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    Amplifying Pattern Will Allow More Cool Air to Finish July

    7/23/2013, 7:17:38 AM

    Tuesday, 11:30 a.m.

    Let's face it; July has been pretty toasty. Most places from the northern Plains and Midwest to the mid-Atlantic and New England have been a few degrees above normal so far this month. At times, it's not been all about the heat, but also the humidity, too, which has been unrelenting in the mid-Atlantic region. The upper-level ridge that blossomed out of the Atlantic and over the mid-Atlantic more than a week ago baked most of the Ohio Valley and East last week. The only relief came in the form of a cold front that did knock down temperatures quite a bit across New England back into the Great Lakes and Midwest, with a pretty substantial reduction in the humidity as well. However, that front never cleared the mid-Atlantic and much of the Ohio Valley, so they've remained in the tropical air mass.

    Things are changing, though. A stronger cold front is now moving out of the Midwest and into the Great Lakes and western Ohio Valley. This front, unlike its predecessor, will completely clear New England and most of the mid-Atlantic region tomorrow afternoon and tomorrow. In turn, drier air will move into these areas with time tomorrow and tomorrow night, finally reducing the humidity that has been nothing short of unrelenting since the summer solstice!

    Here's a look at the 12z Tuesday, July 23, NAM 500mb forecast for tomorrow morning:

    Note the position of the upper-level ridge over the Rockies, as well as the one farther off the East coast than we've seen of late. This gives room for the trough digging into the East to move in and promote a westerly flow to the coastal plain, something that should push the front to the coast, then offshore with time.

    Where it clears effectively back across the Midwest, tonight will be as cool as it has been in weeks, with 50s commonplace, and some of the cooler places slipping into the 40s across Wisconsin and Minnesota. That should also take place in the Great Lakes and Ohio Valley Wednesday night and Thursday morning, with 50s showing up across a large part of Pennsylvania and upstate New York into northern New England as well. While it will be cooler farther south in the mid-Atlantic than it has been in five weeks for overnight minimums, there may be enough clouds to keep temperatures in the 60s from the I-95 corridor south and east.

    It's not the only front. There's another one on the way late this week, thanks to the development of an upper-level low pressure area over western Ontario. The 0z July 23 GFS 500mb forecast for Friday evening:

    This will represent a pocket of chilly air aloft for this time of the year, and it will help to drive another cold front southeastward across the northern Plains and Midwest. Look at the 12z July 23 NAM 850mb forecast for Friday evening, and note the close packing of the red temperature lines in the Midwest:

    That one contour on the back side of the closed low is 6C, which is mighty cool for late July! It makes sense, then to see a chilly weekend on tap for the region with such a strong upper-level low! Here's the projected temperature anomalies for Saturday:

    It's one of the more impressive cool downs I've ever seen near the peak of summer heat! All in response to amplification the upper-level pattern. This should begin to flatten out again next week.

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

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