Mesoscale Precipitation Discussion 0899
NWS Weather Prediction Center College Park MD
956 AM EDT Wed Sep 26 2018

Areas affected...central MS into northern AL, northwestern GA and
eastern TN

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

Valid 261355Z - 261845Z

Summary...Maximum rainfall of 3-5 inches is expected for a couple
of locations across central MS into northern AL, northwestern GA
and eastern TN through 19Z. Overlap with wet antecedent conditions
may produce flash flooding.

Discussion...Earlier heavy rainfall near Tupelo, MS  was reported
to be 4-7 inches since 04Z with rain rates between 1 to 2.5 in/hr.
Regional radar imagery as of 1330Z showed that heavy rainfall had
shifted east of Tupelo to the north-central MS/AL border, with
repeating rounds of heavy rain extending east-northeastward into
southern TN and northwestern GA. Radar estimates and ground truth
supported localized rainfall rates of 1-2 in/hr across southern TN
into northern AL. This activity was occurring just ahead of a
pre-frontal 850 mb trough axis extending southwest to northeast
across MS/AL/TN at 1330Z according to VAD wind plots. Precipitable
water values ahead of the 850 mb trough axis ranged from 2.0 to
2.4 inches via GPS and RAOB data from 12Z, with 20-30 kt 850 mb
flow from the southwest. The wet bulb zero height at JAN was just
shy of 15 kft, supportive of efficient warm rain processes. Aloft,
12Z upper air data at 250 mb showed a max in diffluence over the
tri-state region of TN/AL/GA.

Short term forecasts show the 850 mb trough axis slowly pivoting
eastward with a slight weakening of 850 mb flow. However, deep
moisture and only 20 kt of 850-300 mb mean flow supports near zero
Corfidi Vector magnitudes which would be supportive of training
across portions of the Gulf Coast states into the foothills of the
southern Appalachians. Forecast instability over eastern TN of
250-750 J/kg may limit rainfall rates across northern locations of
the MPD threat area, but in the vicinity of Chattanooga, 4-6
inches of rainfall over the past 48 hours has saturated soils
increasing the threat for flash flooding. Farther south into
central MS and central AL, Flash Flood Guidance is higher, but
greater instability should help to support rainfall rates in
excess of 2 in/hr which may cause flash flooding in more sensitive
 locations.

Otto

ATTN...WFO...BMX...FFC...GSP...HUN...JAN...MEG...MRX...OHX...

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

LAT...LON   36238383 35598371 34768432 33838529 32658717
            32008966 32509030 33318951 34248798 35528662
            35928552