Snow spread from Illinois and Wisconsin all the way to Massachusetts between yesterday an this morning. Meanwhile. Places from Washington, D.C., to near New York City stayed dry. It is pretty rare to see a snowstorm start in Boston without first affecting cities to the southwest. However, in this case, the moisture that will bring snow to the areas that missed it so far won't arrive from southwest until later this afternoon and this evening. Even then, we don't see a uniform shield of precipitation, so there will be areas that receive more or less than the forecast says. This is a draft version of our snowfall prediction map from early morning:
However, plan on slow travel in the I95 corridor from eastern Virginia on northeastward tonight. Accompanying the snow will be a frigid air mass straight from eastern Canada. This video has more.
Earlier this morning, the pressure pattern showed interesting features that could explain what was happening around the Northeast.
This enhanced infrared satellite picture shows the cold front in the Northeast and the moisture wrapping around Odille on the southwest part of the map.
Across the Central and Northern states, thunderstorms are less common at this time of year than in late spring and summer. One area that has had more thunderstorms than usual recently is across the Desert Southwest.
Last week, I mentioned that longer range computer models were suggesting a major warmup by next weekend. More recent runs have backed off on the that idea. However, there is extreme uncertainty beyond the next 7-10 days. This can be seen by looking at the following map.
n the forecast office, we often track cold fronts with pressure maps like these. The examples are from 4AM and 10 AM today. You can see that the northern part of the front is moving more quickly than the southern end. The arrival of the front signals the start of the cooling trend that is spreading east.
The upper air flow over the East is from the west-southwest now, and some of the moisture associated with the cold front can be traced back to tropical storm activity off the Mexican west coast last week. The same moist air mass set the stage for recent flash floods in Arizona. By early next week, the upper-air flow will be coming to the Northeast region from well up in western Canada, as seen on this forecast map forecast map for next Tuesday:
As chilly air pours into the northern Plains and backs into the Rockies, snow can break out at higher elevations. Gillette, in northeastern Wyoming at an elevation of just over 4,000 feet, could have a close shave with snow on Thursday. This map shows the setup.