Despite the recent run of cold weather over much of the U.S. and Canada, a change in the weather patterns could bring warmer temperatures in the second half of December.

The U.S. and Canada have been under the grip of an early burst of winter weather over the last few weeks. Since the week of Thanksgiving, frequent bursts of arctic air have pushed through much of North America. But the mechanism that caused this cold intrusion has now shifted, with a warmer overall weather pattern setting in and possibly lasting for the rest of December.

Recent spate of cold weather

Cold air can come from the Arctic in several ways. The most consistent way to do it is for an upper-level ridge to form over Alaska. That usually forces a trough to develop near Hudson Bay, and the flow from the ridge to the trough brings down cold air from the Arctic.

Such was the case for most of the last three weeks. An upper-level ridge has been a consistent feature since right before Thanksgiving, causing pushes of cold air across the U.S. repeatedly since then.

This weather pattern resulted in a rather active weather pattern, whereby multiple storm systems have moved through, mainly in a clipper-like fashion to build up an early snowpack for the Northern Plains through much of the Midwest. It has also been a rather cold period with temperatures significantly below normal.

Minneapolis, MN, has not seen a temperature above freezing since Nov. 25, a run of 20 straight days. It’s unusual to see a run of days this cold for this long without interruption.

Warming up as pattern changes

Minneapolis’ cold streak is breaking this week as a low-pressure system moves through Canada, causing gusty winds to warm as they flow off the Rocky Mountains and into the Plains and Midwest. This air is sourced from the Pacific Ocean and not the Arctic, resulting in more widespread warmth to return to the country.

The reason for this shift in the source region for air is due to a change in the upper-level weather patterns.

The ridge that has been a stubborn feature across Alaska in recent weeks has pushed westward into the Bering Sea and northeastern Russia. Instead of forcing a trough over central Canada, the trough is now being forced over western Canada, which is allowing another ridge to develop over the rest of the U.S. This is the feature bringing the warmth to the U.S. while the western trough is likely to keep the cold, arctic air locked up in the western half of Canada until that Bering Sea ridge shifts.

Image: An upper level ridge of high pressure has shifted west of Alaska, allowing another ridge to spread through the U.S. and bring warmer temperatures. Source: TropicalTidbits.com

The weather pattern that is setting in for the second half of December will likely keep low pressure systems and cold air further north in Canada instead of the U.S. However, cold fronts should still swing through the U.S. in the coming weeks, causing bursts of precipitation.

One of these fronts is forecast to move across the country this Wednesday through Friday, December 17 to 19. As temperatures are rather warm ahead of this system, most of the precipitation will be in the form of rain, with thunderstorms possible in the Midwest. Snow will be relegated to far northern areas.

Behind this front, temperatures are indeed forecast to drop quite significantly, tapping into some of the cold in western Canada. But it will only last a day or so before rising again.

Several other systems will move through near the Canada-U.S. border this weekend and next week, but the results will likely be limited precipitation and overall warmer temperatures. The northern tier of states may catch some of the cooler temperatures at times, but in general, this is a warmer pattern with limited cold disruptions.

Warm week of Christmas

Computer models have some unseasonably warm temperatures forecast for the week of Christmas. While forecasts may change in the coming week, temperatures 20-30 degrees above normal are possible for most of the U.S. outside of the Northeast in the few days leading up to the holiday. The forecasts are particularly high across the central and southern Rockies through the Southern Plains and into the Mid-South. If these forecasts hold, some records may be broken. Those with snow on the ground will likely see that area being reduced and the chances for a white Christmas have fallen for much of the Northern Plains and Midwest.

But this warmth will not last forever. The ridge will eventually shift eastward again and cold air will come rolling back through the middle of the country. Current model runs suggest the last couple of days of December or more likely early January for that to come back into the conversation.