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    A Pre-Christmas Snowstorm, But Unimpressive Cold Follows

    12/19/2012, 7:18:32 AM

    Wednesday, 11:30 a.m.

    I'm sure a few cages will be rattled with the latter part of the headline, but that's really the thought that came to mind when examining everything today. Yes, it is going to snow, and I'll delve into that off the top. And yes it will be colder behind the storm, with that cold charging down the Front Range all the way to Deep South Texas then progressing quickly across the South into the East. However, when you look at the big picture, the cold won't be all that impressive by typical late December standards.

    Here's the most recent snowfall projections from AccuWeather.com:

    If you look at a current pressure analysis, the developing low sits over northeastern New Mexico:

    It's clearly cold in the Rockies, with temperatures in and around Denver right now hovering around 20 degrees. And it's cold enough out across northern and western Kansas into Nebraska for this to become a raging snowstorm, only with wind, too. The storm will steadily deepen in the next 24 hours, with the center low pressure going from around 997mb now to about 985mb late tomorrow. This will help to pull the colder air into the circulation of the storm, and there will be many places with rain for a while eventually going over to snow as moisture gets wrapped all the way around the storm and back into the cold air.

    Severe weather is also a concern with the storm, starting late tonight in the middle and lower Mississippi Valley, then charging across the Tennessee Valley and the Deep South tomorrow and off the Southeast coast by early Friday. The contrast between the hard charging cold air behind the front and the warm, increasingly humid air ahead of it will be large enough to trigger thunderstorms with damaging winds and hail, and I could see a few isolated tornadoes as well.

    As stated yesterday, some snow will wrap around the back side of the storm as the cold air flows over the Great Lakes and into the Appalachians. Some of the high ground of western Maryland, southwestern Pennsylvania and West Virginia can pick up 3-6 inches of snow Friday and Friday night before tapering off on Saturday. Meanwhile, similar snow amounts are likely in the lee of the Great Lakes, with some localized amounts over a foot in some of the traditional snow belt areas of northeastern Ohio, northwestern Pennsylvania, western New York and the Tug Hill Plateau areas. This is wonderful news for ski resorts that have been lacking snow thus far this season, as they'll have plenty and be able to make more with it being cold enough in the days leading up to Christmas.

    But what about this cold? Why do I think it's 'unimpressive'? For one thing, if you go back to that pressure analysis image, there's also a weak low in the Red River Valley. It is has held up what arctic air there is north of a place like Grand Forks, where temperatures remain in the 20s this morning, above average for the latter half of December.

    Then look at the thicknesses Friday morning, after the front has reached the East Coast, and the cold is established across the South:

    For most of the Ohio Valley and Midwest, we're looking at 528dm thickness values. Meh. Many an arctic outbreak in December we're talking sub 500dm thicknesses. How about the 850mb temperatures? Here's what they look like Friday morning:

    That's an impressive area of -5C, but just a small area with -10C over the Appalachians. Seriously? Is that it? "Where's the beef??" So the 850mb temperatures are in the -7, -8C range Friday afternoon across Virginia and the Carolinas. With a westerly downsloping wind and some sunshine, that's equivalent to highs in the mid-40s. In the second half of December, that's a handful of degrees below average in Richmond and Greensboro. Yeah, it'll be cold, and it'll feel even colder because of the wind and how mild it has been, but please. It's December. This is all the atmosphere has?

    In reality, this is much more a top-down kind of cold, or an 'unstable' cold. By that I mean the cold is a result of low heights aloft, not arctic air in the boundary layer. It'll be more easily moderated, especially where there isn't any snow cover, and that'll be much of the Ohio Valley and all of the Tennessee Valley and South, as well as virtually every place east of the Appalachians. With the cold being more aloft, the wind will more effectively mix out the atmosphere, and even though the sun angles are low, that should be enough to get as much as you can out of the air aloft, and that's why low and mid-40s seem reasonable. As the jet stream retreats some over the weekend, areas over the Plains and the South will steadily rebound back above normal.

    Now, if you want some more serious cold, let's head north. To Alaska. No, the gold rush isn't on, but the arctic freeze is. Yesterday, Northway had a reading of 43 below zero... for the HIGH; 51 below zero for the low early yesterday. Even by Alaskan standards, that's cold, some 35 below normal for the date. It's been very cold more than it hasn't for weeks now in much of Alaska and northwestern Canada. Fortunately, there's been little way of bringing that cold south into the U.S. thanks to the persistent western troughiness. And going forward, that's not likely to change through Christmas.

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


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