A brief spell of warmer temperatures are not uncommon across parts of the U.S. around the third week of January. The common term for this is a “January thaw“. What we have coming up later this week across much of Canada and the eastern two thirds of the U.S. is a January thaw to the extreme, as temperatures will be well above seasonal averages.
Alternating Weather Patterns
Since the beginning of December, we have seen alternating weather patterns of cold and mild temperatures across most of the U.S. In an earlier article, we indicated that after a mild spell around the Christmas holiday, we would see a return to much colder temperatures across much of the nation in early January as the polar jet stream dipped southward into the U.S. Indeed, some very cold air from the northern latitudes descended southward.
The cold air intrusion began in the northwest section of the U.S., and gradually spread to the south and east. By the end of the first week of January, much of the nation was experiencing much below average temperatures. Winter storms accompanied the cold for many areas and by January 8th, all of the Lower 48, except for Florida, had at least some snow on the ground.
Setup For Widespread Warming
The polar jet stream that moved southward into the U.S. in early January has retreated to the upper reaches of Canada, so the most frigid air is now confined to the highest latitudes. In fact, most of Canada will be experiencing well above average temperatures for this time of year.
A southern branch of the polar jet stream is digging southward into the western U.S., but then it is forecast to move northward back into Canada. This brings in relatively cold, but not frigid, air from the North Pacific (cold enough to allow snow to fall in the Intermountain West) but keeps temperatures above seasonal averages, elsewhere.
The southern U.S. experienced a head start on this warmer weather pattern as the subtropical jet stream bulged extremely far north, for January. This allowed air from latitudes much father south, to push northward.
Extensive January Warmth
What is unusual about this January thaw is that it is so widespread. Temperatures will range from 10 to 25 degrees above average across most of the eastern two thirds of the nation from Thursday through the coming weekend. In Canada, some of these temperature anomalies are an eye popping 30 to 40 degrees above late January averages and the coverage areas of these anomalies is quite extensive.
What we will be experiencing is extremely rare. Outside of higher elevations, the vast majority of the U.S. will see high temperatures above freezing. The snow cover that we had in early January has been gradually eroding, and this process will be in overdrive for awhile.
If you live in northern sections of the nation, please don’t attempt to walk or skate on ponds or other ice covered surfaces because the ice can be very thin.
Current forecast guidance is indicating that this massive thaw could lose its grip by the end of the month. We will, however, continue to monitor this situation as future data sets are initialized throughout the next several days.