Joe Lundberg

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Summer Opens With a Volley of Potent Thunderstorms

June 21, 2013; 10:03 AM ET

Friday, 11:30 A.M.

One thing that was generally lacking last summer were Mesoscale Convective Complexes - MCCs for short. These are typically clusters of thunderstorms that form near the apex of an upper-level ridge, or even a little downwind of them. Very warm to hot and pretty moist air is carried northward in some manner by strong winds in the low levels of the atmosphere. For a while, it's almost unnoticed, especially if the air aloft over it is hot and dry - "capping" the unstable air underneath.

Eventually, though, either that ribbon of tropical air outruns that cap, or it starts to get lifted by some sort of upper-level feature approaching from the west. And at times it may simply run into some sort of a road block, a front or a cooler, denser air mass that's just not giving ground as fast as the very warm, humid air is advancing. Then the air is forced upward, and leads to a pretty rapid development of many thunderstorms that can then morph into a larger area of thunderstorms. This "complex" can and often does produce severe weather, but it is more known for tremendous amounts of rain.

In fact, MCCs tend to bring a fairly significant portion of spring and summer rainfall to the northern Plains and Midwest. Last year, they all but did not exist, and the subsequent drought let to excessive heat that sent temperatures soaring to record levels on a few occasions.

Not this year. The complex that formed late yesterday dumped over 6 inches of rain on one town in North Dakota, and several inches of rain into parts of South Dakota and Minnesota overnight. That same complex is still on the rampage late this morning:

It will soon run out of gas, but as it fades, the seeds for the next one are already being sewn in South Dakota, as you can also pick out from that satellite image. Again, much like we saw late yesterday into this morning, this will produce a lot of rain over the course of the next 24 hours as it rolls across the Dakotas toward Minnesota.

With the upper-level flow tilting a little more to the west-southwest, it would appear that this particular thunderstorm cluster or, assuming it grows to become one, MCC, will track more toward the central and northern counties of Minnesota late tonight and tomorrow.

I highly doubt its the last one. With an upper-level disturbance rolling out of the Northwest and across the northern Rockies toward the Dakotas tomorrow and tomorrow night, it will help trigger yet more thunderstorms across the Dakotas. They may not congeal into an MCC, but they will drop yet more rain over saturated ground and could trigger more flooding, as well as severe weather.

Next week, with the prospects of the upper-level ridge sharpening over the Rockies, there will be a lot of hot air on the playing field. With southerly low-level winds likely to prevail, and the humidity apt to remain high up and down the Plains to the Mississippi Valley, I am pretty confident there will another couple of thunderstorm outbreaks, though they may be much closer to the Canadian border initially, if not in southern Manitoba and southwestern Ontario.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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Joe Lundberg
Joe Lundberg, a veteran AccuWeather.com forecaster and meteorologist, covers both short and long-term U.S. weather on this blog.