Subj : MESO: Heavy Rain/Flooding
To   : All
From : Mike Powell
Date : Tue Nov 21 2023 09:24:00

AWUS01 KWNH 211027
FFGMPD
FLZ000-ALZ000-MSZ000-211615-

Mesoscale Precipitation Discussion 1182...Corrected
NWS Weather Prediction Center College Park MD
527 AM EST Tue Nov 21 2023

Corrected for minor updates to discussion wording

Areas affected...east-central Gulf Coast into southern AL

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

Valid 211015Z - 211615Z

SUMMARY...A highly localized flash flood threat will exist from
the MS/AL/FL Gulf Coast into southern AL over the next 3-6 hours
ahead of an approaching cold front. Training with peak rainfall
rates of 2-3 in/hr will be possible where cell training is able to
persist the longest.

DISCUSSION...Trends in infrared satellite imagery over the past 3
hours have shown a decrease in the coverage of colder cloud tops
and lightning over the central Gulf Coast, indicative of weakening
convective intensity. MRMS showed similar trends with a decrease
in the coverage of hourly rainfall in excess of 1 inch. However,
anomalously high moisture remained ahead of a cold front extending
through the lower Mississippi Valley, coincident with a broken
line of convection that was observed from southwestern AL into and
across the Mississippi River Delta of southeastern LA at 10Z. The
10Z SPC mesoanalysis showed PWATs of 1.5 to 1.9 inches (GPS
observations confirmed) and MLCAPE of 500-1000 J/kg along the Gulf
Coast region, extending ~75 miles inland into southwestern AL.

Therefore, despite the overall weakening trend in convection,
there remains low end potential for training of heavy rain with
potential for 2-3 in/hr rainfall rates should cells manage to
align with the SW to NE steering flow. This potential will focus
over a relatively small region of the central Gulf Coast into
southwestern AL where continued 925-850 mb winds of 35-45 kt will
maintain low level moisture transport and a relatively narrow axis
of 500-1000 J/kg MLCAPE over the next few hours, with some
increase in instability into portions of southern AL. However, dry
antecedent conditions and generally high flash flood guidance
values should limit any runoff concerns to where heavy rain
overlaps with urban areas or other locations of poor drainage.

Otto


ATTN...WFO...BMX...LIX...MOB...TAE...

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

LAT...LON   32248781 32238678 31548610 30588589 30058623 
            29998736 30208901 31508861 

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