Mesoscale Precipitation Discussion 0782
NWS Weather Prediction Center College Park MD
228 PM EDT Thu Sep 06 2018

Areas affected...Mississippi

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

Valid 061826Z - 062330Z

Summary...Bands of heavy rain with embedded thunderstorms are
expected over Mississippi this afternoon, to the east of the
circulation center of Tropical Depression Gordon. One of these
bands should affect a portion of northwest Mississippi near
Interstate-55 that has already received significant rainfall
earlier today. This will likely lead to flash flooding in those
areas, with flash flooding possible elsewhere across the state.
Rain rates could reach 2 in/hr in narrow swaths near the center of
some of these rain bands.

Discussion...Since 17Z, GOES-16 satellite imagery has shown a band
of increasingly organized convection along the I-55 corridor in
Mississippi, with several other convective bands in southeast
Mississippi also becoming stronger. The Cloud Phase Distinction
RGB showed increasing ice phase cloud tops with the I-55 band, and
the deeper convection was beginning to produce some sporadic
lightning and increasingly heavy rain rates. The Air Mass RGB and
Simple Water Vapor RGB both show some pockets of drier mid-upper
level air over central Mississippi (as opposed to the area near
Gordon's circulation center), which may allow for some greater
convective instability, but also reduce some of the rainfall
efficiency with the convection in the area. Nevertheless, with
PWATs still around 2 inches, the environment should still be
favorable for rain rates to around 2 in/hr. Furthermore, the
largely uniform wind direction through the lowest 6km should help
channel convective bands into training patterns.

The convective band near I-55 was intersecting an area that
received substantial rainfall in the past 24 hours, over Holmes,
Carroll, Montgomery, and Grenada Counties. NSSL QPE showed a
pocket of over 8 inches of rainfall near the Carroll and Holmes
County borders, and this was precisely the same area receiving
heavy rain now. Radar trends suggested that this may not persist
through the whole afternoon, but even an additional 1-3 hours of
rain (falling at the rate of 1-2 in/hr) on saturated soils would
likely produce flash flooding. Significant flooding occurred
earlier this morning, with some water rescues reported. The
redevelopment of significant flooding, perhaps in new nearby
areas, will be possible in this small region of NW MS,
particularly if the convection can persist for 2 hours or more.

Elsewhere, the potential of training rain bands likewise exists,
but antecedent conditions may be less favorable for flash
flooding. Nevertheless, flooding issues will be possible over much
of Mississippi this afternoon. The Air Mass and Simple Water Vapor
RGBs from GOES-16 did show a pocket of deeper moisture in
southeast Mississippi, which could favor a greater concentration
of convection in that area.

Lamers

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

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

LAT...LON   34738990 34688925 33918863 32908863 31988871
            31128871 31068971 31899055 32649056 33579028
            34279034