A southwesterly flow of unseasonably mild air moving into the Middle and North Atlantic states will reach its peak tomorrow. Friday may start mild in the I-95 corridor, but a cold front will move through, and the first of a series of downward temperature steps will be next. Here is the morning video:
The first cold front will stall in the Carolinas. A low pressure area will form along the front and move northeastward. For most of the I-95 corridor, it may be cold enough for snow and sleet at the start. However, it appears the low pressure area will track inland, pulling in mild air to make most of the precipitation fall as rain.
Once the Monday low pressure areas goes by, much colder air from Plains and central Canada will be poised to come in. The warm-cold boundary will move farther south than the first time, meaning there will be a larger areas ion which it turns cold. Then, another low pressure area will form and track northeast along that frontal boundary. The operational Euro model brings snow to the Middle Atlantic states next Thursday, and Henry talks about that in his tweets and videos. However, the GFS ensemble mean (the GFS is run repeatedly, each time with slightly different starting conditions, then averaged) .
This table shows the ensemble means for the next two weeks at Philadelphia: It suggests that whereas it does turn cold, any snowfall looks quite limited. You can see these items by looking at the temperatures (in this case in degrees Celsius) in the column headed by 2 M TMP. That's the predicted temperature at an elevation of 6 1/2 feet above the ground. The precipitation forecast is the third column from the right. The precipitation value forecast is given in hundredths of an inch. Multiply by 10 to get an approximate snow accumulation. While the ensemble mean can be helpful in forecasting, it smoothes out individual extreme solutions. This works well much of the time, but obviously if the extreme winds up happening, the ensemble mean will have been clueless.
GFS ensemble mean forecast for Philadelphia based on Monday night model run:
Cooling aloft and heating moist air closer to the ground should trigger strong thunderstorms from eastern New York and much of New England southwest through parts of the Middle Atlantic states.
The front will move into a region with high humidity as it approaches the I95 corridor tomorrow. This is the basis for SPC's forecast of thunderstorms approaching severe limits tomorrow.
Tropical Storm Colin is caught in the southern stream while the northern stream is helping to send unseasonably cool air out of central Canada.
Then, as the cold front arrives, there may be violent thunderstorms. This map shows the early morning SPC assessment of the severe weather risk on Sunday:
Farther east on Sunday, rain is likely to be more extensive, and there is a severe thunderstorm threat from the Middle Atlantic region on south.
With the second front, shower activity may be spotty at first as the system comes through Chicago on Saturday but could be wetter and more stormy than the first front by time it reaches the I-95 corridor Sunday.