An earlier heavy band of rain in Delaware County was due to interaction between low level moisture lifting over the Fall Line as it was pushing in off of the Atlantic, triggering some heavy thunderstorms along an old trough boundary that existed north of the humidity front. That band of heavy rain produced two to three inches of rain in an hour's time in parts of Delaware and Chester Counties before it weakened.
Now, a band of steadier, heavier showers is pushing towards Philadelphia from the east, coinciding with a shield of rain across South Jersey and Delaware. The stuff south of us is pushing northwest, with the stuff east of us generally pushing west or west-northwest. All of this will work through the region tonight, with the heaviest and steadiest rains south and southwest of Philadelphia. The stuff that's down near Vineland, Dover, Smyrna will push northwest towards Wilmington, Delaware County, Chester County, and to Lancaster County as the night progresses assuming short-range modeling and radar trends hold.
The band of showers across Central Jersey will push west through Bucks, Montgomery, the city, and perhaps Allentown later this evening.
Rain will be heavy at times in spots, perhaps an inch of rain will fall in some locations through the evening hours as this band of rain moves through. South and west will pick up a bit more than that in the heaviest spots. Flash flood watches line up from the immediate Philly metro (except Bucks County) on south and southeast to include South Jersey and Delaware.
The band of rain should push through over the next few hours, with the shield of rain likely to work through southern and western sections for a good chunk of the evening hours beyond that point.
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