With a frontal boundary slicing slowly into the current heat and humidity during Wednesday, the threat for heavy rainfall and severe weather exists. The Storm Prediction Center has Pennsylvania and the northern two-thirds of New Jersey under a slight risk for severe weather on Wednesday as the front slides into the region.
This is not a fast moving front -- it may take until Thursday night or early Friday to get the threat for precipitation south of the city as the push of "relief" is slowing (not surprising given it's July) but there will be enough of a push from this front to finally get the threat of thunderstorms south of town in time for the weekend. More on that in a bit.
First, tomorrow's threat for storms. It looks like most of us have a shot at seeing thunderstorms develop during the afternoon, with development taking place across Northern Pennsylvania before storms slip southward during the course of the day. Thunderstorms will approach the Philly metro after 4 PM, crossing the city during the PM rush or early evening hours (earlier north, later south). Wind is going to be the primary severe threat with any storms that slide southward...however, hail will probably accompany the strongest of storms as well so one can't rule out the possibility of some large hail in isolated cases. In addition to both, heavy rains will push in as PWAT values (how much "potential" rain could fall in an hour's time) approach two inches ahead of the front across our region. Given we're still pretty dry in many locations despite the needed downpour on Sunday, the threat for flooding is low tomorrow but the threat for thunderstorms is not.
The front will slowly slide southward on Thursday, pretty much sliding over the top of Philadelphia during the course of the day. Thunderstorms are possible, especially from the city on south where moisture and instability will be higher. We can't rule out a thunderstorm north of Philly but the front's slow slide south will gradually shut off the potential for storms as the day progresses. There could be a shot at severe weather for the Delmarva and perhaps the southern portions of New Jersey on Thursday as well...but the threat with the front nearby could be more in the form of heavy rainfall and less in the form of severe weather.