Mesoscale Precipitation Discussion 0878
NWS Weather Prediction Center College Park MD
1143 PM EDT Fri Sep 21 2018

Areas affected...Central Tennessee, Southern Kentucky

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

Valid 220342Z - 220915Z

Summary...Slow-moving thunderstorms will continue to develop and
meander around portions of central Tennessee and southern Kentucky
tonight. These storms could produce rain rates to around 2 in/hr,
which may lead to some localized areas of flash flooding.

Discussion...Regional radars show scattered convection across the
Mid South from central Tennessee into far southern Kentucky. RAP
analysis showed a plume of 500-1000 j/kg MLCAPE along a surface
front in the same region, with a slight increase in precipitable
water values to around 2 inches. This is likely due to a plume of
mid-upper level moisture, evident above 700mb on the CIRA layer PW
product, beginning to push east. Therefore, the rainfall
efficiency in some of the convective bands and thunderstorms may
be on the increase in the past hour or so. Rain rates to around 2
in/hr seem likely, and locally higher amounts couldn't be ruled
out. The RAP forecast sustains a plume of at least 500 j/kg MLCAPE
well into the night, so there should be sufficient instability to
sustain convection for several hours. The area was also in a
weakness in the mid-level flow, with a 700mb ridge centered
nearby. Therefore, observed storm motions were fairly slow.
Corfidi vector magnitudes were generally under 10 knots. Given a
lack of strong low-level forcing, larger convective clusters may
begin to propagate in different ways based on the effects of
convective outflow and chaotic backbuilding storm motion vectors.
This could lead to periods of effectively stationary convective
bands through storm mergers or anomalous motion. The combination
of fairly high rain rates and slow storm motions should create the
possibility of some localized flash flooding tonight, even with a
lack of significant rainfall in recent weeks (soil moisture not
appreciably high in this region).

Lamers

ATTN...WFO...HUN...JKL...LMK...MEG...OHX...PAH...

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

LAT...LON   37628538 37428426 36908441 36158573 35108656
            34678746 34948862 35998865 37108689