Mesoscale Precipitation Discussion 0897
NWS Weather Prediction Center College Park MD
1129 PM EDT Tue Sep 25 2018

Areas affected...Eastern Tennessee, Eastern Kentucky

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

Valid 260328Z - 260830Z

Summary...Thunderstorms should persist overnight across parts of
eastern Kentucky and eastern Tennessee, and could produce rain
rates around 1-2 in/hr in some cases. This could lead to flash
flooding, given relatively wet ground conditions already.

Discussion...Regional radars showed loosely organized convective
clusters and line segments stretching from eastern Kentucky into
eastern Tennessee, situated in an axis of high precipitable water
values. RAP analysis showed PW around 2 inches. The convection was
also positioned just downstream from an instability axis, with
MLCAPE values in excess of 1000 j/kg nosing into the greater
Nashville area. Winds at 925-850mb were out of the southwest (per
OHX and HTX radar VWPs), and should favor some backbuilding given
the upstream position of stronger instability (with minimal CINH).
And this has been observed to a certain extent already, with only
a slight drift to the east noted over the past 90 minutes with the
average axis of the highest reflectivity in eastern Tennessee.
Hi-res models reflect this same pattern, and thus show a fairly
strong signal for heavy rainfall in the next several hours
(particularly 03-06Z). The 00Z HREF has neighborhood probabilities
of over 2 inches of rain from 03-09Z as high as 50-60 percent in
eastern Tennessee. This is noteworthy given the reduced flash
flood guidance across the area, with 6hr FFG as low as 1.5 to 2.0
inches in some areas, and 1hr FFG as low as around 1 inch. These
sort of rain rates should be achievable in an environment with
significant deep moisture available, coupled with moderate
instability (for some storm organization). The lack of a strong
low-level boundary should limit the overall organization somewhat,
though, but enough backbuilding is being observed that localized
heavy rainfall totals in excess of 2-3 inches appear possible, and
that could lead to flash flooding.

Lamers

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

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

LAT...LON   37658383 36558358 35038474 34988598 35868597
            37348476