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.

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.