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Posted at 11:00 AM ET, 12/06/2011

Weather pattern overview: Colder air coming East but only briefly

Cold Canadian air now occupies the central United States. Just yesterday, a strong high pressure system slid into the western high Plains and brought temperatures 10-20°F below average from the Dakotas all the way to the Texas Gulf Coast.


Monday’s weather map, showing sea-level pressure and precipitation (Penn State)
A band of light snow moved through places as far south as Texas, Oklahoma, and Arkansas on Monday. Though this may not concern residents in the eastern quarter of the country, with exceptionally warm temperatures in the 60s expected to stretch from Raleigh to Boston today, these locations will likely get a taste of winter-like temperatures by the weekend when colder weather moves in from the Plains.

In the mid-Atlantic and Northeast, the transition from spring-like weather today to temperatures 5-10°F below normal will be complete by Sunday.


Temperature departures from average expected today (left) and Sunday (right). (Penn State)

The maps shown above illustrate the eastward progression of the cold anomalies (blue shading) and the impending disappearance of the warmth along the East Coast during the next few days. They also indicate that by the time the cold air reaches the coast over the weekend, it will have lost considerable intensity.

Though this latest penetration of frigid Canadian air struck the western Plains the hardest (by far), all is not lost for those hoping for winter weather soon in the mid-Atlantic. Present indications suggest that the eastward shift of cold air will likely be accompanied by a round of snow for the higher terrain of central Appalachia … one that will perhaps extend to the I-95 corridor.

The upper-level trough that helped produce the snow over the Southern Plains yesterday will move toward the Carolinas during the next couple of days and trigger the development of a surface low-pressure system underneath it. Once this feature makes it to the Coast late Wednesday, the multi-day cooling trend for the Eastern third of the country will begin.

Though the low pressure system is not expected to be particularly strong, enough cold air may filter into northern side of the precipitation shield to produce a narrow swath of accumulating snow over interior sections of the mid-Atlantic late Wednesday and early Thursday.


GFS forecast of sea-level pressure, temperatures aloft, and 6-hour accumulated precipitation for Wednesday night. (NOAA)
The picture to the right is a forecast from one of the weather models for Wednesday night, with the area at greatest risk for snow (as it appears now) outlined in orange.

The Capital Weather Gang will closely monitor this situation and provide detailed forecasts regarding any developing potential for snowfall in the D.C. area. (Stay tuned for an update later today)

As the low pressure system moves northeastward off the Carolina Coast late in the week, colder air will slowly move into the East. Seasonable temperatures will arrive Friday, replacing the excessively mild conditions in place right now, and the coldest weather will show up for the weekend. Daytime highs on Saturday will likely be in the 30s and 40s from New York to Richmond.

But as we discussed last Friday, the westerly jet stream is freely swirling around the high latitudes of the Western Hemisphere without a blocking pattern in its way. Regardless of whether or not this mode will dominate this winter, it’s likely to be commonplace the rest of this month at least.

Overall, the global circulation is much differently configured now than it was during much of the past two boreal winters. Reasons for these differences are rooted in the different ways the jet stream has interacted with the traveling troughs and ridges in the westerlies, which in the previous years led to frequent blocking patterns. The image below shows the jet stream flow (outlined by red lines) averaged over multi-week periods from last year (left) and this year (right). The green, yellow, and red shading indicate the fastest winds.


Time averaged winds at high altitude from last year (left) and the past few weeks (right). (NOAA/ESRL)

Strong west winds aloft and much colder air now occupy the same large regions near the North Pole that were much warmer and less windy just one year ago. In the current setup, weather systems more freely move from west to east than they do from north to south. This will help limit the unseasonably cold air’s visit to the Lower 48 this week to just a couple of days, and introduce a significant warm up nationwide next week.

By  |  11:00 AM ET, 12/06/2011

Categories:  Latest, U.S. Weather

 
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