We are well into the summer severe weather season, but it's been a pretty quiet start. It's a great time to talk about how severe weather forms, what ingredients are needed, and finally, what may inhibit formation of thunderstorms, which then become severe.
INGREDIENTS
There's general agreement within the weather community (forecasters, meteorologists, or just weather enthusiasts) that there are 3 ingredients needed for development of thunderstorms, and as a result, severe weather.
1) Moisture
Of course, moisture is needed for severe weather. Generally, dew points need to be above 55° in the perfect conditions, but dew points in Wisconsin during severe weather events are often above 65°. Moisture directly, or lack thereof, impacts instability directly.
2) Instability
While talking about instability, I'll often refer to the amount of instability in the atmosphere in terms of CAPE. CAPE stands for 'Convective Available Potential Energy', which is a great measurement of the amount of "juice" in the atmosphere to fuel storms. Instability allows air in the low-levels of the atmosphere (just above the surface) rise into the upper levels of the atmosphere. Without instability (energy), convection (thunderstorms) simply cannot be supported.
3) Lift
Lift is often referred as the 'trigger'. Here in Wisconsin, the trigger is most often a front. Another trigger (which is most often seen in the Plains) is a dry line. A dry line is where there's a sharp cutoff between moist, unstable air and dry, stable air. A dry line can fire off storms, given that the cap breaks.
What can stop thunderstorm development?
There are a number of things that can stop thunderstorm development. If any of three ingredients are lacking, that can inhibit the development of severe weather. The cap is something you've probably heard a lot about, which is the main 'blocking' mechanism.
On a hot and humid day, there's often unstable air above the surface, which rises through the atmosphere. However, is the mid-levels of the atmosphere, there's warm, but stable air. The warm, stable air is called the cap. It stops the unstable air from rising through the atmosphere, stopping the development of thunderstorms. The cap can break by a trigger (lift) or enough unstable air that it breaks on its own, to name a few.
Regardless, we can prepare you for severe weather, but in the end it's out of our hands. I've only had a few trolls that were upset that I was "wrong" and didn't see severe weather back on May 10 (weird right?). I certainly was happy the forecast was wrong, especially when we're dealing with severe weather, but I guess not everyone thinks that way.
Although there may be other comments I've gotten that I'm "always wrong" and whatnot, here's a couple comments that I got on May 10.
Meteorologist Chad Roethlisberger from WFRV in Green Bay explains what happened on May 10.
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