NWS Philadelphia/Mt.Holly's take on Fridays heavy rain and severe weather potential.
On Friday, the main synoptic system will begin impacting the local
region with deep southerly flow advecting considerable moisture
poleward from the Gulf of Mexico. There does not look to be a well-
defined cold front with this system, but it will probably have more
of a convective cold pool driven surface boundary slowly approaching
the area later in the day. Convection is expected to be ongoing
along this boundary as it approaches throughout the day. As
previously mentioned, the general trend in the latest guidance has
been overall slower with this progression, and thus I have slowed
the onset of the higher PoPs from west to east.
As far as impacts go, we are still anticipating that there will be
at least a marginal severe weather threat with any more robust
thunderstorms that are able to organize. Deep layer effective shear
values will be around 30-35 kts, which will support at least some
organization of convection, especially into the afternoon and
evening when surface based instability will likely be the highest.
Instability will likely not exceed 1000 J/kg or so, but there will
be plenty of forcing. So even a few hundred J/kg of instability will
be sufficient in supporting organized/sustained convection. Wet
microbursts with locally strong to damaging wind gusts will be the
most probable severe weather threat. The deep stacked southerly flow
will support training and back-building of convection as well. With
PWats rising into the 1.4"-1.8" range (potentially record breaking
values for May 27th), heavy rain and thus flash flooding will likely
become an increasingly probable threat with this system somewhere
across the Mid-Atlantic region on Friday. However, there is still
too much uncertainty regarding placement and timing of this potential
threat to get into the details. We are currently forecasting
widespread QPF around a half inch, but totals could locally exceed
one inch where ever convection trains over the same area. SPC
has introduced a Slight Risk for severe thunderstorms in our
area for Friday and WPC has a Marginal Risk of excessive
rainfall.
