Mesoscale Precipitation Discussion 1180
NWS Weather Prediction Center College Park MD
516 PM EST Mon Nov 20 2023

Areas affected...Central LA through Central MS

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

Valid 202215Z - 210415Z

SUMMARY...Areas of training showers and thunderstorms will drive
concerns for isolated to widely scattered instances of flash
flooding heading into the evening hours from areas of central LA
northeastward through much of central MS.

DISCUSSION...Satellite and radar imagery continue to show a
gradual increase in the coverage and intensity of convection
across portions of the Lower MS Valley and the Mid-South as a deep
upper trough continues to amplify upstream over the southern
Plains and into the Arklatex region.

The arrival of additional strong shortwave/jet-energy rounding the
base of the upper trough will be interacting with a strengthening
southwest low-level jet of 40 to 50 kts over the next several
hours which will continue to foster a relatively widespread and
organized convective outbreak. MLCAPE values have risen to as much
as 1500 J/kg along and just ahead of a frontal zone draped over
the region from central LA up into west-central to northwest MS.
This airmass is becoming increasingly moist with time given the
low-level jet, and PWs are forecast to rise to near or a bit above
1.5 inches this evening as the front along with multiple waves of
low pressure help to concentrate and focus the convective threat.

Strengthening shear profiles coupled with the increasingly
moist/unstable airmass should continue to favor convection that is
characterized by a combination of supercells and multicell
clusters heading into the evening hours. However, the activity
should become increasingly aligned with the deeper layer mean
steering flow along and just ahead of the front, and this will
strongly favor concerns for at least some cell-training.

The 18Z HREF guidance along with several runs of the HRRR suggest
rainfall rates may reach as high as 1.5 to 2.5 inches/hour with
the stronger cells, and the training of this activity over the
same area may favor some storm totals heading into the mid to late
evening hours as high as 3 to 6 inches. Areas of central LA into
especially central MS are generally the corridors of greatest
concern.

The antecedent conditions across the region are generally very
dry, with high FFGs, low streamflows per USGS data, and NASA-SPoRT
data showing anomalously low soil moisture percentiles. However,
the degree of cell-training that is expected coupled with the high
rainfall rates that may get into some urbanized locations suggests
that at least some threat of flash flooding will exist over the
next several hours.

Orrison

ATTN...WFO...JAN...LCH...LIX...SHV...

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

LAT...LON   33678951 33638883 33148847 32538875 31828946
            31289063 31089241 31239312 31699310 32499177
            33059103 33449027