The threat for showers will be increasing through this afternoon as the upper low over the Upper Mid West and Ohio Valley starts to slowly move east.
Clearly, the one factor that is not missing for the development of showers is moisture as fog, locally dense, blankets much of the Philadelphia and New York City metropolitan areas this morning. The fog in some locations has reduced visibility to below one mile, while most locations has visibility between 2 to 4 miles. The fog will lift later this morning with overcast to broken cloud cover expected.
The one factor that has been missing to produce rainfall over the region is lifting or upper level disturbances. The northern Mid Atlantic has been sitting in a “dead” zone in terms of lifting where the combination of disturbances off the East coast and over the Ohio Valley has lead to sinking air over the Philadelphia and New York City metropolitan area. This set up has lead to overcast skies, a few isolated showers, fog in the morning, and very humid conditions in the afternoon.
However, now the upper low is starting to finally move east and is expected to track towards New England and become entrained in the Polar jet stream over Canada between now and Thursday. While the exit of the upper low will be welcomed to break this muggy, dreary weather pattern; the result of this upper low lifting to the northeast likely won’t be welcomed. Showers will increase in coverage through the day, initially over much of eastern Pennsylvania this afternoon and then expanding to the east slowly as the upper low starts to move towards western New York. Showers will become more wide spread tonight on through tomorrow and Thursday as the cold front associated with this upper low slowly moves through the region. Initially, not all locations will experience rainfall, but as the cold front approaches, the potential for showers and isolated weak thunderstorms will increase. The most significant impact will be the very heavy downpours. As for fog, the locally dense fog will remain a threat through Thursday morning. Temperatures will remain in the mid to upper 60’s for lows and highs in the mid 70’s to lower 80’s in the afternoons.
A stronger cold front will move through the region on Friday with additional scattered showers and much cooler air. Drier air will invade the northern Mid Atlantic with temperatures falling back to near normal levels with lows in the 50’s and highs in the upper 60’s to lower 70’s.
However, another upper low is expected to develop, this one along the Mid Atlantic coast by Saturday morning. The first part of this weekend is expected to be dry and very comfortable with low humidity, scattered clouds, and temperatures in the mid 40’s to lower 50’s for lows and mid 60’s to lower 70’s for highs. By Saturday night though, a coastal low is expected to develop off the Maryland and Delaware coast and slowly move northeast roughly 150 to 250 miles off the New Jersey coast between Saturday night and Monday morning. At this time, the best lifting is expected to be focused off the coast with only an increase in clouds and a few isolated showers expected on Sunday and Monday if any. However, just a slight shift in the upper low to the west would mean a far wetter forecast, which means there is significant uncertainty in the forecast for the second part of this weekend. Regardless of rain or not, temperatures are going to be very cool with well below normal values. Temperatures will range from the upper 30’s to mid 40’s for lows and mid 50’s to mid 60’s for highs. This weekend will really feel Fall like.