Flash Flood Watches are out for the Philly metro starting tonight and continuing through Monday. The rains that move through starting later tonight through Tuesday may not be widespread nor continuous -- there will be lulls in the action and there will even be moments of sunshine. However, a moisture rich feed in the atmosphere combined with a couple of ripples of disturbances in the atmosphere will produce the opportunity for scattered soakings and tropical torrents -- showers and storms -- that will move through the region.
PWAT's (which is an indicator of how much water is in a column of air in the atmosphere if one were to squeeze it out) will be above two inches in much of the Delaware Valley between late today and Tuesday. This results in the potential of any thunderstorm that fires up to produce heavy rainfall. It won't storm at all times but when storms do pop up, they will be tropical in nature and could produce very heavy rainfall.
Having that potential out there and the fact that some parts of the region picked up five inches plus of rain in the last week does not help. Some parts of the region could reach flash flood benchmarks with less than an inch of rain in an hour's time (the purple shading) or with less than two inches of rain in that same time (the darker blue/green hue).
Even stretching it out to three hours results in some potential flood issues in those areas that picked up a lot of rain last week provided rainfall totals of two inches or so fall in a short period of time. Those type of rainfalls are possible, especially on Monday and Tuesday, with the bulk of moisture feed transporting overhead.
Modeling does indicate this potential does indeed extend into Tuesday.
In terms of who is most at risk...southerly winds pushing up against the Fall Line boundary (where the hills north/west of the city end separate from the relatively flat terrain from I-95 on south and east) probably yield the heaviest rainfall threats in the northern and western burbs, New Castle County in Delaware, as well as across Central New Jersey (generally just above the Fall Line). Basically, areas that have very little tolerance for rain are at the most risk for flash flooding.
The Shore may not see much in the way of storms over the next couple of days -- they aren't completely out of the woods but in terms of how heavy the rain is, how much falls -- coastal areas may fare relatively well. We will keep an eye on that trend.