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.
There's a west to northwesterly flow of basically dry air from south-central Canada to the northern part of the Middle Atlantic states. The flow changes orientation farther south. A light northerly flow is helping slightly less humid air to seep southward.
In parts of the Northeast, a sky looking as blue and clear as the one in the picture will be quite a departure from the look of the rainy skies that were much more common earlier in the summer.
The steering winds aloft are changing, and the next phase looks like it will feature a northwesterly flow of cooler air this weekend and much of next week.
It's going to be hot hazy and humid in the much of the Northeast today through Thursday. The recalescent, sudorific, canicular, pyrogenic frying heat gains our attention.
A line of persistent showers and thunderstorms stretched from Long Island Sound to Cape Cod. Some areas in this zone had more than 3 inches of rain this morning, and there can be highway flooding in localized heavy showers this afternoon and evening. This radar shows the rain zone just after 10 a.m. ET.
This map shows the predicted upper-air flow for tomorrow night. Our timing estimates suggest showers and some thunderstorms could affect the I-95 corridor from Portland and Boston to New York City (and perhaps Philadelphia) on Sunday.