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    A Break for Some as Winter Arrives

    12/18/2013, 7:22:53 AM

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    Wednesday, 11:30 a.m.

    Let's face it. December has been cold and snowy in a lot of the country up to this point. Many locations from Chicago to Philadelphia to Boston have had nearly a foot of snow for the month to date. More may be in store for the Midwest, especially this weekend with the bigger storm tracking from the lower Mississippi Valley into the Ohio Valley and eastern Great Lakes.

    For some, though, a break in the wintry weather pattern is about to unfold. The storm that dumped snow on the Northeast yesterday into last night is now escaping out to sea. It is cold behind the storm to be sure, but the storm is actually drawing the cold air through the Midwest, the Great Lakes and the Ohio Valley into the Northeast, and it will be exiting rather quickly in the next 24 hours.

    The biggest changes will initially occur in the Ohio and Tennessee valleys over to the mid-Atlantic states, particularly in places that are devoid of snow cover. Here's the latest snow depth analysis:

    We're already starting to see the change yesterday and today, as places that had some sunshine, no snow on the ground and a decent breeze yesterday managed to outperform the projected highs. That's a given this afternoon once again, and this isn't the mild air! Wait until the mild air starts to move in tomorrow!

    And just how mild is this air? Well, let's examine in in terms of the 850mb temperatures first. They can give you a decent idea of the potential of an air mass, especially as move away from winter and into the warm season when you're dealing with higher sun angles and better mixing potential. Nevertheless, it can be a good indicator even at this time of the year of how mild it can get given the right circumstances. Let me start with the 850mb temperature forecast for this afternoon from the 12z Dec. 18 NAM model:

    You can clearly see the lobe of cold air rolling through the Northeast and mid-Atlantic states this afternoon. By the same token, look at the mild air over the central Plains! If you had a perfect environment, such as no snowcover, bright sunshine and full mixing, you could boost temperatures into the 80 over parts of Kansas!! Now, that will not happen, but that's the full end potential of the air mass. As it is, we'll see many places in the 60s across Kansas and points south this afternoon, with a few locations topping 70.

    Let's crank the clock ahead to the end of the day tomorrow, and look at the same forecast chart:

    With the upper-level low rolling southward into California, some of the arctic air just about to enter the Northwest and northern Rockies this afternoon will be pulled down the Front Range of the Rockies and into the central Plains to cool things off starting tomorrow. At the same time, however, the warmth in the central and southern Plains today will get squeezed eastward across the Mississippi Valley, resulting in a nice rebound in temperatures in the afternoon - especially where there is sunshine, a good pressure gradient and no snow cover. Tomorrow, 50s will reach to the Ohio River, and some 60-degree readings will occur in the Tennessee Valley. There will also be some 50-degree readings showing up near Washington, D.C. - a taste of things to come.

    That's just the beginning of the warmup. With the upper-level low becoming the anchor to the pattern over the Southwest then the southern Rockies Friday into Saturday, the heights downstream will steadily rise, especially over the Southwest Atlantic, Florida and the eastern Gulf of Mexico up into the Southeast. Look at the 500mb forecast for Saturday evening:

    Note also the confluence zone, or the area where the northern stream meets the southern stream. In the image above, that runs from Nebraska east-northeastward toward Michigan then northern Maine. With it that far north, it will be hard for any front trying to press southeastward late this week into the start of the weekend to make much progress, especially south of the Great Lakes and New England! So, with southerly winds persisting, it should give free reign for the warmth to expand northward - and not just warm air, but increasingly moist air as well, as the Gulf of Mexico opens wide for business and stays open.

    Now let's look at the corresponding 850mb forecast for Saturday evening:

    The 10C isotherm now runs from near St. Louis to Charleston, W.Va., to Dover, Del. Again, if this were April, full sunshine, sea level and a good breeze blowing, you'd expect readings in the 70s! Instead, we'll get the winter solstice version - lots of clouds, wet ground, low sun angle and, in some places, some rain. Even then, we'll see 60s creeping up into southern Ohio, West Virginia, Virginia, parts of Maryland and Delaware.

    This is before the storm in the lower Mississippi Valley really gets cranking!

    By Sunday afternoon, as the main low cranks up up and cross the Ohio Valley into the eastern Great Lakes, the warm surge will push the springlike air up into New York state and much of New England to record highs. When combined with high dew points and rain, the snow melt will be impressive and can lead to flooding issues, all after an extended run of cold and snow for many.

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


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