Mesoscale Precipitation Discussion 1012
NWS Weather Prediction Center College Park MD
1245 PM EST Wed Nov 07 2018

Areas affected...Southern Louisiana, Southern Mississippi,
Ctrl/Srn Alabama

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

Valid 071744Z - 072330Z

Summary...Thunderstorms will increase in coverage into the
afternoon hours, particularly from southern Louisiana into
southern Mississippi. Rain rates to around 2 in/hr will be
possible in the strongest storms, which could lead to flash
flooding. The locations most at-risk for flash flooding would be
urban areas or places that receive sustained heavy rain rates for
a couple hours.

Discussion...Regional radars and GOES-16 IR satellite channels
showed two primary areas of convection as of 1730Z: (1) a forward
propagating, broken convective line pushing into central Alabama,
and (2) loosely organized convective clusters increasing in
coverage in southern Louisiana. The convective line pushing into
Alabama has been well-established for several hours, and has a
history of producing rain rates of 1-2 in/hr (per MRMS, WSR-88D
dual pol estimates, and surface observations). The progressive
nature of the convective line may reduce the overall flash flood
potential given the shorter duration of the attendant heavy rain
rates, but some localized flash flooding could occur where leading
convective cells merge into the line, or the trailing edge of the
convective line begins to take on more of a WSW-ENE orientation
(more parallel to the deep layer mean wind and surface front).
This trend with the trailing edge was already occurring per KMOB
and KBMX radars. Furthermore, the southern edge of the line should
be favored as instability is stronger further south and this would
be able to sustain higher rain rates. The southern end of the
convective line (moving into Choctaw and Marengo Counties in
Alabama) was in a region with MUCAPE around 1000 j/kg per RAP
analysis.

The mesoscale environment should favor a higher probability of 2+
in/hr rain rates and flash flooding in southern Louisiana and
southern Mississippi. A surface convergence boundary was evident
in the observations in the I-10 corridor, and a surface front
further north was showing signs of pushing south on GOES-16
visible satellite. These features should act in concert to lead to
a continued increase of convective coverage in that region.
Instability across southern LA/MS was higher than areas further
northeast, with MLCAPE values generally 2000-3000 j/kg. Given the
overlap with PWs around 1.7-1.9 inches (per CIRA blended TPW
product), this environment should allow for elevated rain rates in
the strongest and most organized convective clusters, possibly
exceeding 2 in/hr on a localized basis. The orientation of the
surface convergence zone and the major axis of some of the larger
convective clusters was generally parallel to the deep layer mean
wind vector. And with the low-level inflow (850mb winds around
20-25 knots) roughly equal to the deep layer mean wind speeds,
some backbuilding could also be favored in addition to the
potential for training. Therefore, flash flooding does appear
possible into the afternoon hours, and would be most likely in
urban areas where surface runoff would be greater.

Lamers

ATTN...WFO...BMX...JAN...LCH...LIX...MOB...

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

LAT...LON   33428683 33198566 32308564 31088749 30308918
            29599039 29719168 29829263 30059341 30469319
            30819197 31329053 32028885 32938791