December Outlook Released: Colder Than Average in the Northwest, Northern Plains; Warmer Than Typical in the Southwest, Southeast

Jon Erdman
Published: November 30,2016
December 2016 may be colder than average in parts of the northern half of the United States, according to an updated outlook released by The Weather Company, an IBM Business.
The coldest air will likely be focused in the West, with occasional bursts of very cold air across the major energy demand centers of the northern and eastern U.S.,  Dr. Todd Crawford, chief meteorologist with The Weather Company, said in a news release.

Pictured here is the December 2016 temperature outlook. Areas not shaded have an equal chance of above- or below-average temperatures.
(The Weather Company, an IBM Business)

One instigator may be La Niña, the periodic cooling of eastern Pacific equatorial water which can exert some influence on the atmospheric pattern, particularly in the colder months.
(MORE: La Niña is Expected to Last Through Winter | Winter Storm Central)
Crawford said blocking high pressure aloft near the Aleutian Islands, forced by La Niña, is setting in earlier than anticipated.
"The upcoming pattern should drive the coldest air into western Canada, but it will likely 'smear' eastward often, resulting in shots of very cold weather across the northern into the eastern U.S.," he said.

Potential pattern over the Pacific Ocean and North America later in December 2016 that may force persistent, cold air into the western and parts of northern U.S.

Winter seasonal forecaster Dr. Judah Cohen of Atmospheric and Environmental Research agrees that a big change is on the way in December.
The  blocking high pressure near the Aleutians will allow cold air to pool in Alaska and western Canada for the first half of December, Cohen said in his most recent outlook.
(MAPS: 10-day National High/Low Forecasts)
"More expansive cold air should also favor a more rapid expansion of snow cover across the continent (North America)," Cohen added.

Average December Highs

In contrast, December is expected to be slightly warmer than average overall in the Southeast, including most of Florida, as well as parts of the Desert Southwest near the Mexican border.
This will be a sharp contrast to one of the nation's warmest Novembers.

Departures from average temperature (degrees Fahrenheit) from Nov. 1-28, 2016.

But it's possible, as of this writing, that dozens of U.S. cities could have one of their top three warmest Novembers on record.
MORE: Winter Storm Argos