Mesoscale Precipitation Discussion 0776
NWS Weather Prediction Center College Park MD
915 PM EDT Wed Sep 05 2018

Areas affected...Northern and Central Mississippi...

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

Valid 060115Z - 060715Z

SUMMARY...Increasing convection with back-building/training
component associated with T.D. Gordon poses flash flooding risk
over the next few hours. 

DISCUSSION...01Z surface analysis denotes the center of Tropical
Depression Gordon across West central MS at this time providing a
generally confluent low level flow environment for moisture
convergence along northeastern outer banding to enhance.  Ample
deep moisture with sfc TDs in the mid-70s and 00z JAN sounding
supports saturated environmental to 450mb with total PWAT near
2.5".   While profile is saturated, this also limits the available
instability, which is in the range of 500 J/kg of MLCAPEs, still
wedge of more unstable air is lifting northward in the low level
confluent flow across SE MS off the Gulf with values up to 1500
J/kg. 

GOES-16 WV suite, depicts an elongated SW-NE trof axis though the
ML/LL WV channels due suggest a strengthening SW jet on the SE
quadrant.  While this would leave the NE quadrant in the left exit
of the jet streak...the larger scale diffluence is sufficient to
support increased upper level evacuation.  As such, low level
winds have responded with a slight uptick increasing instability
advection and moisture convergence.   This speed max is likely
also related to a lobe of increased vorticity and therefore DPVA
that will rotate through the east side of the circulation over the
next 4-6hrs allowing for increased convective development.

As such, 10.3um IR channel shows cycling deeper convection at the
best interface near Montgomery/Grenada counties. Regional RADAR
mosaic and 3.9um IR also denote with the strengthening low level
inflow, propagation vectors have swung around and support
back-building, and are generally near or opposite of the mean cell
motions (given cells are fairly shallow).  GWX radar 1hr rates are
increasing over 1.5" but given the lack of lightning and depth of
the cells, there should be more tropical warm cloud processes
occurring that are poorly handled by RADAR even in tropical mode. 
Rates of 2-2.5" should be expected.  As such a narrow band 3-5"
totals are possible from Tallahatchie to Attala/Winston counties. 
The good news, the band appears to be oriented favorably between
areas affected more significantly where FFG values are higher.  As
such flash flooding is possible hedging toward likely over the
next 4-6hrs.     

Gallina

ATTN...WFO...JAN...MEG...

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

LAT...LON   34798979 34238903 32948893 32188920 32348980
            32979013 33479078 33969090 34509046