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    Joe Lundberg

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    Heat Wave to End This Weekend as Much Cooler Air Enters Pattern

    August 30, 2013; 10:45 AM ET

    Friday, 11:20 a.m.

    Today will be the first day since last Friday with temperatures failing to reach 90 in Minneapolis, an incredible run of late-summer heat that has been combined with some gruesome humidity. If you go back to Aug. 19, it's now 12 days in a row with temperatures well above average, a period that has pushed temperatures to more than 3 degrees above normal for the month despite the first 18 days of the month averaging 2.5 degrees below average!

    A similar turnaround has taken place in Bismarck, N.D., as well. The first 16 days of the month were 2.5 degrees below normal. As of midnight, the current monthly departure is 1 above. I could go on, picking on any number of locations from the northern Plains and Midwest into the Ohio Valley, where a cool first 16 to 18 days of the month have been more than erased by the persistent warmth ever since.

    The heat wave is about to come to an end, however, as a strong cold front will move through the flow to chase the heat and the humidity away from northwest to southeast. The front will move into Montana and North Dakota to start the weekend as an upper-level disturbance comes over the top of the upper-level ridge. This feature was at one point an upper-level low off the Northwest coast. The 12z Aug. 30 NAM 500mb forecast for early tomorrow afternoon:

    Ahead of the cold front, temperatures will again reach to 90 or better across Nebraska and into parts of Iowa, though clouds and some showers and a couple of stronger thunderstorms farther north will prevent it from getting that warm.

    The front will then move farther downstream Saturday night and Sunday, likely reaching Chicago by Sunday evening:

    As the flow amplifies, the downstream trough will deepen, and that means a somewhat slower arrival of the cold front east of the Appalachians. Nevertheless, that front will reach the New England and mid-Atlantic coasts sometime Monday night and Tuesday morning, effectively ending the run of above-normal warmth there.

    What follows is a much, much cooler air mass, and one that will also be much, much drier. Dew points that have been persistently in the 60s and lower 70s over much of the past week will suddenly be in the 50s and 40s as a decent high pressure area first builds into the eastern Dakotas and Minnesota Sunday and Sunday night, then the Midwest Monday and Monday night. This will push temperatures quite a bit below normal. Instead of being 8 to 15 degrees or more above normal, temperatures will be a solid 5 to as much as 10 degrees below. Look at the departure forecast for Monday:

    By Wednesday, most of the cooling that will be taking place behind this front will have occurred:

    Less of this cooling will show up along the Eastern Seaboard, but that's nothing really all that new. The proximity of the ocean and all of its warmth will serve to modify the air mass coming toward the coast greatly. There will be a second cold front later next week, and the main impact of that front will be to either stop the warmup on the back side of the surface high or reinforce the cool air already in place. Either way, it means cooler-than-normal conditions could well extend from the eastern Plains and Mississippi Valley to the Appalachians and beyond for much of next week!

    In other words, if you're a fan of summer's heat, then make the most of the remaining few days of it! This kind of heat may not repeat itself in 2013!

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


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