- The December forecast is now colder from the Great Lakes into the Northern Plains.
- Much of the East has a chance to see a somewhat colder-than-average first month of winter.
- The Southwest is the most likely region to have a warmer-than-average winter.
December’s temperature outlook has trended colder from the Great Lakes into the Northern Plains, according to an updated outlook released Wednesday by The Weather Company, an IBM Business, and Atmospheric G2.
An area from Michigan, northern Illinois and northwest Ohio to eastern Montana now has an increased likelihood of seeing below-average temperatures in the month ahead. Keep in mind this is an outlook for the month as a whole, and that periods of milder temperatures will happen at times.
Elsewhere, odds are tilted toward December being colder than average in much of the Northeast to as far south as the Tennessee Valley, northern Georgia and North Carolina.
The Southwest still has the greatest chance to see temperatures that are above average for December in the new update.
Greenland blocking is one of the reasons for the colder forecast in the northern tier, as well as the odds being tilted toward colder-than-average temperatures in much of the East.
The setup involves a weather pattern that forms from blocking high pressure that sometimes develops near Greenland.
When that happens and persists, it typically opens the floodgates for cold air to spread from Canada into the central and eastern states through a southward-plunging jet stream.
It’s also referred to as the negative phase of the North Atlantic Oscillation, or NAO, as the graphic below illustrates.
One fly in the ointment on this forecast is how the large-scale weather pattern might evolve over the Pacific Ocean in December. It’s an uncertain factor that could determine how much cold air is readily available for this so-called negative NAO pattern to tap into and send southward into the U.S.
For now, expect December to kick off a cold start to winter in the nation’s northern tier, and keep those heavier winter coats handy.
The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.