AWUS01 KWNH 071927
FFGMPD
NJZ000-PAZ000-DEZ000-MDZ000-VAZ000-DCZ000-WVZ000-080030-

Mesoscale Precipitation Discussion 0882
NWS Weather Prediction Center College Park MD
326 PM EDT Mon Aug 07 2023

Areas affected...Western & Central MD...Central & Eastern
PA...Northern DE...Northern VA...DC...

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

Valid 071925Z - 080030Z

SUMMARY...Very intense rainfall to accompany severe thunderstorms,
cell orientation or even mergers may allow for increased duration
to result in localized instances of flash flooding through the
evening.

DISCUSSION...GOES-E WV shows core of upstream height-falls are
starting to press into the Mid-Atlantic with numerous strong
updrafts already forming along the front range of the of the Blue
Ridge Mtns as well as into south-central PA. Discrete supercells
are currently the mode across PA into West-central MD/NW VA
starting to approach more unstable axis of 2000-2500 J/kg of
MLCAPE from central VA into central PA. CIRA LPW and RAP analysis
suggest, a WSW-ENE wedge of lwo to mid-level enhanced moisture is
crossing N WV across the Mason-Dixon line. This has resulted and
likely to continue, a more expansive area of convection that
continues to be favorably oriented for multiple rounds of heavy
rainfall across this area, slowly sinking southward with time as
southeasterly return moisture off the Atlantic lifts northward as
well combining toward 1.75-2" values by mid-evening. This should
increase rainfall efficiency with time with rates of 1.5-2"/hr
(though sub-hourly rates would be much higher with 1-1.5" totals
in 15 minutes suggested by HRRR or .35-.5" in 5 minutes by
numerous WoFS members); however, forward cell motions are likely
to limit totals by any one given storm to about 2". Stronger
rotating updrafts may enhance this a tad given Bunker's right
moving propagation vectors suggest slightly reduced forward speed
combined with isallobaric acceleration of moisture flux into the
supercell itself.

The key to greater flash flooding potential will be the
requirement for additional upstream thunderstorms. Hi-Res CAMs
and WoFS solutions, combined with current observational trends
suggest this is more likely across E WV, Maryland and far Southern
PA, though there are a few guidance members that suggest a
enhanced potential of back-building environment over north-central
VA as well, particularly later into the evening. This is less
likely further north in central to northeast PA, but overall, the
FFG values are a bit lower relative to MD/NoVA to suggest perhaps
one really strong round could exceed these lower values and induce
a localized flash flood instance or two. As such, a broad
possible risk of spotty/localized flash flooding will accompany
the severe weather risk through the evening across the
Mid-Atlantic.

Gallina

ATTN...WFO...AKQ...BGM...CTP...LWX...PHI...RNK...

ATTN...RFC...MARFC...OHRFC...NWC...

LAT...LON 41487566 40897508 39707547 38477641 37887780
37817897 37947969 38387964 38857930 39237899
39787889 40137892 40607850 41157768 41387679


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