Mesoscale Precipitation Discussion 0884
NWS Weather Prediction Center College Park MD
655 PM EDT Sat Sep 22 2018

Areas affected...Extreme Northeast TN....Far Northwest NC...Blue
Ridge/Piedmont of Western and Southwest VA

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

Valid 222255Z - 230345Z

Summary...Slow-moving showers and thunderstorms will continue to
develop and focus along and near the Blue Ridge through the
evening hours. Flash flooding will remain likely.

Discussion...Radar imagery shows numerous slow-moving showers and
thunderstorms across the Blue Ridge and adjacent areas of the
Piedmont involving western and southwest VA. The convection has
been  focusing along an inverted trough north of a
quasi-stationary frontal zone where there is a notable axis of
moisture convergence into the higher terrain. Coinciding with the
moisture convergence has also been a weak instability axis with
MLCAPE values of as much as 500 to 750 j/kg as seen in the latest
RAP analysis.

GOES-16 1-minute clean IR satellite imagery has actually been
showing some cooling of the convective tops over the last 30 
minutes and suggestive of a bit more organization with stronger
forcing. There is some very modest right-entrance region jet
dynamics aloft which are likely helping to facilitate some of the
ascent, but much of the forcing is tied in with the aforementioned
moisture convergence, instability and orographic ascent.

The PWATs across the region remain a solid 1.5 to 2 standard
deviations above normal with values of 1.5 to 1.6 inches, and this
is expected to continue through the evening and will be conducive
to highly efficient rainfall processes given the focused moisture
convergence.

Rainfall totals again are expected to locally exceed 2 inches/hr,
and the latest HRRR-tle probabilistic guidance suggests this
threat will continue through at least 03Z. The thunderstorm
activity will tend to be maximized over southwest VA, but there
will be a threat also over far northwest NC and extreme northeast
TN closer to the front and where instability profiles are a tad
more favorable along with proximity of higher terrain as well.

Additional rainfall amounts of as much as 4 to 6 inches will be
possible through 03Z which is likely to cause more flash flooding,
and especially given the overall wet antecedent conditions and
locally steep terrain.

Orrison

ATTN...WFO...GSP...MRX...RAH...RNK...

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

LAT...LON   37928004 37877961 37687932 37227896 36557903
            36167952 36258021 36378084 36308112 36258159
            36188192 36188223 36328225 36578206 37058146
            37228127 37438102 37518069 37628034