Mesoscale Precipitation Discussion 0891
NWS Weather Prediction Center College Park MD
109 PM EDT Mon Sep 24 2018

Areas affected...eastern TN into central/eastern KY and southern
OH

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

Valid 241708Z - 242230Z

Summary...Rainfall rates of 1-2 in/hr are expected through the
afternoon across portions of eastern TN into central/eastern KY
and perhaps southern OH. Given low Flash Flood Guidance (FFG),
flash flooding will be possible.

Discussion...17Z surface observations showed a slow moving front
draped from the southern Mid-Atlantic states into the TN and lower
OH Valleys with extensive cloud cover to its north. 12Z RAOBs
indicated abundant moisture is in place with precipitable water
values near 2 inches from the lower MS Valley into the lower OH
Valley and modest 850 mb flow of 20-30 kt over KY and TN. Regional
radar imagery over the past 1-2 hours has largely shown radar
estimated rainfall rates below 1 in/hr over the region, with the
exception of northwestern GA where estimated and observed rates
were in the 1-2 in/hr range. Instability was a significant
limiting factor farther north where rainfall was more stratiform
in nature.

Short term forecasts from the RAP indicate that as a shortwave
(currently over the lower OH Valley) lifts toward the northeast
through 00Z, a diffuse warm front will lift north with greater low
level moisture advecting into eastern KY and OH, supporting
increasing MUCAPE into the 500-1500 J/kg range by 21Z. In
addition, an axis of 850 mb confluence will become better defined
across eastern TN/KY, which should help to support areas of
south-north training of heavy rain. Aloft, the right entrance
region of a 50-70 kt 250 mb jet streak over IN/OH may help to
focus added lift across the OH River Valley through the remainder
of the afternoon. As instability increases, even slightly given
the abundant moisture, rainfall rates of 1-2 in/hr will be
possible but cannot rule out locally higher rates if training sets
up favorably, leading to 6 hr rainfall maxima of 3-5 inches
through 23Z. Given low 3-hr FFG values of 1-2 in/hr, flash
flooding will be possible, although coverage of exceedance within
the MPD threat area appears like it will be scattered.

Otto

ATTN...WFO...FFC...HUN...ILN...JKL...LMK...MRX...OHX...

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

LAT...LON   39228442 39088361 37678366 36638374 35148426
            34688464 34838559 35698631 36758616 37348598
            38488541