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Posted at 10:57 AM ET, 12/02/2011

Arctic air poised to strike U.S. next week, but core may miss mid-Atlantic and Southeast

Earlier this week, a strong cold front moved from the Plains to the Southeast U.S. Accompanied by temperatures more than 10 degrees below climatology, this system brought snowflakes to parts of Arkansas, Tennessee, Mississippi, and Alabama.

An infrared satellite shot from this past Tuesday shows the jet stream and the location of the core of cold air. (NOAA)
It is rare to see snow in locales that far south in November, and one may wonder if unusually cold air outbreaks that affect the South like that one will be a common theme this winter. With another short-lived cold shot taking a similar aim at the Ohio Valley and Southeast early next week, and a more fierce blast of frigid air expected to strike parts of U.S. late next week, it might be tempting to think just that.

But next week’s shots of polar air appear likely to spare the Southeast and mid-Atlantic from the brunt of the chill and instead target the Northern Plains and Great Lakes.

Some of the drivers behind the recent events can be traced back to changes in the weather over the tropics. In the Pacific Basin, for example, the westerly jet stream has slowed considerably at latitudes close to the equator. This has been associated with an increase in amplitude of the troughs and ridges over that sector of the globe.

High-altitude pressures and winds from Dec 1. (Penn State)
In particular, one very large high pressure center (tall ridge shaded in red) recently developed near western North America. During the past week, this feature has helped steer chilly air masses from Canada into the southern U.S (small blue shades in the Southwest and Southeast), and certainly played a role in producing the snow event in the mid-South a couple of days ago.

As we move through next week, a fresh batch of Arctic air will build over Alaska and essentially slide along the east side of that same ridge into the northern U.S. This will likely be the coldest air we’ve seen over North America this season.

A forecast of the jet stream flow next Friday. (NOAA)
Its initial impact on the lower 48 will be felt in the Northern Rockies. Highs will likely be near zero in that part of the country by Thursday. National Weather Service forecasts for northern Montana predict highs in the single numbers next Thursday. The image to the right is a forecast of the jet stream flow one week from today.

The frigid air will have no problem oozing southward at least half way down the Plains and eastward toward the Great Lakes, giving the local populations a multi-day stretch of absolutely miserable conditions with highs way-below average on the heels of frosty north winds.

But as the frigid air expands further southward and eastward over the weekend, very strong southwesterly winds aloft –in association with the strengthening of an upper-level ridge off the Southeast coast- will probably help protect that region from the punishing cold by essentially weakening it and deflecting it back toward the northeast. This contrasts at least somewhat from recent weeks, where the hearts of (albeit weaker) cold air masses had no problems reaching Florida latitudes for short periods.

The greatest uncertainty in this forecast relates to the southward and eastward extent of the Arctic blast. The cold air will be shallow enough to slide right underneath the high altitude jet and conceivably reach the Gulf and Atlantic coasts in full form. But at this point, it appears as though it will lose its punch on its southeastward journey and not make it that far.

There is, however, a significant, glaringly obvious, component of the circulation that will play a large role determining the fate and geographical extent of not only this cold outbreak, but also ones that may come down the road later this month. It is one that suggests that as we move toward the middle of the month, any unseasonably cold air that reach the East Coast will do so in spurts which are interrupted by significant moderating trends.

As Wes discussed yesterday, because of the expected lack of high-latitude blocking over the western hemisphere in the coming weeks, the jet stream will be able to flow relatively freely from west to east across Canada on a journey that circumnavigates the Pole. As a result, cold air intrusions into the U.S. will have a harder-than-otherwise time lingering on for more than a few days, before being eroded by the next weather system coming in from the west. I agree with Wes’ sentiment that those living in the mid-Atlantic and Northeast hoping for a prolonged cold and snowy pattern in the near future will be probably be disappointed

By  |  10:57 AM ET, 12/02/2011

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