Tuesday 9 a.m.
The upper-air current arriving in Pennsylvania this week originates over the Gulf of Mexico, so it is no surprise the main weather theme is warmer and more humid than typical for the season. However, computer models suggest that on Sunday, the flow reaching Pennsylvania will have originated in western Canada. Translation: warmer, then chillier. This video has more:
The details of the weather pattern are far less clear-cut than the overall trend. For example, there has been rain, some heavy, in a corridor extending from Pennsylvania to Virginia. Pittsburgh has been near the western edge; Philadelphia just east of the eastern edge. Farther south, violent thunderstorms developed in east-central North Carolina. Here is a map showing where precipitation was at 10 a.m. EDT.
Rain with areas of fog should spread from Virginia to New Jersey Monday or Monday night then spread into New England for Tuesday. From the mountains of northeastern Pennsylvania into the interior of New England this could at least start as snow or ice. The GFS for 1 AM New Year's Day looks interesting. See the map below. Whether or not this storm develops and where it will snow or rain cannot be precisely predicted two weeks in advance using these models.
This map is a spaghetti plot showing the upper air currents predicted by members of last night's GFS ensembles. In looking at each line, you see variations, but they all show the idea of major trough centered east of the middle of the country on Christmas Eve.
Any storm in the Northeast could be disruptive for travel, whether it turns out to be rain and fog or snow and ice. If a strong storm develops, the best chance for snow on Wednesday will be over the central or northern Great Lakes region. This map is last night's GFS operational solution for 7 p.m. ET Christmas Eve.
The map below shows the low pressure area and cold front now moving into the East. Looking at the temperatures, truly cold air is well behind the cold front. This is in line with the idea that the front itself is the leading edge of the change to colder conditions.
The following map shows why areas to be affected by the cold front will not have a long period of rain. However, along and north of the storm center, the precipitation will last longer.
The satellite picture shows a deck of clouds all the way along I-80 from Nebraska to New Jersey with few holes along the way. However, nobody in the cloud zone is having any substantial precipitation. There was some spotty freezing drizzle this morning.