The air masses over the eastern two-thirds of the United States are at war over the next two days with the potential for strong to possibly even severe thunderstorms for much of the northern Mid Atlantic tomorrow night and Thursday morning.
Today, the old Arctic air mass is still putting up a right with high pressure anchored over Maine producing a north to northeasterly wind over Connecticut and eastern New York keeping temperatures in the 30's and 40's when they were supposed to be in the 50's. As the low pressure system over central Oklahoma lifts towards the eastern Great Lakes tomorrow morning, the cold front currently to the south of the Philadelphia metropolitan area is expected to race to the northeast. This will allow very warm air to invade the region leading to temperatures rising into the mid 50's to lower 60's through the region. A few locations may push into the mid 60's over the Delaware River Valley if some sun breaks out.
The warmer the region gets tomorrow afternoon, the greater the instability will be in place ahead of the cold front. Most locations will remain dry through tomorrow evening, however I can't rule out an isolated shower or thunderstorm tomorrow afternoon between 1 PM and 4 PM. These thunderstorms would be capable of heavy downpours, occasional lightning, and some lightning.
The preview to Spring will come to a crashing end late tomorrow night through Thursday morning as a powerful cold front crosses the region. The cold front will produce a wide swath of very heavy rainfall with a line of thunderstorms as the leading edge. This line is expected to march through the Philadelphia and New York City metropolitan areas between 11 PM tomorrow night over western locations to 10 AM Wednesday Morning. The potential for severe thunderstorms will be between 11 PM tomorrow night through 3 AM Wednesday with strong wind gusts over 40 mph, frequent lightning, and very heavy downpours. This line will be followed by steady moderate to heavy rainfall leading to localized flash flooding, poor visibility below a mile, and temperatures falling through the day. High temperatures on Thursday will be around 1 AM followed by temperatures steadily falling through the 40's and eventually the 30's by the evening hours. Rainfall amounts of one to three inches can be expected.
Thereafter, Polar air will invade for Friday and the weekend with disturbances bringing the threat for scattered snow showers and temperatures near normal with lows in the mid 10's to lower 20's and highs in the upper 20's to mid 30's.