The waffles of a springtime frontal boundary mean larger changes in temperatures over a short distance. In the case of our current weather pattern, a stalled out frontal boundary will lie just to Philadelphia's south tomorrow before nudging back northward tomorrow night and into Wednesday. While the front meanders around the region, various weak systems will traverse east along the front and provide shower chances for the region. The first of these is tracking through and north of the Philly area tonight with showers -- the best chances of these will last through the evening hours before the system pulls away and drags the front down to about Wilmington.
Round #2 comes east tomorrow with rain and some embedded thunder from about midday into the afternoon hours. The NAM (below) shows the potential for the heavier showers and thundershowers generally just north of Philadelphia, an idea that is backed up by the GFS (although the GFS is farther north with the showers than the NAM as it places the frontal boundary near and just north of Philly). The NAM probably has a slightly better handle on the frontal boundary spot but where the front lines up is critical for where temperatures ends up on Tuesday -- it could be 70+ in Dover while in the 50's in Allentown tomorrow afternoon. Showers will also track along and just north of the front, meaning the best chances for rain tomorrow will be Philly and north.
Rainfall should total a quarter to half inch north of Philadelphia, with some isolated higher totals in Central and Northeastern Pennsylvania that could reach three-quarters of an inch. The front will nudge northward on Tuesday night and Wednesday, providing a chance for the region to surge back towards 80 on Wednesday...at least for those who get south of the front!
