The coldest part of this week's cold snap is now moving into the Midwest and will be over the Northeast tomorrow night and Friday. This video has more, including a look at what could be a major precipitation producer at the beginning of next week.
This map shows the pervasiveness of the northwesterly flow of bitterly cold air behind a cold front that is contributing to snowfall from Washington, D.C., to New York City this morning. Clearing will follow the frontal passage.
In previous winters, I have talked about the upper stratospheric cold signal, where the normal vortex over the North Pole weakens or actually reverses to form a high pressure. That reversal has often been a sign of impending blocking aloft. In blocking patterns, storms are forced south of their usual paths and it often turns colder. This winter, we have seen that such a setup is not required to make it get cold or snowy. The vortex has remained over the pole all winter, and is rooted in place now, as shown on this map from the University of Wyoming.
The upper-air flow forecast for this evening shows the trough that helps to support rainfall ahead of the cold front.
After reaching the 80s today from NYC to Boston, it might not be that warm again through much of next week.
A noticeable push of cooler air will spread southward from Ontario and Quebec into the eastern Great Lakes and New England between tomorrow and Saturday.
A cold front from eastern Canada will slide southward along the East coast between late Friday and the end of the weekend. For the area from Philadelphia to Boston, where temperatures will reach the summery 80s each day through Friday, it will mean a noticeable change to cooler weather.
Average high temperatures in Chicago and New York City are in the mid-70s now, but for the next several days, temperatures will run 8-15 degrees above those long-term averages. Supporting this warmth is a flow aloft that originates over the Southwest:
Rain was common in the Northeast this morning, though Boston was still waiting as of 9 a.m. Their summer dry spell has continued.