Mesoscale Precipitation Discussion 0777
NWS Weather Prediction Center College Park MD
308 PM EDT Wed Aug 17 2022

Areas affected...just inland of the Central Gulf Coast

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

Valid 171907Z - 180107Z

Summary...Showers and thunderstorms are forming near a convergence
zone draped across the region.  Hourly rain totals of up to 2.5"
with local amounts to 4" are possible.  This would be most
problematic over eastern areas that experienced heavy rainfall
yesterday in MS & AL and within urban areas.

Discussion...An outflow boundary/convergence zone is acting as the
leading edge of the thermal gradient across portions of northeast
TX, central LA, south-central MS, and southern AL.  Undular bores
are apparent on recent radar imagery across northern LA, implying
some stability/CIN remaining present in that area.  Showers and
thunderstorms are forming near the leading surface boundary at the
present time.  Precipitable water values are 1.6-2" per GPS data. 
ML CAPE -- on both sides of the boundary -- is 1500-3500 J/kg per
SPC mesoanalyses.  The atmosphere is uncapped, as 700 hPa
temperatures are ~9C.

A layer of thicker clouds moving through southern AR and central
MS should act to limit much northward return of the boundary. 
This means that the mesoscale guidance is probably too far north
with its convective placement (by a row or two of counties or
parishes) which fits recent radar imagery trends.  A region of
effective bulk shear of 30 kts lies somewhat north of the forming
convection, which as it approaches, could help organize short,
training bands similar to yesterday.  A southward shift to the
convection is expected, like yesterday -- as forming cold pools
could act to keep the outflow boundary moving.  The worry is that
small waves in the boundary caused by organizing convection could
lead to periods were short periods of training could occur. 
Hourly rain totals to 2.5" with local amounts to 4" are possible
in this air mass.  Portions of the MPD area have experienced above
average rainfall during the past week, particularly yesterday in
MS, AL, and the western FL Panhandle.  These saturating soils in
addition to urban areas appear to be most at risk for widely
scattered instances of flash flooding. 

Roth

ATTN...WFO...BMX...FWD...HGX...JAN...LCH...LIX...MOB...SHV...
TAE...

ATTN...RFC...LMRFC...SERFC...WGRFC...NWC...

LAT...LON   32949445 32719258 32619087 32338907 31648689
            30848521 30268589 30228931 30879286 31869670
            32179688 32599557