Fri 08:18 PM Forecast Review — The storms developed and moved just south of Philadelphia as picked up by the afternoon HRRR.
NEXRAD radar at 6:55 PM

Updated Fri 4:25 PM, highlighted below, based on new HRRR
Updated Fri12:30 PM, highlighted below, based on new HIRESW

Strong thunderstorms expected early this evening. The latest HRRR shows all the ingredients coming together about 4:30 -8:30 PM 5:30 – 9 PM (peak 7-8 PM) in Philadelphia.

Update: The latest HRRR has the heaviest storms just to the south of Philadelphia

CAPE values in the 1600-2200 J/Kg range. Lifted Index moderately negative at -5. Very strong upward vertical motion is predicted to develop (blue shading/white arrow below) along with cyclonic upper air contour (blue arrows).

Heavy rain expected, especially from Philadelphia south and east.

HRRR 12z forecast for 8 PM showing very strong vertical velocity (blue shading/white arrow) right over Philadelphia and cyclonic upper air curvature. (blue arrows) (Click on image for a larger view.)