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    Joe Lundberg

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    Active Weather Pattern Finishing Out April

    April 24, 2012; 9:57 AM ET

    Tuesday, 11:35 A.M.

    On Monday's post I talked about how the nation has become a land of extremes. That continued Monday, with a grand total of 23.2 inches of snow piling up atop Laurel Summit, Pennsylvania. In fact, look at the current visible satellite image, and you can see the stripe of snow centered on the Appalachians:

    That snow extends deep into West Virginia, all remaining vestiges of the storm that has lifted north to just west of Ottawa this morning. There are still a few snow showers in northern Pennsylvania and portions of western New York state into Ontario and extreme western Quebec at this hour. It has temporarily cleared across Ohio and southern Pennsylvania, as you can see from the image above, but the air aloft is really cold, given the look of the 500 mb chart:

    And that means the sun this morning will give way to a lot of cumulus clouds this afternoon, but as this upper-level low lifts out to the northeast over the next 24 hours or so, that instability will go away tomorrow.

    Meanwhile, by looking at the same 500mb forecast chart, you see why it has been so warm in the West and the Rockies of late - a strong upper level ridge poking up into Montana. Records were set from Death Valley, Calif., to Pocatello, Idaho, and Grand Junction, Colo., and even Miles City, Mont., and Dickinson, N.D. Denver is sure to see their record of 85 disappear this afternoon as temperatures head for 90, and they won't be alone in seeing July-like warmth in late April.

    These kinds of contrasts often lead to some sort of trouble in the atmosphere. In other words, with two very different air masses in the playing field, sooner or later one will attack the other. And when that happens, there's usually a change in the weather, and that change is often very turbulent.

    An upper-level disturbance over the west shores of Hudson Bay today will drop southward through Manitoba toward northern Ontario tonight and tomorrow. At the same time, the upper level flow over the northern Plains to the Mississippi Valley will be from the west-northwest or northwest. And at the surface, a southwest flow of warm air will become established over the southern Plains toward the middle Mississippi Valley. The low levels will bring the warmth so far north, but with the upper level flow out of the northwest, that low level warmth will run into increasing resistance. Then, with the approach of the upper level disturbance from the north, the air aloft will begin to cool. This will all lead to the development of clouds, rain, and thunderstorms late tonight and tomorrow across the Midwest.

    This whole 'disturbed zone' of weather will then follow the mean upper-level flow to the southeast tomorrow night into Thursday, spreading clouds rain and scattered thunderstorms through the Great Lakes and Ohio Valley. Where the rain is well-time, it will really dent any warm up that would normally occur with a southwest surface flow. While I think the latest NAM forecast is going a little overboard on the intensity of the rain with this feature, I do believe it has the overall right idea of an area of warm advection rain and thunderstorms streaking over the Appalachians into the mid-Atlantic tomorrow night and early Thursday:

    It will warm some Thursday ahead of the cold front that will come out of all of this, but it will be a very brief warming, and a wet one in most of the Northeast quadrant of the country. Then, behind it, another shot of chilly air comes out of central Canada and by the Midwest and toward the Northeast. Just look at the 500 mb flow for Thursday evening:

    With such a strong upper level ridge poking up toward Greenland, where can the feature coming at the Northeast go? It has to drive off the mid-Atlantic and New England coast. By Friday, the effective cold front will be down into North Carolina and Kentucky, and that sets the table for the next attack of the warm air from the southern Plains.

    The models diverge on the exact timing of that next attack, as well as how far north it can push the warm air back to the north later Friday into Saturday. Given the strength of the blocking downstream, I'm betting on this front making less progress to the north than we've grown accustomed to for much of the late winter and early spring season. Again, it may not be your classic blocking pattern, but the upper levels are certainly amplified, and it should be good enough to force at least a couple of additional disturbances this weekend and early next week to go more west-to-east, rather than southwest to northeast. And that's going to mean more opportunities for stormy weather, and it will remain cool across the Great Lakes and Northeast. In addition, with the weakening of the upper level ridge over the Rockies and Plains, these areas are apt to cool down quite a bit this weekend as well, while the warmth holds on across the South.

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

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