The world very likely isn't ending tomorrow, unless the Mayans have a trick up their sleeve, but one certainty of weather that's ending is the warm regime that dominated most of December. Tonight and tomorrow's front has a pretty substantial amount of pop associated with it, with snow flying in the Midwest and a soaking rain likely in our region tonight and early tomorrow morning.
Rainfall will spread in after sunset, likely after 7 or 8 PM from west to east across the region, with rain becoming steadier and heavier after Midnight. The 1-6 AM range is when most of the rain with this storm system, which coincides with the hours immediately ahead of the frontal boundary passage. The NAM computer model, below, shows the steady and heavy rains across the region as we approach wake-up tomorrow morning.
While thunder is not likely late tonight, it's possible along the frontal boundary itself...and the best (but still small) chances of that occurring will be to our south across Delaware and Maryland as the front pushes through and there's a bit warmer air down there to work with.
Rainfall tonight into tomorrow morning will average around an inch, with some inch and a half totals across the Poconos possible. While flooding is not likely on a widespread basis, some localized ponding of water on roads can't be ruled out up to our north. Winds will increase this evening from the east and southeast, becoming rather gusty overnight in advance of the storm as warmer air moves in. That means a temperature rise kinda night, with Midnight highs today and pre-dawn highs tomorrow morning.
The bigger story will be temperature drops on Friday morning -- the front's passage will be pretty potent and accompanied by a sudden wind shift from the southeast to the west. Temperatures in the pre-dawn hours could be in the mid 50's in Philadelphia and near 60 across South Jersey and Delaware, with a 10-15 degree temperature drop in a couple of hours' time likely once that front crosses through.
We're timing the frontal boundary passage between 5 and 7 AM in the city -- so that means that the early bird crowd will be heading to work with warmer temperatures than those in the latter half of the rush hour...but more importantly, temperatures going home are going to be a good bit colder than they will be heading out the door as temperatures will be on a slow, steady fall after that quick drop tomorrow morning. We will be facing 30's for temperatures tomorrow afternoon as you head home. Adding to this will be a blustery wind from the west that will gust to over 30 mph, which will yield a wind chill factor in the 20's to around 30 degrees.
So, yeah, winter's first official day tomorrow will definitely coincide with a return of chilled reality...one that will last into next week.