December Thaw Update: Arctic Chill Long Gone, but Even Warmer Weather Ahead for Much of Western, Central U.S.

A prolonged spell of unusually warm December weather is expected to cover a sizable swath of the U.S. in the week ahead, and even the occasional interruptions in the mild pattern won't be all that cold.
The warm pattern continues after several rounds of early arctic chill this fall. That subzero cold will virtually vanish from the Lower 48 even as we approach the shortest days of the year.
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December Thaw Setup

 

A jet stream pattern that has encouraged cold air masses from near the Arctic Circle to rush southward into the U.S. has changed, allowing much milder air from the Pacific Ocean and Gulf of Mexico to become the dominant influences on temperatures.
Computer forecast models are in strong agreement that this will allow unseasonably warm weather to take over a large part of the western and central U.S., not to mention a large swath of western and central Canada, for most of the upcoming week.
Record high temperatures are possible early this week in parts of the West, including Portland, Oregon, Sacramento, California and Salt Lake City, Utah.
Late this week record warm low temperatures are possible in Great Falls, Montana, Rapid City, South Dakota and Omaha, Nebraska.
(MORE: National 10-Day Forecast Maps)

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Outlook: Morning Lows This Weekend

Models are in strong agreement on mild weather later this week. Even typically frigid Northern cities may barely dip below freezing on some nights.

Subzero lows and single-digit highs in the north-central states have relented, and high temperatures at or above freezing will continue to gradually expand northward as this week progresses.
This should allow many areas of remaining snowpack to melt. As of Dec. 9, only 18.8 percent of the contiguous U.S. was covered with snow -- the smallest snow cover for this time of year in at least 25 years.  That's a dramatic turnaround from mid-November, when 50.4 percent of the country was blanketed in white -- the largest snow cover on record during November since records began in 1966.
In areas where snow cover melts or has melted completely, temperatures will 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.
That fact will become obvious later this week. Last week, the warming trend was more obvious at night than during the day, as nighttime temperatures warmed up across much of the central U.S. while clouds and fog -- in some cases resulting from warm air blowing over cold ground -- kept daytime temperatures in check.
Now that snow cover has shrunk and the bare ground can absorb incoming sunlight, daytime temperatures will also surge to levels 15 to 25 degrees above mid-December norms, starting over the Intermountain West on Wednesday and slowly shifting eastward into the Plains and Upper Midwest by Friday and the weekend.
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