The humid air mass that has been in place over the Philadelphia and New York City metropolitan area over the past several days is on the way out. That’s the good news. The bad news is that in order to get the humid, tropical air mass out of the region, showers will becoming an increasing threat the next three days.
The upper low that is driving this entire pattern is creeping ever so slowly to the east. The showers and elevated thunderstorms associated with this upper low are now setting up over the northern Mid Atlantic and will move over the region from south to north today on through Friday afternoon as disturbances rotate around the upper low.
The showers and areas of rainfall will remain scattered in nature, but will be far more widespread than the past few days. These areas of showers will be capable of very heavy downpours with already portions of northwestern New Jersey reporting flash flooding due to rainfall amounts of one to two inches. The cold front associated with this upper low will attempt to move through tomorrow afternoon with wide spreads showers and embedded thunderstorms. However, the cold front will collapse before exiting the region and a stronger cold front associated with another upper level disturbance will have to move through on Friday to really clear out the humid air mass in place. Temperatures through the period will continue to range from the mid to upper 60’s for lows and 70’s for highs.
High pressure will briefly build into the region on Friday night into Saturday with scattered cloud cover, much lower humidity, and temperatures falling into the 40’s for lows on Saturday morning and rebounding into the 50’s to lower 60’s for highs on Saturday afternoon. A developing upper low off the Mid Atlantic coast will lead to a weak coastal low pressure system to develop over the New Jersey coastal waters that will track towards New England on Saturday night on through Monday with a threat for widely scattered showers, variable cloud cover, and cool weather conditions. Unlike the previous upper low that drew an air mass from the Tropical Atlantic and Gulf of Mexico, this air mass will be from the northern Atlantic leading to temperatures averaging below normal in the 40’s for lows and mid 50’s to lower 60’s for highs on Sunday and Monday.
Temperatures will rebound by Tuesday as high pressure builds in from the west with clearing skies and temperatures near normal in the 50’s for lows and upper 60’s to lower 70’s for highs.