Today's frontal boundary paves the way for an area of low pressure to organize and move northeast late tonight into tomorrow, spreading what is likely going to be a shield of showers or light rain into a chunk of the region. Modeling varies about the northwest extent of showers but both the NAM and GFS do show them getting as far northwest as the northwest burbs of Philadelphia tomorrow morning.
The GFS has been consistently aggressive on the extent and coverage of the band of light to steadier rain as it pushes northeast...and if it were December instead of October we could talk about the probability of snow as the atmosphere aloft would probably support some snow in some locations. That said, the atmosphere at the surface and in the lowest few thousand feet will be too warm for snow to reach the surface tomorrow; hence, a chilled bout of showers and rain will fall tomorrow morning across Southeast Pennsylvania on northeast into New Jersey.
The NAM (below) is a bit farther northwest with the shield of showers, pushing them as far west as Central Pennsylvania as they push northeast with the area of low pressure. Both of these models are more aggressive with the northwest extent of precipitation compared to the Euro, which is much drier and weaker with the low's development thanks to a confluent flow post-front preventing much development.
Modeling generally is suggesting the most shower activity falls along and just east of Route 202 through Southeast Pennsylvania on through South Jersey. A couple of tenths of an inch of rain can't be ruled out with the morning rainfall.
Wednesday may set up as the coolest day since April 23rd -- a high of 51 that day. We projected 56 in last night's forecast for Wednesday and I feel that could be an early morning high as we could slip into the 40's in the city during rainfall and not recover much from there as northwest winds take over once precipitation ends. It's going to be a crisp, dare we even say raw, day tomorrow.