Subj : MESO: Heavy Rain/Flooding
To   : All
From : Mike Powell
Date : Tue Aug 29 2023 17:23:00

AWUS01 KWNH 292159
FFGMPD
PAZ000-MDZ000-VAZ000-WVZ000-300400-

Mesoscale Precipitation Discussion 1005
NWS Weather Prediction Center College Park MD
558 PM EDT Tue Aug 29 2023

Areas affected...much of WV...far northern VA and western
MD...southeast and central PA

Concerning...Heavy rainfall...Flash flooding likely

Valid 292200Z - 300400Z

Summary...Occasional rainfall rates of 1-2"/hr to lead to
additional localized totals of 2-4" through late evening. Isolated
to scattered instances of flash flooding are likely.

Discussion...Disorganized clusters of showers and thunderstorms
are steadily progressing northeastward across portions of northern
WV late this afternoon, along and ahead of a quasi-stationary
front. Rainfall rates in association with percolating updrafts
have been as high as 1-2"/hr, owing to relatively efficient warm
rain processes. The mesoscale environment is characterized by SB
CAPE of 1000-1500 J/kg, PWATs of 1.4-1.7 inches (near the 90th
percentile, per nearby PBZ/IAD sounding climatology), and
effective bulk shear of 20-30 kts. While low-level moisture
transport values are fairly meager, the nearly stationary front
should continue to provide enough focus for additional development
and proliferation of convection (across more of northern and
eastern WV and into adjacent portions of VA/MD and southeast and
central PA). In addition, broad uplift and longevity of updrafts
should also be provided by increasing upper-level diffluence via
the right-entrance region of a strengthening jet streak over the
Northeast.

12z HREF and more recent HRRR runs are both in fairly good
agreement in depicting localized totals of 2-4" through the late
evening hours. HREF 40-km neighborhood probabilities for 2" and 3"
exceedance are as high as 40-80% and 20-40%, respectively. The
bulk of these totals are expected in a 3-hr period or less, where
associated flash flood guidance (FFG) ranges widely from 1.0-4.0",
but generally ranges from 1.0-3.0" (with a good amount of the
western outlooked area across the more vulnerable Appalachians
ranging from only 1.0-2.0"). Given the vulnerability across much
of the area and already ongoing convective activity, isolated to
scattered instances of flash flooding are considered likely.

Churchill


ATTN...WFO...CTP...LWX...PBZ...RLX...RNK...

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

LAT...LON   41447761 41367723 40947705 40677724 40347740 
            39847774 39217752 38897743 38617806 38267888 
            37947957 37678103 38028139 38868070 39568052 
            39967998 40317938 40727883 41057845 41257812 
            
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