There is no doubt that heavy rain is on the way as a significant surge of moisture races north ahead of a powerful cold front.
Clouds will continue to increase and lower through the day as a strong southwesterly flow from the surface to mid levels pushes a warm and moist air mass into the region. Temperatures will average above normal through the next 24 hours with temperatures rising through the 40's and into the 50's this evening and overnight. Rain will begin to push into the Philadelphia metropolitan area late tonight around mid night and continue to move east through the New York City metropolitan area early tomorrow morning.
The rain will be very heavy at times, producing flash flooding and rapidly reduced visibility through the morning rush hour. Combined with the heavy rain threat will be embedded thunderstorms within the rain shield that will lead to strong wind gusts possibly pushing over 40 mph at times. A strong 850 MB jet streak around 3o KT to 45 KT will support isolated downbursts within these thunderstorms. The rain will be heaviest from 6 AM to 2 PM with rainfall amounts ranging from one to two inches of rain. Flooding of streams, small rivers, and urban locations are likely from this cold front passage. The cold front will exit by tomorrow afternoon between 2 PM and 5 PM with temperatures rapidly falling through the 50's, 40's and 30's with morning lows on Tuesday morning in the 20's and 30's.
Once the cold front passes, a new pattern will become established with a return to much colder conditions mirroring the temperature profile of early January. Temperatures on Tuesday will still average near normal with the immediate coast approaching 40 degrees. However, from Wednesday through Friday temperatures will average below normal with highs in the 20's and 30's throughout the region. A series of weak cold fronts will keep the threat of scattered snow showers in the forecast through the week with some minor accumulations in isolated locations.
The main story is the developments at the end of the week. Model guidance is strong converging on a solution that will bring a potentially significant snowfall to much of the northern Mid Atlantic. This storm appears to develop in two parts. The first is an isentropic lifting driven snowfall with a thermal gradient at 850 MB over 12°C and a surface gradient forecasted to range from 20° F on Friday afternoon to over 30° F on Friday night from the coastal waters to the interior. This strong thermal gradient will be set up ahead of the actual coastal low development on early Saturday morning, and strongly argues for an impressive isentropic lifting environment. Model guidance is attempting to forecast the potential snowfall for this scenario, but likely is under-forecasting the potential influence of frontogenesis and mesoscale lifting parameters like banding of precipitation. This gradient also strongly argues for a rapidly unstable atmosphere and thunder snow may have to be considered on Friday evening given the atmospheric set up.
The coastal low takes over as the dominant feature of this storm on Saturday morning with heavy snow focused along the immediate coast. The exact track of this storm is still uncertain and will be an important detail, however a significant majority of the guidance strongly suggest that the atmosphere will support an all frozen if not all snow event for this storm. A significant snowfall from Friday afternoon through the day on Saturday is growing and looks likely.
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