Mesoscale Precipitation Discussion 1072
NWS Weather Prediction Center College Park MD
454 PM EST Thu Dec 27 2018

Areas affected...Central Louisiana into Central Mississippi

Concerning...Heavy rainfall...Flash flooding likely

Valid 272153Z - 280353Z

Summary...Increasing risk for significant flash flooding as a line
of storms stalls or slowly moves east this evening. Potential
exists for an additional 3-4" (locally higher possible), bringing
event totals as high as 8" in places.

Discussion...As of 22z, a mesoanalysis showed strongly backed flow
off the Gulf of Mexico across portions of LA into central MS,
while a cold front marched east across eastern TX. The highly
convergent flow in the lower levels has continued to funnel in
anomalously high moisture (mid/upper 60 dewpoints; PWATs near
1.7") into the area. Instability remains sufficient and is
expected to do so through the evening hours with MUCAPE 1000-1500
J/kg. While the northern end of the squall line, tied to the
better mid/upper level forcing, races eastward across northern MS,
TN, and KY, the flow will remain more relaxed and parallel to
storm motions across LA and southern MS. Finally, GOES-16
mesoscale sector IR imagery over the area shows cloud tops
continuing to cool in pockets of deeper convection.

Hi-res models still are not capturing the southern end of the
squall line well, being too fast (east) or too north with the axis
of heaviest rainfall. Conceptually, based on the environment and
current radar trends, the heaviest rainfall is likely to be across
central LA into central MS and eventually work its way into
portions of west-central AL by mid-evening. While not spatially
correct, several hi-res members paint several inches of QPF (HRRR
6-hr QPF ending 04z showing corridor of 3-4" with locally higher
amounts), which seems realistic given the reports so far and the
environment that is expected to be maintained for several more
hours.

As such, flash flooding is likely across the area and could be
locally significant where additional amounts of 3-4" (locally
higher possible) are not out of the question. So far, several
mesonet observations have reported 1-hr totals in the 1-2" range
with 6-hour totals ending 22z showing a widespread 2-4" with some
locally higher amounts.

As the soils saturate, any additional rainfall will become
problematic especially over sensitive and urban areas. The risk of
flash flooding will continue past 04z and additional MPDs may be
necessary downstream.

Taylor

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

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

LAT...LON   33398758 33208638 31438631 30708776 30488808
            30118927 29189036 29169122 29459161 29499205
            29819186 30459127 31009067 31668941 32258896
            32868873 33378841