Yesterday's thunderstorms were definitely high producing, tropically soaking in nature for large portions of the region. Rainfall varied from a few tenths in Allentown and Atlantic City to as much as seven inches of rain in Frankford, DE (lower Sussex County). Rehoboth picked up 6.77" of flood-inducing and traffic-crawling rain, leading to street flooding along Route 1 across the Delaware Beaches.
Even locally, a swath of heavy thunderstorms that developed just above the Fall Line in the morning hours produced three inches of rain across Delaware and Chester Counties, added with another inch or so of rain last night with the bands of precipitation pivoting through the region.
The cause of all of this was the interaction of a boundary (humidity front as I coined it) with an upper level disturbance that cut off from the jet stream and stalled over Maryland and Delaware. The humidity front was essentially a boundary that separated tropical air from tolerable...you might remember if you reside in Philly and points north yesterday morning was somewhat tolerable outside as dew points were in the lower 60's. Lower Delaware had the tropical air. As the upper disturbance spun, the interaction of it with the surface front helped generate showers and storms across the Mid Atlantic that slowly pushed northwestward. The morning storms in the western burbs were caused moisture lifting over a weak surface boundary that resided across the suburbs, enhanced a bit by geography (Fall Line).
We weren't alone in the soaking -- parts of the Washington, DC, metro picked up three to five inches of rain on Thursday night as the disturbance fired up initially over Virginia and slowly moved across that area.
The upper level disturbance will pivot back west today, with the surface boundary across the region weakening but still able to act as a trigger for some scattered thunderstorms this afternoon...any of which could produce soaking rains.