Mesoscale Precipitation Discussion 0004
NWS Weather Prediction Center College Park MD
1100 PM EST Mon Jan 08 2024
Areas affected...portions of southeastern LA...southern and
east-central MS...southern and west-central AL...far western FL
Panhandle
Concerning...Heavy rainfall...Flash flooding likely
Valid 090400Z - 091000Z
Summary...Numerous instances of flash flooding are likely
overnight with additional localized totals of 1-6" expected. There
will also be a threat for locally significant flash flooding in
low-lying and poor drainage areas, particularly across portions of
southeastern MS into southwestern AL.
Discussion...An MCC (mesoscale convective complex) is in the
process of maturing across the Deep South this evening, in
association with a low pressure system (located near the LA/MS
border) and accompanying strong baroclinicity. This low pressure
system itself is associated with a much larger cut-off low and
upper-level trough to the northwest (centered over OK), and an
exceptionally large area of cold cloud tops (-50 degC or colder
via GOES-East IR imagery) extend from a broad area of the
Southeast northward through the TN/OH Valley and Midwest.
Honing in on the mesoscale and our low pressure system of
interest, an embedded area of very cold cloud tops (-70 degC or
colder) are evident near and just east of the low center (along
and ahead of the warm front). These tops are in association with
the strongest convection, where MRMS hourly estimates have ranged
from 1-3" (despite rather fast storm motions). As the warm front
continues to lift northward from the Gulf coast, increasing
amounts of instability (expanding 1000-2000 J/kg of MU CAPE) will
be on tap for the convection to take advantage of, and certain
locations will end up with repeated 1-2"/hr rainfall rates due to
intense convection along and ahead of the warm front followed by
an intense squall line from the west (favoring southern MS/AL, a
bit inland from the coast). All of this heavy rainfall will be
facilitated by immense moisture transport from a 60+ kt low-level
jet, evident by strong isentropic lift across a broad range of
isentropic surfaces (from 285K through 305K). When this moisture
transport (the product of wind speed and mixing ratio) is analyzed
via a single isobaric surface (i.e. 925 mb), the traditional scale
is quickly maxed out across the warm sector with 30+ m/s of
moisture transport across southeast LA and southern MS/AL (as well
as a massive portion of the Gulf of Mexico).
Going forward, the new (00z) hi-res guidance has fairly good
agreement with regard to the location for heaviest rainfall
(generally southeastern MS into southwestern AL, a bit inland from
the coast). While the magnitudes of the various HREF members vary,
the probability matched mean (PMM) indicates this region has could
see localized totals of 3-6" through 10z. Some guidance indicates
a bit less than this (including most of the recent HRRR runs which
cap out at 4-5"), while one outlier member (the ARW2) indicates
the potential for 6-9" totals. The HREF PMM solution is considered
to be the most likely scenario, indicating localized totals of
1-3" prevailing outside of the hatched area.
Given that streaks of 1-3" of rainfall have already occurred
across much of the area (with some places having already been
warned for ongoing or imminent flash flooding) with additional
1-6" amounts forecast, numerous instances of flash flooding are
considered to be likely with a locally significant flash flood
threat continuing through the overnight hours. The threat for
significant flash flooding is most likely in low-lying, poor
drainage areas (including across hydrophobic urbanized terrain).
Churchill
ATTN...WFO...BMX...HUN...JAN...LCH...LIX...MEG...MOB...TAE...
ATTN...RFC...LMRFC...SERFC...NWC...
LAT...LON 34068738 33768609 33008576 32068580 31298605
30568657 30348768 30358883 29859020 29449138
29669228 30179212 30469190 31209116 31959047
32758972 33398898 33928818
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