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Posted at 02:10 PM ET, 01/24/2012

Winter flirts with U.S., but remains uncommitted

A fairly robust arctic air mass crossed the border last week and gave parts of the Northwest, Northern Plains and Great Lakes their first dose of real winter. Though it was very late coming, it definitely reintroduced the Northern Tier to the supremely uncomfortable conditions winter is famous for.

Seattle received more snow last Wednesday than they typically get in an entire winter. Cut Bank, Mont., plummeted to -29°F, and even Chicago joined the act with near blizzard conditions on Friday as a feisty low-pressure system moved along the arctic front to its south.

But, as we have seen so often this season, the chill didn’t last. In fact, as we move into early February, the weather pattern across the nation as a whole will instead be marked by bouts of mild conditions, rain and thunderstorms more reminiscent of early spring.

Winter lovers were thrilled when the weather models a couple of weeks ago were predicting the development of an exceptionally high-amplitude flow (tall ridges and deep troughs) across the Pacific Basin. In association with this pattern over the Pacific, some of the forecasts advertised a prolonged January spell of arctic air for much of the Lower 48. But, to the delight of winter haters, it didn’t quite work out that way. And maybe, in the end, everyone got some of what they wanted.

There’s no doubt that a fragment of frigid arctic air spilled into northern parts of the country late last week. Temperatures were 10-20°F below average across the Northern Plains for a few days. The blue shading in the picture below roughly outlines the initial extent of the arctic air’s penetration into the U.S. Shortly thereafter, a modified piece of the cold shot worked its way to the Northeast and even brought a few inches of snow to the I-95 corridor over the weekend.

Temperature anomalies (°F) averaged Wed-Fri last week. (Image courtesy of NOAA/ESRL)
It is also readily apparent that in addition to the recent cold, the snow pack has also grown markedly during the last 10 days. The images below highlight the increase in snow cover from Jan. 11 (left) to Jan. 22 (right) that accompanied the arctic air’s visit.

Snow depths on Jan 11 (left) and Jan 22 (right). (Image courtesy of NOAA/NOHRSC)
However, as harsh as it was, this brush with fierce winter weather didn’t last long. Just as the models anticipated, the cold setup was accompanied by development of a spectacularly large ridge near the dateline (the warm bubble aloft outlined in yellow in the left image below).

High-altitude temperatures and winds from one week ago (left) and 2 days ago (right). (Image courtesy of The Pennsylvania State University)
As our medium-range outlook at CWG discussed last time, this ridge moved quickly westward and weakened, and redirected the southward flow of polar air over the ocean instead of into the U.S. In association with this evolution, westerly winds aloft recently began to spread across the country, bringing with them milder maritime air to much of the States.

By late last weekend, temperatures had already surged back above normal along the Canadian border and to near 60°F in the Central Plains. The numbers in the map below are yesterday’s temperature anomalies, with red (nearly everywhere) representing above-average highs and blue likewise below.

Temperature departures from average (1981-2010) on Monday afternoon. (Image courtesy of The Pennsylvania State University)
Amid this unseasonable warmth, a round of big thunderstorms rolled across Kansas City Sunday night, while severe weather with deadly tornadoes raked the mid-Mississippi Valley and Deep South.

As we move forward, generally mild conditions will likely continue into early February at least. There will be occasional interruptions of below-average temperatures, such as the chilly air mass poised to visit the East for a day or two beginning late in the weekend. Highs in the 20s in New England, and in the 30s and 40s in the mid-Atlantic are possible on Sunday. But the data suggest that relatively warm weather - by midwinter standards - will be the rule rather than the exception.

The temperature anomaly forecast for Feb. 1 (shown below, from the GFS weather model, where reds are warm and blues are cold) suggests the Lower 48 will continue to observe the well-worn theme we’ve seen so often this winter. And that is, on average, unseasonably mild pretty much everywhere.

Temperature anomalies predicted by the middle of next week from the GFS ensemble mean. (Image courtesy of The Pennsylvania State University)
One feature worth monitoring over the next couple of days - that may produce big thunderstorms across the South once again - is an upper-level low currently over New Mexico (shown below by the blue L). It will propagate slowly eastward and may produce severe weather when it moves over the Gulf Coast states late in the week.

Current upper-level winds and pressures. (Image courtesy of NOAA)
As we saw over the last few days, with dozens of tornadoes across the South, severe-weather makers in this part of the country are not particularly unusual in late January. Though this system may not pose the kind of threat as the previous one, it nonetheless reminds us that spring is not far away. Indeed, parts of the country may feel like it’s already here.

By  |  02:10 PM ET, 01/24/2012

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