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    Joe Lundberg

    Early Week Storm a Prelude to a Bigger Storm Later This Week

    12/17/2012, 6:25:06 AM

    Monday, 11:30 A.M.

    There's a lot of mild, moist air on the playing field today, stretching from the mid-Mississippi Valley to the East Coast, and from the Great Lakes and southeastern New England to the Gulf Coast. There are few places in that amoeba-shaped area without a lot of clouds, and in the absence of high pressure across the region, there's not a lot of air movement, either. Look at the 10 a.m. pressure analysis:

    The main feature is a relatively weak area of low pressure in the lower Ohio Valley. There's another one north of the Lakes in northern Ontario, with a relatively strong surface high in Labrador, and a weak one over southern Florida.

    With the developing low in the Ohio Valley, there will end up being two main areas of precipitation. One will be with the trailing cool front (I can't really call it "cold," as the air mass behind it today in the southern Plains is still milder than average) in the form of showers and thunderstorms. This is showing up now from the mouth of the Mississippi River all the way to northeastern Georgia. This area of rain and thunderstorms will cross the Carolinas late this afternoon and tonight, and be gone by morning.

    The second area of precipitation is still rather fractured over the Ohio Valley to West Virginia at this hour. Eventually, that will congeal into a more concentrated area of rain that will move through western and then northern Pennsylvania into upstate New York, and the vast majority of that will be in the form of rain, not snow.

    As it runs into the larger high up to the northeast over Labrador, there will be enough cold air in the low levels of the atmosphere for snow this afternoon to grudgingly give way to sleet and freezing rain, then rain from south to north in the northern half of New England. That said, there may well be a few inches of snow and sleet over interior Maine, if not six inches, before the storm winds down on Wednesday in Maine.

    All that is just a precursor to a much-larger and better-organized storm that will spread some mountain snow from the Sierra to the Continental Divide tonight and tomorrow. By Wednesday morning, low pressure will clearly be organizing over southeastern Colorado, southwestern Kansas, and the Oklahoma and Texas panhandles, a feature that will bring colder air southward through the Rockies. As the storm organizes, there will be a period of cold air forcing in Colorado, resulting in several inches of snow in the foothills out into the northeast Plains of the state.

    With the storm then tracking toward Michigan by the end of the day Thursday, there's no doubt about the fact there will be accumulating snows on the northwest flank of the storm, which will include northern and western sections of Kansas into Nebraska, northern and western Iowa, extreme southeastern Minnesota, and a significant portion of Wisconsin. In general, the outer portions of this broad area can expect to see a solid 3-6 inches of snow, and the core of that area will pick up a good 6-12 inches of snow. There could easily be places in Wisconsin that exceed a foot of snow by the time it stops falling late Thursday and early Thursday night. Complicating it will be wind, causing some blowing and drifting snow and further reduced visibilities.

    Even in Chicago, a couple of inches of snow may fall on the back end of the storm as the cold air drills in behind the storm on Thursday.

    To the south and east of the storm, however, it will be very, very mild with nothing but rain. Even in New England, there will be little resistance to the arrival of warmer air from the south and southwest, with just a limited amount of snow and ice restricted to interior Maine. Severe thunderstorms cannot be ruled out ahead of the attending cold front as it sweeps across the Ohio and Tennessee valleys into the Deep South, and into the Southeast Thursday night.

    Once the storm and the front bypass the Lakes and sweep through the South and East, it will turn much colder; though, by December standards, it won't be anything unusual. Below normal, yes, but not severely so. Nor should it stay more than a couple of days. If you look at the big picture, the Pacific still holds most of the cards at this point. So as long as there remains an upper-level trough near the West Coast, much of the time just off the coast, it will be hard to drain large pieces of cold air out of western Canada into the central and eastern U.S. Moreover, on the occasions where you can pull it into the pattern, such as late this week into this weekend, there's virtually no way of keeping it in place.

    So, if you're hoping for a white Christmas this year, and you don't live in the Midwest or downwind of the Lakes in the preferred lake-effect snow areas, your chance of getting one is getting lower and lower with each passing day.

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


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    Joe Lundberg