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.
Before we assess the ambiguity of the medium-range forecast, and what it means for the States, let’s start with an overview of this week’s cold front (outlined in blue in the picture to the right).
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.
This cold front will reach Northeast and mid-Atlantic by Friday. When it does, however, the strength of the chill behind it will have weakened somewhat. Though significant lake-effect snows are again possible in the northwest winds over, for example, western Michigan, highs during the weekend will be in the 30s from Boston to D.C. (near normal) , and in the 40s southward from there through the Carolinas (slightly below normal).
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.)
It is possible that the recent (dynamically-driven) warming of the stratosphere (noted by some forecasters last week, including the CWG’s Wes Junker), and the downward-development of blocking into the troposphere that can accompany it, are helping to trigger the amplitude (waviness of the jet stream) increase over the Northern Hemisphere. The polar vortex (as defined by ring of winds around the Pole, pictured to the right) is definitely being knocked around a bit at many altitudes, and is losing circular symmetry in the process. The chart to the right illustrates today’s not-so-annular upper level westerlies (outlined by the black line).
During the next several days, as large-scale wave activity piles up near the dateline, a very tall ridge is expected to develop over the Central Pacific. The picture to the right is a 7-day forecast of the buckling of the jet stream (orange line), and the development of a large high pressure system (red H) at upper levels near the Bering Strait.
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.
However, ensemble means from the global models strongly suggest that the high-altitude high pressure system near the Bering Strait will retrograde (shift westward) late next week and gradually redirect the flow of Arctic air farther west over the Pacific Ocean. This will likely allow much milder west winds of maritime heritage to occasionally overtake the much of the Lower 48 and, as posted last week, interrupt Arctic air visits to only a few days.