Storms are firing in the Midwest this evening and will work east as the night progresses. While our earlier forecast mentions a few storms after midnight, I do think we have a pretty good shot of seeing at least some showers and storms working into Central Pennsylvania after 3 or 4 AM. The kicker is that those storms will likely not be severe tonight -- that activity is reserved for the Midwest and Lakes...some of that stuff will be particularly nasty from Rockford east to Toledo from a wind and tornado standpoint, with damaging wind possible even as far east as Columbus and Pittsburgh.
Some modeling does show the approach of rain and thunder to our western doorstep by daybreak. This is where the forecast gets very tricky tomorrow because a warm front will be the highway many of these storms drive east along tonight, with warm air to the south and less humid air on the northern side of the front. The front will lift slowly north but slow further as it does so...and the approach of the low pressure center and disturbance that is propelling this severe weather will all coincide with our area tomorrow afternoon.
If the thunder complex to our west holds together and gets into our region during the morning hours, it may be tough for the warm front to push further north as the decaying storm complex stabilizes and cools the air as the disturbance pushes through, setting up a stabilized boundary that will make it tougher for the warm front to push farther north. If those storms fizzle or move through quick enough, the warm front will have an easier time pushing north and thus will increase our risk for severe weather here in Philly and Southeastern Pennsylvania.
A few caveats for tomorrow --
1) North of the warm front is where the risk of heaviest rains will be. If you're north of the PA Turnpike, the risk for a few inches of rain can't be ruled out. The possibility of this is higher in Allentown, Mount Pocono, Reading, Scranton than across the northern burbs but one can't rule out a two or three inch rain event in the Philly suburbs as well. Those who get the heaviest rains will also get flooding of varying degree (usual suspect roads, streams, creeks).
2) South of the warm front is where the best severe chances are, especially for those close to the low pressure center track. The low will generally ride east along the warm front...so it may track east-southeast through the region, dependent in part on what the MCS/storm complex does. Those in the warm sector south of the warm front will probably get less rain overall than north of the warm front although they will run the risk for severe weather. That means the possibility of damaging wind and/or tornadoes. It doesn't mean it's likely -- it just means it's a possibility (remember the "coin flip" odds I mentioned earlier). The Philly metro and points south are under a moderate risk for severe weather as of this evening for tomorrow. This may change tomorrow morning and perhaps change a couple of times during the day, all dependent on how the warm front behaves. Even if we get nudged into slight in the city the possibility of severe weather will be relatively higher than usual in the Mid Atlantic (one in three type odds within 25 miles of a spot).
Regardless of where the front lines up tomorrow, it will be an active weather day. If you don't get the severe weather aspect from tomorrow, the heavy rain aspect is legit and another round of localized flooding can't be ruled out.