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    Expansion of Heat on Cue for Summer's Arrival

    6/20/2013, 7:22:22 AM

    Thursday, 11:45 a.m.

    The summer solstice is hours away, at 1:04 a.m. EDT. While a relatively cool, dry air mass covers the greater Northeast now thanks to high pressure over Pennsylvania, it will be quickly modified over the next couple of days as it slides off the coast. That will lead to a gradual expansion of heat and humidity in the coming days, starting in the Plains and Mississippi Valley, and eventually reaching the East over the weekend and early next week.

    Temperatures in the 90s will be common over the eastern Colorado, Nebraska and Kansas down into much of Texas this afternoon, with 90s also affecting parts of Missouri down into Louisiana. Tomorrow, some of that kind of heat will try to wedge itself into parts of the Midwest and across the Mississippi Valley, thanks to a southwest flow aloft. Here's a look at the 20 June 12z NAM 500mb forecast for tomorrow evening:

    This kind of action will almost assuredly generate strong to potentially severe thunderstorms across the Dakotas and parts of Nebraska into Minnesota and Iowa this afternoon and tonight. The passage of an upper-level disturbance across the Upper Midwest late tonight will drag a weakening cold front out of the northern Rockies into the Upper Midwest, but it will stall tomorrow from Minnesota back to southern Wyoming tomorrow. Along that boundary, yet more thunderstorms will flare up tomorrow, keeping temperatures in check anywhere near it and to its north.

    In fact, that weak disturbance is just a little piece of a bigger upper-level low rolling through the Northwest right now. The air mass in and of itself is not chilly in its origins, but when you look at the low heights combined with the clouds and showers coursing through Washington, as well as gusty winds, you get a day that is more out of early May or even late April rather than the second half of June for the region. Look at the projected temperature anomalies for tomorrow:

    Contrast that to the weather over the South into the Southeast, which is being forecast by the modeling to be relatively close to normal in the coming days, if not a bit below it. The reason? Clouds and scattered showers and thunderstorms, largely occurring during the afternoon and evening hours. This will keep daytime highs a little below average for this time of the year, while at the same time the nighttime lows won't be far out of range of normal. In essence, the South and Southeast will not be underneath the upper-level ridge, bur rather to the south and east of the mean upper-level ridge.

    This will force the hand of the heat coming out of the Plains this weekend. Examine the 20 June 12z NAM 850mb heights and temperatures forecast for Sunday evening:

    The strongest flow is dictated by the closeness of the height lines on the forecast chart. Sunday evening, that runs in an arc from the western Gulf up into central Texas and Oklahoma, then arcs across the Midwest and Great Lakes before turning across upstate New York to New England. If that is to be true, we should expect to see the biggest thrust of the heat be aimed toward the areas with the strongest flow. That should mean the heat will eventually move into the Great Lakes and Ohio Valley this weekend, with 90 degree heat likely in Chicago, and probable in Detroit.

    It may take until Sunday to reach the mid-Atlantic and southern New England, or even Monday in some instances. The heights aloft are exactly high on the front end of this pattern in the Northeast, so it wouldn't surprise me at all to see temperatures struggle to get to 90 initially, with some scattered shower and thunderstorm activity over upstate New York into New England.

    The heat won't be denied, however. It's not as if we're talking 100-degree heat. Most of the I-95 corridor from the Carolinas to at least southern New England will see it reach 90, with some mid-90s showing up through the middle of the week. If you look again at that image above, though, it has a look to it of the classic Bermuda High - high pressure off the East Coast, anchored near Bermuda. It will circulate heat and humidity from the Mississippi Valley into the East, enough to spark some afternoon and evening thunderstorms on an almost daily basis through the middle of next week. In other words, some areas of low clouds and fog to start a given day; otherwise, sunshine mixing with building clouds with temperatures surging to 90 and beyond with dewpoints in the 60s to near 70. Then thunderstorms start popping in no organized fashion, only to dissipate long after dark as the pattern could well repeat itself deep into next week.

    Just in time for summer's arrival.

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

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