Today's cooling is brought to you by a backdoor front that slides down the coast a bit. That same frontal boundary is attached to a storm system in the Midwest that will push east and impact us late tonight and tomorrow. Before those rains get here later tonight, the prospect of drizzle and some fog looms later today in parts of the region due to onshore flow off of the Atlantic dragging low level moisture into the region. Tonight could be a pretty drizzly, not to mention crisp, evening around the region as east winds cool do their work.
Later tonight, the storm system in the Midwest pushes on in and showers will increase in coverage and intensity, breaking into a steadier rain in the predawn hours. Some of the rain will likely be steady and heavy on Friday morning in parts of the region -- modeling varies in specific timing and detail, with the GFS faster than the Euro in sending this heavier shot of rain through, but odds generally support rains being heavier in the 8 AM to Noon timeframe across the region.
During this timeframe, a steady round of rain with perhaps some thunder will work through the region. Modeling generally agrees that the heaviest rains associated with this will line up across Eastern Pennsylvania, to the northwest of Philadelphia. Someone northwest of the city could pick up an inch to inch and a half of rain on Friday morning if modeling is correct. Not enough to cause flooding problems given the amount of dry weather we've had in general (last night notwithstanding), but still a pretty decent chunk of rain.
The backdoor front does stall out to our south overnight and then begins to nudge back north as the storm system moves in, which means portions of our region could get back into the warm airmass at some point on Friday afternoon before the cold front moves on through. This also means some thunderstorms are possible on Friday afternoon (probably in the early to mid afternoon timeframe) with the frontal boundary itself. It doesn't seem like severe weather would be a threat but locally gusty winds could occur, especially down in Delaware or Maryland where some warmth should occur.
Temperatures tomorrow could be pretty ugly, like today, in terms of forecasting. North of the city will likely be quite cool -- 40's and 50's for much of the day. The city has a shot of getting to 60 but it's dependent on whether the warm front makes it far enough north. Parts of Delaware and even some portions of South Jersey could get to 70 or above on Friday afternoon before the cold front crosses through.
Rainfall for most generally stays at or under an inch, exceptions being northwest of the city where that heavier axis of rain seems to be more likely on Friday morning. Northwest of city could get an inch and a half or so in a few spots.