Mesoscale Precipitation Discussion 0804
NWS Weather Prediction Center College Park MD
204 PM EDT Sat Sep 08 2018

Areas affected...Ern Kentucky, Srn West Virginia, Wrn Virginia,
Far Nrn TN/NC

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

Valid 081802Z - 082330Z

Summary...Scattered thunderstorms should develop from central and
eastern Kentucky, into the central Appalachians, and into southern
Virginia this afternoon. Some of the storms will likely produce
locally heavy rainfall, and could lead to flash flooding. Rain
rates around 1 in/hr will be common in most of the thunderstorms,
with rates to around 2 in/hr possible in the strongest storms.

Discussion...GOES-16 visible satellite showed a relatively sharp
southern edge to an expanse of stable cloud layers over the Ohio
River Valley and Mid Atlantic. This generally marked the position
of a surface front, and the air mass to the south of this front
was becoming increasingly unstable in the early afternoon hours.
Sufficient low-level moisture exists across the warm sector from
eastern Kentucky into western Virginia, with surface dewpoints in
the upper 60s to around 70 on a widespread basis. MLCAPE values
were around 1500 j/kg and this should be sufficient to support
some scattered stronger thunderstorms during the afternoon and
early evening hours. As the convection will be developing in the
warm sector south of the front, there may be a lack of focusing
mechanisms for more concentrated bands of convection. However,
relatively slow storm motions are expected, and Corfidi vector
magnitudes are generally less than 10 knots. Therefore, although
convection should be scattered, some localized heavy rainfall is
expected and this may lead to flash flooding. Flash flood guidance
across a considerable percentage of the outlined area is less than
2 inches in one hour, and the 12Z HREF shows probability of
exceeding flash flood guidance around 20-30 percent this
afternoon. Therefore, there is certainly a chance of flash
flooding, with the highest chances likely in urban areas or basins
with steeper terrain gradients.

Lamers

ATTN...WFO...GSP...JKL...LMK...LWX...MRX...RAH...RLX...RNK...

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

LAT...LON   38687984 37627941 37137865 36447900 36268011
            36078158 36468226 36828311 36828402 37088478
            37458551 37958557 38178509 38188437 38068369
            38098247 38608096