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AWUS01 KWNH 220316
FFGMPD
CTZ000-NYZ000-NJZ000-PAZ000-220800-

Mesoscale Precipitation Discussion 0655
NWS Weather Prediction Center College Park MD
1116 PM EDT Sun Jul 21 2019

Areas affected...Pennsylvania, New Jersey, Southeast NY and Long
Island

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

Valid 220315Z - 220800Z

Summary...Thunderstorms moving slowly along a stationary boundary
will increase in coverage in response to a shortwave lifting north
from Virginia. Rainfall rates of more than 1"/hr are likely in a
very moist column, and as storms train along the front this may
produce enough rain for flash flooding.

Discussion...Recent radar from KCCX and KOKX depicts a line of
convection oriented east-west along a stationary front draped
across the region. This convection is intensifying as noted by an
increasing in cooling cloud tops on recent GOES-16 IR imagery, and
is supported by a subtle shortwave lifting northward out of
Virginia. This ascent is driving increasing coverage of
thunderstorms as the environment remains highly favorable due to
MUCape of 1000-2000 J/kg, and PWATs of 1.5-1.8 inches. The 00Z U/A
soundings from KRNK and KPIT, the moisture source region, had
PWATs of 1.47 and 1.66 respectively, with mid-level lapse rates
near moist-adiabatic values with warm cloud depths over 12 kft.
The multiple forcing mechanisms acting upon this favorable
thermodynamic environment are likely to persist convection
overnight, especially as instability/moisture is resupplied on a
modest W/SW 850mb LLJ.

High-res CAMs are not handling the situation well tonight, and
this creates a somewhat lowered confidence as to the flash flood
potential. Even the most recent runs of the HRRR are severely
underestimating the coverage of rainfall across Pennsylvania.
Despite a lack of model agreement, current observations of the
environment remain highly favorable for continued thunderstorm
development, at least into N PA before the instability drops off
quickly in the more stable air north of the stationary front. RAP
mean 0-6km winds are aligned with the front, with Corfidi vectors
forecast to become increasingly aligned with the mean wind as
well. This suggests training of echoes is likely, so despite storm
motions of 15-20 kts, 2-3" of rainfall is possible as rain rates
exceed 1"/hr. In some locations, this could create flash flooding,
especially across urban areas or in locations that have FFG as low
as 1"/1hr due to recent rainfall of more than 200% of normal, and
75-85% saturated 40cm soils according to the NWM.

Weiss

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...ALY...BGM...CTP...OKX...PBZ...PHI...

ATTN...RFC...MARFC...NERFC...OHRFC...

LAT...LON   41757651 41747585 41637499 41537438 41407383
            41277351 41037326 40757325 40557335 40357378
            40107448 40017519 39957600 39967656 39967728
            40017809 40047863 40137919 40297952 40627968
            40977959 41307924 41527859 41737744


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