Monday will be the warmest day of the year so far and warmest day for a while afterwards -- temperatures will trend back moreso to reality over the course of the week in the wake of an early Tuesday frontal boundary passage through the Northeast. However, Monday will flirt with, perhaps graze record high levels in a number of spots as temperatures approach levels we last saw ten years ago.
The warmest temperatures in the region might not be around Philly tomorrow -- they could occur up around New York City. High resolution version of the NAM used below shows the potential of 90 in/around Newark or Midtown Manhattan tomorrow afternoon at the peak of heat. Newark's record, for what it's worth, is 92. Boston could also flirt with the upper 80's, if not approach 90, tomorrow afternoon as well. Philly could come close to their record -- the wind trajectory is forecast to be west-southwest (which is a great downslope wind around here as well as all along I-95).
No matter how it's slice -- Monday will be a warm, summery day. As a cool front approaches from the west, a few thunderstorms are possible late in the day near Pittsburgh but the storms should fizzle as they approach us overnight.