AWUS01 KWNH 051218
FFGMPD
SCZ000-FLZ000-GAZ000-051800-

Mesoscale Precipitation Discussion 0809
NWS Weather Prediction Center College Park MD
817 AM EDT Mon Aug 05 2024

Areas affected...Florida, southeast Georgia, far southeast South
Carolina

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

Valid 051216Z - 051800Z

Summary...Heavy rain associated with Hurricane Debby will continue
to pivot northward across Florida, Georgia, and into South
Carolina through the afternoon. Rainfall rates in excess of 3"/hr
are likely at times, which through pronounced training could
produce 4-8" of rain with locally more in the most persistent
bands. Flash flooding, some of which could be significant to
locally catastrophic, is expected.

Discussion...Hurricane Debby made landfall this morning near
Steinhatchee, Florida according to the National Hurricane Center,
and continues to move northward near the Big Bend of Florida.
Rainfall associated with Debby has already reached 5-10" with
locally higher amounts according to local mesonet observations in
portions of the northern and central peninsula, and multiple flash
flood warnings are in effect. PWs as measured by GPS in the
vicinity of Debby are around 2.5-2.7 inches, above daily records
for the region, which combined with warm cloud depths near 17,000
ft and tall-skinny CAPE of 500-1000 J/kg supports tremendously
efficient tropical rain rates which have been estimated at
2-3"/hr.

Through the afternoon, the high-res CAMs are actually in good
agreement that heavy rain will continue to push north within the
central dense overcast (CDO), and within spiral rain bands lifting
northward from the Gulf of Mexico. Both HREF and REFS
probabilities for rain rates within the CDO are high for 2"/hr,
while rain rates in spiral rain bands and within the greater
instability could reach 3-4"/hr are progged by HRRR 15-min
rainfall accumulations. Mean 0-6km winds will remain strong at
40-50 kts, but propagation vectors aligned anti-parallel to this
mean wind combined with generally unidirectional flow at any
location around the hurricane will result in pronounced training
of cells to extend the duration of these intense rain rates.
Additionally, mean 850mb winds of 50-60 kts are around 1.5x the
mean wind, which will lead to additionally enhanced ascent across
the northern peninsula and into GA. The continuous rainfall and
spiral bands will cause 4-8" of rain with locally higher amounts
nearing 10" as reflected by both HREF and REFS 6-hr PMM. This will
cause widespread significant flash flooding, with locally
catastrophic impacts possible where the heaviest rain occurs
across any urban areas or soils already saturated.

Farther north into eastern GA and southeast SC, increasing onshore
flow will gradually spread higher moisture and instability
northward to support an expansion of heavy rain showers.
Impressive rain rates of 2"/hr are likely within these expanding
cells, which through training could result in 2-4" of rain,
especially in the vicinity of Savannah, GA, where isolated flash
flooding is possible through this afternoon. However, there will
likely be far greater impacts and much heavier rainfall
necessitating additional MPDs after this initial discussion period.


Weiss


ATTN...WFO...CHS...FFC...JAX...MLB...TAE...TBW...

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

LAT...LON   32538111 32508040 32288034 32118053 31648095 
            31198122 30638133 30068125 29628115 29078099 
            28638097 28198113 27848136 27558169 27298198 
            27208237 27298270 27578294 28048304 28588296 
            29098311 29678355 30038428 30658401 31398340 
            32048250 32408176 
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