Mesoscale Precipitation Discussion 0585
NWS Weather Prediction Center College Park MD
557 PM EDT Thu Aug 02 2018

Areas affected...Nrn Georgia, Upstate South Carolina, Western
North Carolina

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

Valid 022156Z - 030355Z

Summary...An area of heavy rain may be focused in north-central
Georgia this evening, from the Atlanta Metro Area into locations
just to the northeast. Additional moderate to heavy rain appears
likely to develop further northeast in upslope regions of the
southern Appalachians. This may lead to rain rates of 1-2 in/hr in
the heaviest rain bands, and the potential for flash flooding as a
result.

Discussion...Per regional radar VWPs, an 850mb low appeared to be
situated just north of Columbus, GA, and just northeast of where
it was this morning. Surface observations also suggested a weak
surface low with a closed circulation in the wind field just south
of Peachtree City, GA. This weak low was likely helping to focus a
band of heavy rain, moving through the immediate Peachtree City
area as of 22Z, and drifting in the general direction of Atlanta
Metro area. The rain band was likely being sustained by focused
low-level convergence in the presence of a very moist boundary
layer, and this could continue to the northeast through the
evening -- into Atlanta Metro and even further northeast toward
Gainesville, GA. KFFC radar ZDR in areas with reflectivity at
least 40 dBZ was reduced under 1 db. The combination of ZDR
approaching zero with high reflectivity is indicative of numerous
small water droplets and dominant warm rain processes. A mesonet
site near Newnan, GA reported an hourly rain rate to around 1.6
in/hr, which is higher than is being estimated by MRMS and dual
pol radar. The efficient rainfall processes are producing more
rain than would be implied by the weaker low-level reflectivity
structure as well. This rain band could lead to areas of flooding,
particularly if it drifts over the Atlanta Metro area with a large
amount of impervious surfaces.

The other trend will be an increasing low-level jet and moisture
transport into the southern extent of the Appalachian Mountains,
which should allow moderate to heavy rain to gradually redevelop
in upslope areas from far northeast Georgia, into far upstate
South Carolina, and into western North Carolina. Portions of this
area have reduced flash flood guidance to around or less than 2
inches in 3 hours, which is certainly achievable given the
expectation for 1+ in/hr rain rates in embedded convective bands.

Lamers

ATTN...WFO...FFC...GSP...MRX...RNK...

ATTN...RFC...LMRFC...OHRFC...SERFC...

LAT...LON   36208152 35848132 35148189 34428253 33428345
            32938445 33818507 34908462 35828300 36158216