Increasing Storm Threat PA/MD/DE/NJ....

[weathermanchild]

A cold pool driven event again seems possible from C PA to the Atlantic Coast...

This has no SPC mesoscale discussion, probably because it would be such a marginal threat. However, this setup is very similar to yesterday.

The attached image (I don't know how to make it appear without it being an attachment, forgive me) shows the existing/ developing squall line in W PA with the purple dash-dot line. This line will continue to propagate across PA driven by the mean wind (about 20 kts toward the east). It will encounter SB Cape in excess of 2000j/kg and ML CAPE > 1500 in C PA (in a few hours or so). Additionally, it will move away from the favorable shear, causing the updrafts to occur very close to the downdrafts. The Downdraft cape is > 1000 and the DCAPE ridge can be seen on the image as a light brown dashed line. When the squall line nears this DCAPE ridge, the cold pool will begin to take over. The cold pool will likely form inside the green circle and propagate towards the coast while expanding slightly.

The main similarity with yesterday which has usually been missing this season so far is a source for synoptic forcing which can help to sustain the updrafts during the cold pool formation stage. A vorticity maximum exists in NE OH and will track east into C NY, providing forcing the the squall line. The boundary layer from C PA to NJ and DE is deep and very well mixed, and extremely warm and moist. Just above the B/L, there is a dry layer with dewpoint depressions ~10-15 C to support the downdrafts. Therefore, I suspect a few scattered wind reports are likely as this area moves toward the coast. the most intense cells will also likely have hail.

this is supported by several runs of the HRRR according to reflectivity and updraft helicity plots (not shown here). similar to yesterday, I would not be surprised if there was another vivid lightning display, especially from SE PA into N DE.