Joe Lundberg

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Blowtorch to Spark Severe Thunderstorms From Midwest to Ohio Valley Ahead of Dramatic Turnaround

July 24, 2014; 10:11 AM ET

Thursday, 11:45 a.m.

The more I look at the setup going into the coming weekend, the more impressed I am becoming at this blast furnace of heat coming out of the southern Plains tomorrow and heading toward the East Coast this weekend. Not everyone will be scorched by this blowtorch, but those that do will certainly remember it. However, along the fringe of the heat surge and ahead of a strong cold front, severe thunderstorms seem almost certain to erupt. Once that front passes, though, it will go from above and much above normal to much below normal in a heartbeat. And once the flip has been made, look for it to remain cooler than normal for several days.

If you look at the current weather map, there's not necessarily a good hint of what may come. There's a sprawling area of high pressure over the Upper Midwest and Great Lakes, and it has pushed a front to the western Plains and down to near the Red River Valley and the Tennessee Valley. Temperatures underneath the high and surrounding area will easily be below average this afternoon across the Midwest and Great Lakes into the Ohio Valley as well as the mid-Atlantic states into New England. However, the western side of this high will quickly moderate this afternoon thanks to a strong upper level ridge over the southern Rockies that will expand out into the southern Plains. Here's the 12z July 24 NAM 500mb forecast for this evening:

That will send temperatures soaring into 90s over the eastern Rockies and Plains, with some over 100:

Note the orientation of that ridge. It's not so much a north-south ridge any longer, but more of an east-west ridge. The reason for that is the vigorous upper-level low rolling out of Washington into Alberta that's pulling a strong cold front through the Northwest into the northern Rockies. Those features will be two triggers to the severe weather that will ensue tomorrow night into Sunday.

Let me talk about the heat some more. I've showed you this chart the past couple of days, but it's worth throwing up there again, the 850mb forecast for tomorrow evening:

850mb temperatures away from the Rockies can be a good indicator of surface high temperatures. For locations not too far above sea level, if you take the 850m temperature in Celsius and add 15, you get an approximation for the high temperature potential. So, for an area in western Missouri which is projected to reach, say, 27C, add 15 (42), then convert to Fahrenheit, and you'll come up with a number like 108! Now, will it get that hot this time around? No, I don't think so. There may be too many clouds around for one thing. They are generally 1,000 feet or more above sea level for another. And the mixing may not be as efficient in late July as it would be in June and early July. All that aside, it is still going to be exceptionally hot tomorrow afternoon from Missouri back to the eastern Rockies and from Nebraska to Texas.

Now, go back and look at that 850mb forecast chart. Do you see the wind barbs on there? Note carefully the 30- to 40-knot winds they are portraying a few thousand feet above the ground from eastern Kansas up into eastern Iowa and western Illinois. That tells me those winds are trying to shove that heat across the Mississippi Valley, where the current cool, dry air mass is residing. As fast as those winds are aloft, something has to give. Either the heat moves in, and all is quiet, or that blowtorch wind forces the hot, humid air more violently upward, causing strong to severe thunderstorms to break out. Judging by the modeling, it strongly suggests the latter happens from eastern Iowa into Illinois tomorrow afternoon.

The heat doesn't stop trying to come eastward. On Saturday, look at where that ribbon of steamy air is aimed:

The heat will literally and figuratively 'run out of steam' at some point, but it should be able to ignite more thunderstorms Saturday and particularly Saturday night across the Ohio Valley - all before the cold front can enter the picture from the northern Plains. It will get warmer and more humid Saturday east of the Appalachians, with an attempt at heat on Sunday in Virginia up into Maryland and Delaware. Sooner or later, though, the heat will no longer expand northward across the mid-Atlantic, not really reaching New York and New England.

By this time, however, the trailing cold front will be charging in from the Midwest and across the Great Lakes and Ohio Valley with another round of strong thunderstorms and perhaps yet one more round of severe thunderstorms in the Ohio Valley and parts of the eastern Great Lakes.

Then the dramatic turnaround behind the front. Look at the projected temperature anomalies for next Thursday:

This cooling actually begins on Monday, but it lasts all week long in the Mississippi Valley as well as in the Great Lakes and the Ohio and the Tennessee valleys. Quite a dramatic turnaround from 90s with oppressive humidity Saturday and perhaps into Sunday to 70s with low humidity for a couple of days next week!

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

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About This Blog

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