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    What to Watch for in the Weather in the Days Leading up to Christmas

    12/13/2012, 7:31:03 AM

    Thursday, 11:30 a.m.

    Some days it is easy to pick up on a theme. The writing comes easy, as the pattern, the storm or the story more or less writes itself in a way. Then there are other days where you have to think harder, look harder and work harder to come up with something that is still appealing. Sure, the story starting to grab more traction now is the potential for a significant storm next week, one that could impact the eastern half of the country in one way or another, but that is just one of many things to watch for in the weather in the days leading up to Christmas, which makes the story or blog in this case more of a challenge to write.

    I have come to embrace challenges, rather than run from them, so what I've decided to do is go with a series of highlights on the weather over the next couple of weeks - really, the 12 days leading up to an including Christmas.

    1) SoCal storm to kick off a parade a storms. Here's the morning IR still:

    This feature is bringing rain through Southern California this morning, making a place like Palm Springs not a top 10 vacation pick today. As we follow this system eastward tonight, it will spread rain and some mountain snow through Arizona into western New Mexico, with some snow in the southern mountains of Utah and Colorado overnight into tomorrow. Then, as the storm rolls out onto the Plains tomorrow nigh and Saturday morning, most of the precipitation will go with it, though west-facing slopes in the Rockies will still get some snow.

    Initially, the vast majority of the precipitation with the storm out on the Plains will be in the form of rain, but as it tracks to the Great Lakes Saturday night, it will dump a few inches of snow on portions of the Midwest. Then, on Sunday, the storm will be forced to turn eastward, eventually causing a secondary area of low pressure to form south of New England later in the day.

    Ahead of the storm in the Northeast, there will be a sneak attack of arctic air tomorrow night into Saturday. Weak high pressure will move into Quebec to set up the low-level chill, and it will be deep enough so that when precipitation spreads into upstate New York and central and northern New England later Saturday night and Sunday, there will be some snow and ice, while areas to the south get a cold rain in southern New England. The rain amounts will be much lower farther south in the mid-Atlantic.

    2) Second storm in the pipeline may be the strongest of the bunch. This storm will have more working for it. It really won't get its act going until late in the game, so as the upper-level support for it slides across the Plains, there may not be a lot to show for it. Eventually, though, a wave of low pressure should develop along an old front in the South, heading toward the mid-Atlantic coast Tuesday or Tuesday night. The timing is a little hard to nail down, as the features that will really set this all in motion are out in the Pacific right now, making them harder to properly initialize. Regardless, the precipitation shield associated with it should expand with time in the East later Monday or Monday night.

    Right now, the amount of cold air available to this system is quite limited. If, however, it can deepen rapidly and earlier enough, it may be able to either cool the entire column to the point at which it can snow, or it will drain enough cold air in from northern New England to change rain to snow in some interior areas of the entire Northeast. It should be all rain along and even near the I-95 corridor, though.

    3) Unusual block will eventually lead to some chill in the East. I say unusual, because we normally think of blocking in the Atlantic as being over or near Greenland. If you look at the 500mb forecasts, though, and some of the model ensemble forecasts, you don't really see that. Look at the GFS ensemble 500mb height forecast for next Wednesday morning, along with the projected height anomalies:

    The block is actually over northern Quebec! What it does is force the second storm next week underneath it, and my belief is that the jet stream flow is strong enough that storm should get off the coast quickly. Once it rapidly deepens offshore, it will definitely slow down, but its lingering impacts on land areas are largely going to be on New England and southeastern Canada, not so much the mid-Atlantic.

    Really, if you step away and look at the Northern Hemisphere as a whole, you could say it's all plugged up! Lots of areas with big anomalies, both positive and negative.

    Anyway, with that weak but notable upper-level high in place over northern Quebec deep into next week, it will force the jet stream around it. While one part goes north, another part will be diverted south, and this will allow some of the chillier air to drain into the East over time heading toward Christmas and beyond, even as the mild air remains in place on the Plains.

    4) Snow cover still limited. There will be many places that traditionally have a white Christmas looking out at a barren landscape come Christmas morning. There's just not a lot of cold air around on the Plains, nor will there be, and most of the storms that come through the area will have very limited moisture with them. Instead, they'll wait to develop until farther east, which means a lot of places that have no snow now may have little chance of getting any kind of precipitation, including snow, between now and Christmas.

    There are some hints at a change in things after Christmas, but just hints at this point. The big thing that remains constant is the low heights, representative of troughing, persisting in the West right through Christmas. Until that changes, it will be hard to see the big picture changing much.

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

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