Mesoscale Precipitation Discussion 0876
NWS Weather Prediction Center College Park MD
847 PM EDT Fri Sep 21 2018

Areas affected...Arkansas, NW Mississippi, Far SW Tennessee

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

Valid 220046Z - 220546Z

Summary...Slow-moving thunderstorms will persist over Arkansas
this evening and into the early parts of the overnight, possibly
extending east along a front into the Mississippi-Tennessee border
region. These thunderstorms will produce heavy rain rates, which
could reach 2-3 in/hr in the strongest storms. This would be
likely to lead to flash flooding, and appears most likely in
central Arkansas over the next several hours.

Discussion...KLZK radar shows slow-moving thunderstorms and
scattered convective rain showers slowly drifting NNE in Arkansas.
These were generally following the 850-300mb deep layer mean wind,
but were also exhibiting a gradual drift to the east and southeast
with time toward a maximum in low-level thetae and MLCAPE (to
around 1500 j/kg) in southeast Arkansas and northern Mississippi.
One prominent band of convection was situated from about 30NW of
Hot Springs, to near Hot Springs, to near Camden. KHOT ASOS
measured 1.49 inches of rain in around 30 minutes with this
convective band, confirmation of the significant rain rates in
central Arkansas. Although KLZK ZDR does not indicate extremely
small droplets (ZDR clearly 1-2 dB in most of the strongest
convection), elevated KDP values around 2-3 deg/km still indicate
a high concentration of water droplets and a strong likelihood of
very heavy rain rates. Other than a few of the strongest cells,
MRMS vertically integrated ice suggests that most of this is
likely to be liquid water with a lack of much hail mixing in.
Therefore, rain rates up to 3 in/hr seem achievable, especially
when considering an environment with PWATs on the order of 2.0 to
2.2 inches.

Flash flooding seems likely given these potential rain rates,
particularly near where the stationary front draped across central
Arkansas will intercept the convective band lifting slowly north.
Radar already shows some shallow convection closer to the front,
and there may be enough coalescing to support a couple hours of
heavy rain at certain locations from Hot Springs and Pine Bluff
north into the Little Rock metro area. This will be near the
convergence of some mesoscale boundaries and at the northwest tip
of a low-level thetae axis, with weak low-level southeasterly
inflow, which should all favor a more focused area of heavy
rainfall in central Arkansas. Additional heavy rain and flash
flooding cannot be ruled out further east along the surface front
with slow-moving storms, as they may be able to tap into greater
instability, which could offset some lower PWATs in that area.

Lamers

ATTN...WFO...JAN...LZK...MEG...SHV...TSA...

ATTN...RFC...ABRFC...LMRFC...

LAT...LON   35568942 35088829 34008969 33239183 33329346
            33919452 35299447 35299212