Bitter arctic air will plunge into the western U.S. and Plains this upcoming week. A bubble of warm air (ridge of high pressure) along the Southeast coast will battle with that cold air to determine our weather during the first 12 days of December.
Given these competing air masses, the first 10 days of December temperatures are likely to average out near normal in our area (highs in the 40s and lows in the 30s). However, those averages are likely to be the end result of a roller coaster ride. We favor near average temperatures to start the month with warmer than normal temperatures taking control December 5 to December 7-8. After that, temperatures may cool down to normal to below normal levels.
How “warm” the above normal air is and how cold the air is after the warm spell ends is very much up in the air. This period has much more uncertainty concerning the pattern than was present last week. There is lot of spread in the temperature forecasts between the various model runs starting during our possible warm period and extending through December 12.
Precipitation will probably average above normal. The models are now hinting at a wave developing off North Carolina that could either skirt us to the south or bring us a little cold rain Monday or Monday night. Other chances are likely to follow but are still quite nebulous.
For those liking snow, models only suggest modest potential. Of the ten “analog” patterns similar to the one simulated around December 10 by the GFS superensemble mean, only one shows snow (the pattern from 1962: a multi-snowstorm December). However, the 06Z GFS model and a few ensemble members are showing a potential storm approaching the coast towards the end of the period (December 10-12) once the arctic front has shifted to our south. The problem with patterns like this one is that a strong surface low might still track to our north or on a westerly enough track to pull in warm air from the Atlantic. There is too much uncertainty in such a long range projection to have a lot of confidence in any scenario.
So why is there so much uncertainty?
The pattern is one that is likely to bring really cold temperatures southward into the United States. The 5-day mean pattern (Dec. 4 – Dec. 9) of the upper level pattern simulated by the 50 members of the European ensemble forecast system is shown below.
Note how the lines on the map bulge northward (the red area) and then the lines dive southward towards the trough of blue shaded area on the figure. The arrow on the figure depicts how that weather systems would be steered from Canada into the western U.S.
That huge ridge over Alaska usually results in very cold high pressure system building over the Yukon which then plunges southward into the western United States with the coldest temperature relative to normal usually ending up in the northern Plains. However, having so much cold air in Canada and the U.S. usually means some of it eventually still gets into our region albeit somewhat modified.
Complicating the picture for us is our flow aloft will be from the southwest. The mean flow during those 5-days suggests that any low pressure system that tracks eastward from the southern Rocky mountains would track towards the Great Lakes bringing us our moderating temperatures. However, behind that front lots of cold air would be available to provide cooling once the front manages to work south of the area. How long that cold air holds would be dependent on whether the next low pressure system tracked to our north or south probably sometime towards next weekend.
So what’s the big deal about the cold in the Northern Plains and why does it matter to me?
The European ensemble mean (below) is predicting temperature differences from normal (colder) across Montana of more than 20 degrees F around December 6 (about 5,000 feet up). Daytime temperatures across Montana and portions of the Dakotas during a day or two are likely to remain locked below zero. As the cold air arrives there Tuesday into Wednesday, snow and blowing snow is also likely across Montana and the Dakotas. While the northern Plains shiver, the southwesterly upper flow is expected to sweep warmer than normal air into our region.
Our big problem is low level cold air is denser than warm air so it will ooze eastward with time even as it modifies. However, the upper flow may try to stay southwesterly setting up a situation where warm air overruns the cold air at the surface. That can be a favorable setup for precipitation, but it’s too early to discuss in what form.
So what are you sure of?
Not much except that it will be cold in the west and brutally cold in the Northern Plains. Locally, we should see a short-lived warmer than normal period sometime in the December 5-8 period. During that stint, minimum temperatures will probably show bigger departures from normal than then high temperatures during the day. Then a colder period is a reasonable bet to arrive once the arctic front gets through the area but how soon it arrives and how long the cold air lasts to me still is up in the air. That is the reason for opting for temperatures during the first twelve days of December to average near normal.