AWUS01 KWNH 061122
FFGMPD
NCZ000-SCZ000-GAZ000-061700-

Mesoscale Precipitation Discussion 0817
NWS Weather Prediction Center College Park MD
721 AM EDT Tue Aug 06 2024

Areas affected...Coastal Plain of Georgia, South Carolina,
southeast North Carolina

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

Valid 061120Z - 061700Z

Summary...Tropical Storm Debby will continue to crawl near the
Georgia coast this morning. Rain bands pivoting onshore will
contain rainfall rates that may briefly reach 4"/hr, leading to
6-hr rainfall of 3-5" with locally higher amounts. Flash flooding
is likely, some of which maybe significant to locally catastrophic.

Discussion...Tropical Storm Debby continues to lift slowly
northeast along the Georgia coast this morning as noted in
satellite and radar imagery. The slow motion and persistent
onshore cyclonic flow around T.S. Debby has resulted in heavy
rainfall and flash flooding overnight, especially across portions
of the Lowcountry where 24-hr rainfall of 6-10" has been
widespread in the vicinity of I-95 between Savannah and
Charleston. Although the heaviest rain bands have lifted a bit
north of Savannah this morning according to the regional radar
mosaic, regenerating rain showers moving onshore are likely even
outside of the heaviest bands. This is in response to an extremely
saturated column measured by special 06Z U/A soundings from KCHS
and KJAX which had PWs above 2.6", daily records at both sites,
created by deep column saturation through 200mb and weak lapse
rates. Additionally, warm cloud depths were measured around 16,000
ft, while an impressive theta-e ridge axis noted via the SPC RAP
analysis surged onshore. Within this airmass, warm advection on
850mb winds of 35-45 kts (AOA the mean 0-6km wind to enhance lift)
will continue to cause redeveloping tropical downpours offshore to
pivot onto the coast.

There are some subtle differences in placement of the heaviest
rainfall the next several hours on the recent CAMs, most notably
in the NAMNest which pivots the heaviest rain into the Pee Dee.
Although the HREF EAS probabilities suggest the greatest risk will
continue to focus in the vicinity of Charleston and Berkeley
counties, the most recent runs of the HRRR have also trended a bit
farther northeast, so there is potential that despite modest rain,
so far, north of Winyah Bay, the flash flood risk may ramp up this
morning in those areas. Still, the highest potential for
additional flash flooding appears to be focused near Charleston
where HREF neighborhood probabilities for more than 5"/6hrs are as
high as 40-50% as spiral bands with rainfall rates of 2-3"/hr (15
min HRRR accumulations up to 1" suggesting brief 4"/hr rates)
continue to move onshore. Unfortunately, with the slow motion of
Debby, this is also where the heaviest rain has already occurred
leading to saturated soils and ongoing flash flooding. Anywhere
within the discussion area could experience flash flooding this
morning, but the greatest risk of significant to locally
catastrophic impacts appears to be in the vicinity of the
Charleston metro where both HREF and REFS 6-hr PMM reach 7+" over
extremely vulnerable areas.

Weiss

ATTN...WFO...CAE...CHS...FFC...ILM...

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

LAT...LON 34567935 34437827 34277776 33927775 33847789
33367879 32927928 32627978 32108048 31788112
31998167 32268214 32918226 33728163 34368054

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