What do we mean by a “cap”?

All of Southeastern Minnesota and Northern Iowa has a Moderate Risk (level 4 out of 5) of severe thunderstorms Monday, April 28th. This is a conditional severe weather threat across the area, and not everyone may see severe weather at all. But what does “conditional” mean in this case? Why is this threat conditional?

To answer this, let’s dive into a term many meteorologists often talk about during severe thunderstorm potential…a “cap”

A cap is a layer of warmer air a couple thousand feet up in the atmosphere. This cap can form in a variety of ways, from radiational cooling at night to warmer air being transported aloft over a specific area. In the case of Monday, it is the latter.

Warmer, more moisture rich air will be transported from the south across the region Monday. At the same time, a layer of slightly warmer, and drier, air will be transported from the southwest across the region, overspreading the cooler and more humid layer of air closer to the surface.

This layer of warmer air aloft is the “cap”. This prevents thunderstorm from developing because thunderstorms form from rising air. As long as rising air is warmer than the air around it, it will continue to rise. Once that rising air hits the cap, however, it is suddenly colder than the air around it. Because of this, it sinks, preventing cloud growth and eventually thunderstorm development.

This is what is expected by most model guidance across the region tomorrow. The cap is expected to hold, and prevent widespread thunderstorm development from taking place. However, the cap can be “broken” if enough air is being forced upward by something like a cold front.

If forcing from the cold front is strong enough, it can force air through the cap regardless of how warm it is, resulting in continued cloud growth and eventually thunderstorm development. In the case of Monday, if this happens, storms will quickly become severe due to a highly favorable environment above the cap.

With that said, while widespread severe thunderstorms may not materialize Monday, there is a higher risk that any storms that can break through the cap will become severe.