they stop rising.

Where does this warm air originate? It comes from the higher terrain of Mexico. This warm air originates in the plateaus between the Sierra Madre Occidental and the Sierra Madre Oriental mountain ranges. The air can only be pulled northward or southward. The strength of the cap increases when there is south-southwesterly flow aloft. It also increases in the summer months as the jet stream retreats northward. Strong daytime heating, significant lift near a weather system or front, and colder temperatures aloft (associated with a low pressure system) can "erode" the cap. West Texas and the Texas Panhandle have an easier time eroding the cap due to the higher elevation. Central and Northern Texas have the most difficult time in breaking the cap. Since the cap allows the air below to continue to warm and moisten, the potential instability increases. If the cap can break, explosive thunderstorm development can occur. This is what makes forecasting weather for the I-35 corridor a challenge when the cap is involved.

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