Mesoscale Precipitation Discussion 0063
NWS Weather Prediction Center College Park MD
649 PM EST Sat Feb 11 2023

Areas affected...coastal Georgia & South Carolina

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

Valid 112348Z - 120548Z

Summary...Developing showers and thunderstorms are expected to
continue to expand in coverage and intensity over the next several
hours.  Hourly rain totals to 2" with local amounts to 4" are
possible.

Discussion...The 2300 UTC observation set indicated that a low
pressure system appeared to be forming just northwest of the
Okefenokee swamp in far southern GA.  Divergence aloft is being
sponsored by a cold low near the central Gulf coast.  Recent
indications via radar and satellite imagery showed increasing
concentration of convective activity near the forming low, with
some local hourly rain estimates up to 1".  Other convection
offshore SC closer to the coastal front has been edging closer to
shore over the past couple hours.  MU CAPE of 500-1000 J/kg exists
near the low per SPC mesoanalyses.  Precipitable water values have
risen above 1.75" per recent GPS data.  Effective bulk shear is
30-40 kts across coastal GA.  Flow is nearly unidirectional out of
the southwest.

RAP guidance indicates that the low forming in southeast GA is
expected to track northeast towards the central coast of SC over
the next several hours, which would act to bring the coastal front
closer to the coast, if not ashore, which would increase
instability near and ahead of the cyclone.  RAP guidance indicates
MU CAPE reaching 1000+ J/kg ahead of the low, which would allow
convection to be more robust, and potentially organize and train. 
Low to mid-level frontogenesis is expected to overlap with the
incoming instability from roughly Savannah northeastward, which
would aid rainfall efficiency.  There is a mild signal in the 18z
HREF of a couple rounds of heavy rainfall, particularly near the
lower and middle SC coast.  While the guidance shows greater than
average spread, the expected evolution favors heavy rainfall. 
Hourly rain totals to 2" with local amounts to 4" are seen on a
couple pieces of mesoscale guidance, and befit the environment
expected to evolve this evening into tonight.  Some of this region
has seen rainfall of 100-200% of average over the past week, but
the region is also marshy/swampy.  Heavy rain issues should be
more significant/evident in urban areas.  Considering the model
dispersion, confidence is lower than average.

Roth

ATTN...WFO...CHS...ILM...JAX...

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

LAT...LON   33367975 33207909 31958082 30998144 31108246
            32678141