The days of unseasonably mild air in the eastern third of the nation are numbered. The transition away from the current regime, which has recently featured mild weather over much of the eastern half of North America, towards one that will deliver repeated chilly spells over the Ohio Valley will likely become obvious in the last couple of days of the month.
Before then, however, exceptionally mild air will once again surge from the Plains to the East Coast late this week and bring a very warm start to the Thanksgiving weekend for many locations east of the Rockies.
In recent weeks, the circulation over the Northern Hemisphere has brought excessively mild air in bursts across the eastern half of the country and very cold conditions at times in the West. The net effect of these events is well illustrated by the November 2011 temperature anomaly map shown to the right. Areas shaded in yellow and red have been relatively warm, and regions in blue have been likewise cold.
On multiple occasions, temperature anomalies greater than 15°F above climatology have spread from the mid-continent to the Atlantic coast. Kansas City, Nashville, and Richmond have enjoyed multiple rounds of 70-degree weather this month, while even those in northern New England used to seeing snowflakes this time of year have seen their thermometers uncharacteristically top out into the 60s.
On the cold side of the pattern, deep snow covers sizeable chunks of real estate west of the Continental Divide.
In yet another iteration of the dominant November theme, the weather for the last half of this week will be very warm for the Plains. Places near Kansas City should approach 70°F once again.
As the warm bubble spreads east to the Coast by the weekend, expect temperatures well-above average from the Carolinas to D.C. to Maine on Saturday. But as the holiday weekend comes to a close, changes in the global-scale dynamics will start to produce the kind of weather maps we haven’t seen consistently in about a month.
Many of the global models are pointing toward a pattern that will feature cooler weather in the East. Instead of the southwesterly flow aloft we’ve seen in recent times (as shown by the black arrow on the left), the new regime (on the right) will be accompanied by a northwesterly jet stream over North America.
The temperature anomalies that typically accompany the pattern on the right nearly oppose those observed in November. And as we move from the last few days of November into the very beginning of December, the coldest weather relative to climatology will likely settle over the Ohio Valley and Southeast.
At this point, it’s unclear how far into December the northwesterly flow will stick around. Weekly changes in the tropical weather around the globe will have a say in that matter, as they frequently do. Somewhat related to the background La Nina condition in the Pacific currently underway - which can provide a basic setting that favors a ridge in the Southeast like we’ve seen recently- these variations are highly capable of temporarily rearranging the jet stream over North America. Just as they are likely partly responsible for implementing the upcoming pattern, they will probably play a role in undoing it.
And though a northwesterly flow pattern can deliver fierce chill in mid-winter to southern latitudes, the models suggest that this installation will not tap Arctic cold.
Forecasts of high-altitude temperatures for the next two weeks offer little, if any, evidence that a large warm bubble of air will soon move into northern territories and dislodge polar air from those regions. It should instead remain north of the Arctic Circle in our sector of the globe for a while at least.