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Posted at 01:29 PM ET, 12/05/2012

Atmospheric road blocks keep weather pattern warm for much of Lower 48 U.S.


Temperature different from normal this morning analyzed by GFS model. Temperatures were as much as 30 degrees (or more) colder than normal in Alaska but up to 30 degrees warmer than normal over the Lower 48 states. (WeatherBell.com)
After another warm day in the Washington, D.C. area with temperatures nearly 20 degrees above average in some spots, I decided to do some digging: if we don’t have the cold air, then where is it?

As Jason pointed out Tuesday, over 400 warm weather records were set in the Lower 48 Monday. Then, scores of additional record highs were set Tuesday over this area. I single out the Lower 48, because over the 49th state, Alaska, it was a different story!

On Tuesday, Anchorage recorded a high temperature of 7 degrees which was balmy compared to Fairbank’s -29 (with a low of -38). I think we’ve found the cold air.

Will that cold air eventually make it our way?

As Wes Junker discussed Monday, it doesn’t look likely that cold Arctic air will make it south anytime soon. The reasons are due to what I like to call atmospheric road blocks: the jet stream and various global patterns such as the North Atlantic Oscillation and Pacific North American (PNA) pattern. The jet stream, found at roughly 30,000 ft. (300 mb) drives our weather and separates cold Arctic air to the north from warmer maritime air to the south.

Right now, the jet stream is displaced north right around the U.S.-Canadian border. As long as the jet stream remains up there, it stays relatively warm down here.

Employing an analogy: think of the jet stream as a concrete median on an interstate: the cold air can’t simply cross the median! We must wait for the median to move south and bring the colder air to us.


High pressure blocks currently over the North Pacific and North Atlantic. Both are situated in a way that favors warm flow over large parts of the U.S. (but cold air for Alaska and Europe) (WeatherBell.com)

Where the jet stream goes depends largely on the various oscillations Wes discussed and their associated blocking patterns; the north Pacific and Greenland high pressure areas on either side of the Lower 48 will divert the jet stream and associated storm track to our north keeping us under a ridging pattern characterized by higher pressures (or heights) and thus warmer temperatures.

So the jet stream is a median preventing the cold air from moving south and the NAO and PNA are traffic cops directing that colder air north instead of south. Okay, enough with the analogies!

A weak cold front brings close to average temperatures tomorrow into Friday


uropean model 500mb plots showing the location of the upper-level ridge Tuesday evening, Thursday morning, and Saturday morning (RaleighWx)
Even with the atmospheric road blocks in place, a weak cold front from a slight southerly push in the jet stream made it through the region overnight.

This cold front will bring near normal high temperatures (upper 40s to mid-50s) for the region tomorrow and Friday before another ridge sets up pushing temperatures back above normal and into the 60s by the weekend.

The 500mb maps using the European model to the right show the overall pattern (courtesy of RaleighWx). The first 500mb plot from Tuesday evening shows the warm ridge in place. By early Thursday morning (second 500mb plot) a shallow trough, or slight dip in the jet stream, provides some cooler temperatures closer to average. However, by Saturday evening another ridge (though not as strong) is firmly in place bringing a warmer than average weekend.

A look ahead

I have allayed the warm weather fans and disheartened the snow lovers. As Wes mentioned, the warmer than average pattern looks to persist into at least mid-December and the Climate Prediction Center agrees (see 6-10 day outlook). The cold air is currently bottled up north of the Canadian border while areas in the Lower 48 bask under what feels like a heat lamp; especially to those already acclimated to colder temperatures.


Climate Prediction Center 6-10 day temperature outlook indicating warmer than average temperatures for the eastern United States (NOAA)

We need ridges to turn into troughs before we really see some cold Arctic air. While this mild air may feel out of place for the holiday season, my advice is to enjoy it while it lasts until that freezer door finally opens!

Jason Samenow contributed to this post

By Kathryn Prociv  |  01:29 PM ET, 12/05/2012

Categories:  Latest, U.S. Weather

 
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