Much has been made about the possibility that one or more severe Arctic blasts will hit the United States in the coming weeks (since my post last week). While it is nearly certain that a solid punch of January-like air will briefly visit the central and eastern parts of the Lower 48 late this week, the longer term prospects of a much more intense strain of Arctic chill devouring the country for a prolonged period are much less clear, and, in fact, considerably less likely.
By late tomorrow, a strong north wind will blow a modified Canadian air mass across the Central Plains.
Temperatures late Wednesday and Thursday are, believe it or not, threatening to go below-average (for this time of year) by several degrees from the Dakotas to the Gulf Coast. And actually, the forecast would be notably colder if the airflow trajectories behind the front crossed a sizeable snow pack. But since there is hardly any snow anywhere in the Lower 48, the post-frontal chill in the U.S. this time around will be, for the most part, underwhelming.
A quick cold shot like this would hardly be noteworthy in most Januarys. But, since many places in the Northern Plains are running 15-20°F above normal for the month so far, this front may shock residents in places like Minot, ND. Their January temperature anomaly right now sits at an unbelievable +21.4°F.
But this is not the kind of air mass delivery that has piqued the interest of those hoping for a real winter to show up. What some forecasters are eyeing is a potential confrontation later in the month with an air mass at least 20 degrees colder; one that is born out of an unmistakeable polar pedigree.
The global weather models started hinting at such a confrontation about a week ago. Its origins are ostensibly related to changes in the global-scale dynamics that are currently weakening the jet stream over the Pacific –and to a lesser extent over northern Europe. (Some blame the Arctic Oscillation/North Atlantic Oscillation/Pacific North American pattern etc. for these variations. Yet these statistical constructs are instead pictures of underlying physical behavior, not causes, much like the way a fever measures disease.)
This highly undulating flow will initially drive Arctic air (black aa in image to right) southward into the Gulf of Alaska and northwestern Canada. Without relying on details in a forecast for this far out, the guidance strongly suggests that at least some of the Arctic air will leak into the Plains early next week (see picture below) and eventually spread east-southeastward over the Great Lakes, Ohio Valley, and Northeast a couple of days later. Anomalies of 10°F to 20°F degrees below climatology are possible in these regions with this air mass.