December Thaw: Long-Lasting Break From Arctic Chill For Most Of U.S. | The …

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After several rounds of arctic chill this fall, most of the country will enjoy a long spell of warmer-than-average weather over the next week or two — just as meteorological winter has begun.

(MORE: November 2014 Shattered Cold, Snow Records)


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A jet stream pattern that has encouraged cold air masses from near the Arctic Circle to rush southward into the U.S. will change over the next several days, allowing much milder air from the Pacific Ocean and Gulf of Mexico to become the dominant influences on temperatures nationwide.

Computer forecast models are in strong agreement that this will allow unseasonably warm weather to take over a large part of the Lower 48, not to mention a large swath of western and central Canada, later this week and on into next week, and perhaps even beyond that.

(MORE: National 10-Day Forecast Maps)

This means temperatures will stay well above freezing, day and night, across much of the Deep South this week and probably next week as well.

Meanwhile, subzero lows and single-digit highs in the north-central states should give way to high temperatures at or above freezing at times, allowing areas of remaining snowpack to melt. As of Dec. 2, about 22.5 percent of the contiguous U.S. was covered with snow — a sharp drop from 50.4 percent on Nov. 16.

In areas where snow cover melts completely, temperatures could surge even higher, as snow cover has a refrigerating effect. Snowpack prevents sunlight from warming the ground underneath and reflects more sunlight directly back to outer space, making it more difficult to get the air near the ground to warm up.

Based on current forecasts, the warming trend will be more obvious at night than during the day; by this weekend, high temperatures will be at or slightly above average for most of the country, but low temperatures will be 10 to 20 degrees above average across a huge swath of the West, the Plains, and the Southeast.

Perspective: Cold Spells Dominate 2014

While 2014 has certainly had its warm days, they have tended to be the exception rather than the rule in much of the central and eastern U.S.

In fact, temperatures have been below average for the year to date across most of the country east of the Rockies. Temperatures have been farthest below average in a swath encompassing the entire Mississippi River Valley and much of the Great Lakes. In these areas, temperatures were 2 to 4 degrees below the historical average for the first 11 months of the year, according to data from the High Plains Regional Climate Center.

It’s been the eighth-coldest year to date on record in Illinois, based on NOAA climate analysis for January through October. Missouri, Iowa, Indiana and Arkansas are also having one of their 12 coldest years of the past 120. (State-by-state data through November will be released in mid-December.)

For cities such as Chicago and Minneapolis, even a December with just average temperatures would result in 2014 being the coolest calendar year since 1996.

The pattern is well represented in the number of record highs and lows so far this year. Through Nov. 30, NOAA’s National Climatic Data Center says there have been 33,013 record high temperatures in the Lower 48 this year — but also 47,402 record lows. For just the month of November, there were more than four times as many record lows (8,977) as record highs (2,022).

One part of the country, however, has been consistently warm. California, Nevada and Arizona are experiencing their warmest year to date on record.

And the warm weather isn’t going away there, either — even though sometimes big pattern changes take weather away from one region and move it somewhere else, the Southwest will hold on to its warmth this time even as the rest of the country shakes off the frosty conditions.

The shift to a warmer regime just puts the central and eastern U.S. where most of the rest of the planet has been all year; 2014 is on track to be the warmest year on record worldwide.

MORE ON WEATHER.COM: Urban Heat Islands


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