Tropical Storm Karen looks pretty awful out there this morning, with an exposed center of circulation to the northwest of thunderstorms that are firing over the Gulf of Mexico. Hostile upper level winds across the northern Gulf are contributing to the storm's struggles, which is resulting in a system that is likely to remain weak to landfall and could perhaps weaken a bit further from its 40 mph max winds as of 5 AM. As one forecaster at the NHC put it last night, Karen has been decapitated.
Karen's remnants may play a factor in the upcoming frontal passage early next week -- perhaps enhancing some of the rainfall that comes through. If anything, the influx of humidity and tropical moisture ahead of the front over the past couple of days is probably going to leave a decent mark on the region as the front pushes through on Monday afternoon, Monday night, or Tuesday. Yeah, even this far out there's a bit of discrepancy on timing and impact with the frontal passage, much of it having to do with the strength of the mid level feature that "powers" the front.
There's about a 9 to 12 hour difference between the GFS (top) and Euro (bottom) in terms of when we'd get the heaviest rains through the Philadelphia region. The GFS slows the storm system down thanks to a more potent mid level vort behind the front that moves a bit more slowly, holding the front up a little bit as the mid level feature spins east through the Midwest and Great Lakes over the next couple of days.
In either case, the prospect for a steady to heavy rain for parts of the region remains on tap during this time frame. Modeling suggests a general 1/2" to 2" event through the region, probably heaviest across Central Pennsylvania...some spots out there probably make a run at three inches of rain from this system.
The front out to our west has been rather potent -- over a dozen tornadoes in Iowa and Nebraska yesterday, over three feet of snow in the Black Hills of South Dakota. While that energy fades some between now and Monday, there's still enough moisture around locally to produce a pretty hefty rainfall across the Delaware Valley on Monday night into Tuesday.