AWUS01 KWNH 272131
FFGMPD
DEZ000-MDZ000-VAZ000-NCZ000-280201-

Mesoscale Precipitation Discussion 1079
NWS Weather Prediction Center College Park MD
530 PM EDT Fri Sep 27 2024

Areas affected...Southeast VA, eastern MD, far southern DE

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

Valid 272130Z - 280201Z

Summary...Clusters of showers and thunderstorms with 2-3"/hr rain
rates continue to lift northward this evening. Through training,
these rates could produce 2-3" of rain with locally higher
amounts. Flash flooding is possible.

Discussion...The regional radar mosaic early this evening shows a
cluster of thunderstorms lifting northward out of far northeast
North Carolina towards Richmond and as far north as the Middle
Peninsula of VA. These thunderstorms are lifting northward on mean
0-6km winds of around 40 kts, so remain progressive, but are
containing MRMS measured hourly rainfall accumulations of 2-2.5
inches. These impressive rain rates are being fueled by an axis of
tropical moisture characterized by PWs of 2-2.4 inches, above the
daily maximum for the region according to the SPC sounding
climatology, funneling northward around the periphery of what is
now Post-T.C. Helene over Kentucky. An axis of SBCAPE exceeding
500 J/kg is collocated with this PW plume, providing rich
thermodynamics to support continued convection as ascent persists
through 850mb wind convergence, modest upper jet diffluence, and a
weak shortwave impulse rotating within the flow. This heavy rain
has already resulted CREST unit streamflow responses above 500
cfs/smi, leading to multiple active FFWs.

The high-res CAMs differ in their evolution the next few hours,
but the latest NAMNest appears to be handling the current
convective activity the best, followed by the recent run of the
HRRR which has caught onto to the eastward shift. These runs
suggest convection will persist as it heads north, but will begin
to encounter weaker instability into MD/DE. Some of this will be
offset by thermodynamic advection as low level 850mb inflow pushes
higher PW and instability to the north, but in general this should
result in a slow wane of the intensity of this convection. This is
reflected as well both by HRRR 15-min rainfall accumulations and
HREF neighborhood probabilities for 1-hr rainfall exceeding 1"
peaking around 20%. However, as propagation vectors veer to become
more aligned into the more rich thermodynamics, this could result
in short-term backbuilding and training to lengthen the duration
of these rain rates leading to corridors of rainfall that could
reach 2-3" in some areas.

As long as the intense rain rates persist, they will pose a risk
for flash flooding. However, much of eastern MD and DE has been
dry recently, leading to elevated FFG that only has a 5-10% chance
of being exceeded. This suggests the greatest risk for any impacts
will be across SE VA, or where any training can move across an
urban area through the evening.


Weiss

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!58W3cin_F4YkP2J__5_J-CiJOa7wa4Omsm_-6tOtquI9E3w5FH3fx4cjnChviSZyme61=
z-NO63IO2h5zNFfqtn0m3WA$  for graphic product...

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

ATTN...RFC...MARFC...SERFC...NWC...

LAT...LON   38937572 38847530 38727506 38427502 38047501=20
            37617524 37267557 36987568 36677580 36407595=20
            36147616 36107656 36277695 36557747 37127763=20
            37707746 38437693 38827659 38927623=20

=3D =3D =3D
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