Mesoscale Precipitation Discussion 0888
NWS Weather Prediction Center College Park MD
331 PM EDT Sun Sep 23 2018

Areas affected...Central/Northern MS...TN...Far Southwest to
South-Central KY...Far Northwest AL

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

Valid 231930Z - 240130Z

Summary...Showers and thunderstorms are expected to become more
widespread and concentrated in nature this afternoon and evening.
Heavy rainfall rates and wet antecedent conditions will likely
result in some flash flooding.

Discussion...The latest GOES-16 WV imagery suite is showing a
modest 500 mb shortwave and associated trough axis advancing east
across central/eastern AR this afternoon with an axis of upper
level divergence noted downstream from this over northern MS and
much of western/central TN. Meanwhile, a wave of low pressure is
noted along a front near Tunica, MS. Ahead of this surface wave is
plenty of deep layer moist southerly flow and also an increasingly
unstable boundary layer, with the latest rap analysis showing as a
nose of MLCAPE upwards of 1500 j/kg pooling up ahead of a cold
front entering northwest MS and then extending east along a
stationary front across southern TN to the east of the
aforementioned surface low.

Over the next few hours, stronger instability is expected to nose
up across northern MS and into southwest and south-central TN as
additional diurnal heating further destabilizes the boundary
layer. Given the approaching upstream shortwave energy and focused
low level convergence near the front, a more widespread and
focused area of heavy showers and thunderstorms will be expected
through the remainder of the afternoon and into the evening hours.
While the most concentrated activity should be near the front,
there will be sufficient large scale warm advection and forcing
aloft for at least steady moderate to heavy rainfall with some
embedded elevated convection farther north up across northwest TN
and into southwest to south-central KY.

PWATs across the region remain quite high with values of 1.8 to
2.2 inches, with the greatest moisture concentrated up across
northern MS. Given the depth of moisture, highly efficient
rainfall processes are likely to result in enhanced rainfall rates
with hourly amounts of as much as 2 to 2.5 inches possible within
stronger convective cores.

The recent runs of the HRRR have been supporting more concentrated
heavy rainfall developing across areas of western/central TN and
locally over northern MS over the next several hours which will be
rainfall on top of what has already occurred over the last 24
hours. The antecedent conditions are quite wet here, and with the
HRRR supporting an additional 2 to 4 inches with local amounts to
near 5 inches through mid-evening, flash flooding will become
increasingly likely. Will continue to monitor this closely.

Orrison

ATTN...WFO...BMX...HUN...JAN...LMK...MEG...MRX...OHX...PAH...

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

LAT...LON   36868749 36808663 36668567 36338452 35708433
            35248493 34988645 34768721 34418799 33588898
            33108957 33129030 33529058 34259024 35248998
            35798965 36358921 36768846