As winds aloft become southwesterly, much milder air will move from the Gulf states toward the Northeast. At the same time, a northerly flow of frigid Arctic air will plunge southward into the Rockies and northern Plains. When cold air masses move south into the Plains, the Rockies often act as a barrier to any significant advance westward.
However, when the flow aloft is northerly on both sides of the Rockies and all the way to the West Coast, cold air can make it into normally protected areas from Washington to California. In San Francisco, temperatures could drop well down through the 30s tomorrow night. In 1972, San Francisco had eight record lows, including 24 on Dec. 9. That reading stands as the lowest ever for the airport location (SFO). It has never been below 30 there in February.
At Las Vegas, Nev., the temperatures will tumble into the 20s tomorrow night and again Thursday night. Record lows there in December are all in the teens and 20s. The most recent record low there in December was in 1990. In that year, six records were set, the lowest being 11 degrees on Dec. 23. There have been no new record lows set there in this century. Before making any inferences about that, I observe there has been only one record high in December set in this century (70 on Dec. 24 in 2005).
In the Chicago to Detroit area, it will become milder through tomorrow night (into Thursday in Detroit), then colder air will move in. Farther east, from Washington, D.C., to Boston, temperatures will climb to the 50s (north) to 60s (south), then not begin to turn chilly until sometime Friday. The video has more:
Looking farther ahead, one publication that ventures into climate prediction is The Browning Newsletter (www.BrowningNewsletter.com). The author, Evelyn Browning Garriss, suggests that recent volcanic eruptions contributed to the strength of last week's cold across much of the U.S. She writes that "Historically, winters with similar conditions (i.e; similar to the situation now) in the Arctic and Atlantic have been cold, especially in the Midwest and Northeast."
It is too early to be confident about any forecast for Christmas Day (or even the week before). However, the GFS model does go out 16 days, and it has a cold look for the Northeast exactly one week before Christmas.
A band of heavy rain and locally violent thunderstorms moved from the New York City area at 4 a.m. to just past Boston (distance: 188 miles) by 9:30 this morning. Note the stunning contrast between where it is pouring (dark red) and where it has dried out.
On the map, showers and thunderstorms were located along and ahead of the gray line that cuts through Pennsylvania and along/ahead of the blue line. Both should be off the East coast by Thursday. Drier air from the Upper Midwest should filter into the Northeast later in the week.
The large storm that drenched the Northeast during the weekend has drifted out to sea and somewhat drier air is coming in to replace it. However, another upper air trough extending from Wisconsin to Louisiana is supporting several pockets of showers and thunderstorms.
From northern West Virginia across most of Pennsylvania and western and central New York, there could be several inches of rain with flooded streets and streams.
...will move east to bring rain overnight from parts of Virginia to Southern New England. In the southern part of this area there can be some violent thunderstorms late today and tonight. The rain will depart tomorrow, but a large storm is likely to affect the Middle and North Atlantic states this weekend.
This activity map produced by the National Weather Service Storm Prediction Center overlays existing areas of thunderstorms on the map showing various risk levels.