Joe Lundberg

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Shrinking Heat, Expanding Cool

June 4, 2014; 10:56 AM ET

Wednesday, 11:55 a.m.

Severe weather has reared its ugly head over the past 24 hours, stretching from parts of Montana and eastern Wyoming all the way to Ohio Valley. It's not like the upper-level disturbance that's causing the violent weather is all that impressive, at least not when examining the upper-level charts for this morning:

By the same token, look again at how tightly packed the 500 mb height lines area from the Ozarks to the Great Lakes. Temperatures reached the 90s yesterday from eastern Colorado and southern Nebraska into parts of Missouri and Illinois, while at the same time it stayed in the 70s to the north, even in the 60s in large part of South Dakota. And not only was it hot in the central and southern Plains, but it was also pretty humid, too. That hot, humid air mass was being driven right at the cooler air to the north, and that helped to fuel the potent thunderstorms that left behind a trail of destruction:

Low pressure in east-central Illinois late this morning is spreading rain and strong thunderstorms into the Ohio Valley this afternoon, with the strongest cells closer to the Ohio River proper. Heavy rain may trigger flash flooding across parts of Indiana and Ohio, and the heavy rain will pelt parts of the central Appalachians into southern New England tonight into tomorrow. Here's the 12z June 4 NAM 48-hour precipitation forecast:

While this lead storm is moving steadily eastward, the next disturbance will spark more strong to locally severe thunderstorms this afternoon into tonight over eastern sections of Wyoming and Colorado out into the Black Hills of South Dakota, western Nebraska and western Kansas. While not as volatile of an outbreak of severe weather as what occurred yesterday into last night, the air mass is plenty unstable. A look at the convective available potential energy, or CAPE for short, which is an indicator of how unstable the atmosphere is, for tonight:

This disturbance will also be relatively weak, and the accompanying surface wave of low pressure will ride the old front to the east across the central Plains toward the Tennessee Valley. That front is being pulled south by the lead system, and it will be the same separation between a dry, comfortable air mass and a relatively hot, humid one staying put across the southern tier of states. As such, the unsettled weather will follow suit in a banded area between Kansas and Oklahoma eastward to Virginia and the Carolinas. To the south, a typical early summer air mass will be in place with little more than air mass or sea breeze thunderstorms to contend with, while to the north it will be rather cool and dry by early summer standards.

How cool? Well, look at the anomalies for Friday:

That's not an air mass that would raise concerns for frost, but it may cut into the number of people who plan on using the pool! And if you look again at that image, you'll note an even cooler air mass pushing across the border from Alberta, Saskatchewan and Manitoba into Montana and North Dakota. A cold front pressing southeastward will bring the chilly air into the region late Thursday night and Friday, with yet another opportunity for strong thunderstorms to erupt from parts of Wisconsin and Nebraska to the Upper Midwest.

If you step back and look at the big picture, there really isn't a lot of heat around the country. There will be some severe weather as these various fronts push what hot and humid air there is farther and farther south. The warmest places will be in the South and in the West, while the coolest areas will be in the northern Plains.

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About This Blog

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