Mesoscale Precipitation Discussion 0801
NWS Weather Prediction Center College Park MD
429 AM EDT Sat Sep 08 2018

Areas affected...Southeast PA...Central and Eastern
MD...DE...DC...Southern NJ

Concerning...Heavy rainfall...Flash flooding possible

Valid 080830Z - 081330Z

SUMMARY...Shallow but moisture rich slow moving
showers/thunderstorms may reaggravate flooding in areas that
received flooding rains last night.

DISCUSSION...Surface analysis and regional RADAR suite depict a
secondary push of northerly winds north of the main front across
SE PA into Central NJ.   Cells are very shallow with IR tops
warmer than -20C but surface Tds in the upper 70s and total PWATs
still in the 2" range still will support warm cloud rain processes
for efficient rates.  A weak subtle shortwave/speed max is helping
to initiate the weak convection but as it passes should aid in
strengthening drier air push from the north to further invigorate
these showers with some potential increased vertical growth.  RAP
analysis indicates a ribbon of enhanced instability from S PA with
SBCAPEs up to 1000 J/kg increasing to 2500 J/kg over the central
Delmarva, south of the main front and with strengthened mid-level
flow, upstream redevelopment may support some repeat tracks, or
even hung up cells along the eastern coast of the Chesapeake Bay
or Delaware River (sfc frictional convergence enhancement). 
Allowing for increased localized totals much like the cells that
move through the Bay and Kent/Queen Anne's county recently.  
These cells alone would not induce flash flooding, but as the line
moves south will track through areas that are saturated with last
evening's heavy rains, especially in the DC and Philadelphia Metro
areas.   It is this repeat nature that makes flash flooding
possible over the next few hours within the area of concern.

Gallina

ATTN...WFO...AKQ...CTP...LWX...PHI...

ATTN...RFC...MARFC...

LAT...LON   40267486 40217447 39967401 39607415 39147462
            38867486 38857503 38627503 38257510 38037527
            37977564 37947594 38117617 38367626 38637634
            38787633 38877639 38907647 38907660 38867675
            38817690 38867704 39047719 39107737 39217747
            39357761 39497777 39627789 39827787 40007736
            40247619 40247540