Update, 8 PM: Severe Watch has been dropped in the Delaware Valley but is still out in Central and North Jersey, plus parts of Eastern PA (Northampton and Monroe Counties) where thunderstorms have continued to rock through, bringing flash flood warnings and heavy rainfall from the eastern edges of the Poconos on down into North Jersey. That activity is currently moving east and east-southeast.
Farther north, a line of storms is across Ontario and Lake Ontario, pushing southeast. If anything were to impact us from that line, it would be much later tonight...and may be the line that the NAM was referring to in its midday run (and referenced down at the bottom of this post).
Update, 6:45: If you haven't noticed, the cluster of thunderstorms that looked far more ominous out to the city's west an hour ago than they do now overhead. They weakened quite a bit...again...as we continue to suffer from the "dry begets dry" syndrome with summer thunderstorms fizzling out as they approach the city. Storms are stronger to the city's north (and some storms in the Poconos and Northwest Jersey are severe at this hour). We should see a quiet evening ahead of us.
Update, 5:30 PM: Severe Thunderstorm Watch has been expanded from its earlier position (Lehigh Valley & north) to include Southeastern Pennsylvania and most of New Jersey except for the extreme southern counties (see the map below and the counties in pink). This watch is out until 11 PM.
Thunderstorms are scattered around the region early this evening -- and it might not be the only round of storms the region sees this evening.
Some of the storms are severe -- warnings were issued across Northwest New Jersey for a very slow moving storm that lead to a flash flood warning in Sussex County earlier today. Other warnings were out earlier this afternoon near Lancaster and Lebanon for the storms near Harrisburg earlier and are now out near Reading and Pottstown as those storms move east. Storms could contain heavy rainfall along with gusty winds and some hail. If they hold these storms will move through the city after 6:30 tonight...might be enough to delay the front end of the Phillies game. Storms could bring a quick half inch to inch of rain as they move through as well in addition to the aforementioned severe threats.
Additionally, the threat for additional storms exists later tonight. High resolution computer modeling suggests the possibility of a second line of thunderstorms moving through the region after 2 AM tonight (see above). The GFS does show similar to the NAM but the Euro does not show anything later tonight.
More: Current Weather Page