Colder Air to Usher in December Reality Check

December has not felt much like December so far around much of the country, but there are indications that a blast of colder air will affect parts of the Lower 48 this week.

The seemingly relentless warmth has led to record breaking temperatures across much of the country to start the month of December, but there is an end in sight.

(MORE: Record Breaking December Warmth)

At least for parts of the country, it looks like this recent warmth gives way to a winter comeback later this week.

Pattern Change Across Lower 48

The majority of the cold air so far this month has remained to the north, across parts of northern Canada and Alaska. With the northern branch of the jet stream shifted north into Canada, mild air from the Pacific – and occasionally from the tropics – has been able to spread much farther north than usual for the time of year.

Numerous record high temperatures have been observed this month around most of the U.S., but the relative warmth has been felt just as strongly across most of Canada.

This weekend, dozens of official reporting stations broke daily record high temperatures from the Mississippi Valley to the East Coast. Incredibly, some of those record highs have been set during the morning hours.

All good things come to an end – that is, unless you are a fan of winter weather. In that case, consider the upcoming forecast an early present from the winter season.

(MORE: Will El Niño Jeopardize a White Christmas?)

A southward dip in the jet stream associated with Winter Storm Echo has already dropped temperatures back to average or even below average in parts of the West. Temperatures more typical of December will persist there much of this week.

That cooler air will spread into the adjacent Plains states by midweek, sending temperatures to near mid-December averages from North Dakota to Texas by Wednesday.

Finally, by late week and next weekend, a southward dip in the jet stream will move through the Midwest, South and Northeast, dropping temperatures back closer to December averages in those regions.

How Cold Could It Get?

The invasion of colder air this week is not forecast to be particularly severe for mid-December standards, but it will be a bit of a shock compared to the recent warmth.

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This could bring single digit or subzero temperatures to the coldest parts of the Great Basin and Rockies. Low temperatures may drop to near the freezing mark in Phoenix on Wednesday and Thursday morning where there is a freeze watch in effect.

For daytime highs, most of the West and High Plains may stay below freezing for a few days, outside of the West Coast, Southern California and lower elevations of the Desert Southwest.

(Forecast: Boise | Denver | Salt Lake City)

This cold air will likely shift east with time, later impacting the Midwest to Great Lakes and perhaps the Northeast by next weekend.

With eastward extent, the cold air will tend to modify, meaning the coldest of the cold air will likely stay over the western U.S. The result is that temperatures may only fall to near or slightly below average levels across the East next weekend (Dec. 18-19).

(Forecast: Miami | New York City | Washington, D.C.)

Here are a few examples:

Chicago: Highs retreat from recent 50s and low 60s to the 20s and 30s by Thursday and Friday. The average high in the Windy City is in the middle 30s this time of year.

St. Louis: Coolest air begins to arrive Thursday with highs in the 40s through Friday. Average highs this time of year are in the low 40s. Morning lows will drop to near freezing Thursday through Saturday mornings.

Atlanta: Temperatures topping out in the 60s and low 70s through Thursday will give way to 50s Friday-Sunday. Average highs are in the middle 50s during mid-December.

New York City: Highs in the 60s may hang around until Tuesday. This will be followed by 50s through late week, and then 40s this weekend. Average highs are in the middle 40s this time of year in the Big Apple.

As this return of cold air is still several days away, check back regularly to weather.com for the latest information.

MORE ON WEATHER.COM: Cold Weather Tips