What are Mesoscale Convective Systems?

What makes these storm clusters so special?

DETROIT – If you've lived here long enough, you've no doubt experienced a strong "pop-up" thunderstorm on a hot, humid day, or seen that line of strong storms cross the area and interrupt a sultry summer afternoon. 

The common denominator here is that storms seem worst when they hit during the hottest part of the day, and for good reason: That's when the atmosphere is most unstable and, the more unstable it is, the more violently surface air parcels can rise. Another common denominator is that the storms generally seem to settle down at night, as temperatures cool and the air become more stable.

Lately, however, we've seen a bunch of particularly severe clusters of storms blowing up AT NIGHT. Just last night, one exploded in central Wisconsin and traveled southeast, narrowly missing U.S. Cellular Field where the Tigers played a rain-shortened game (the severe part of it just missed them to the west, but the rain obviously got to them).

Note: The attached satellite/radar loop (above video) from the night of July 22, 2016 shows the beginning of the MCS in Wisconsin that later shortened the Tiger game in Chicago.

These special, nocturnal clusters of severe storms are called Mesoscale Convective Systems (MCS), and they can travel hundreds of miles and last 12 hours. 

But why do they blow up at night and, typically, fade just after sunrise? Because the low-level jet stream strengthens at night, and this stronger band of wind less than a mile above the ground feeds warm, moist air into the system much more efficiently as evening arrives. Another factor that keeps the storms going strong through the night is the fact that, once the sun goes down and no more solar radiation is coming in, the tops of the storms cool even more -- which makes the environment in the storm even more unstable.

All it takes is the smallest trigger in an otherwise unstable atmosphere to get some storms going in the late afternoon or evening and, if the strengthening nocturnal low-level jet is strong enough and feeding into that area, then that massive cluster of strong or severe storms blows up into what looks like a big circular blob on satellite images.  The most intense weather associated with an MCS is usually at its lead edge.

And there's one more thing to know about MCSs:

The more intense ones sometimes create their own little upper level disturbance, that still lives on even after the storms dissipate.  This is called a Mesoscale Convective Vortex (MCV), which can be the trigger for a batch of more "typical" afternoon thunderstorms later on.  So MCSs are sometimes the storms that keep on giving.
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