Mesoscale Precipitation Discussion 0737
NWS Weather Prediction Center College Park MD
1253 PM EDT Thu Aug 30 2018

Areas affected...portions of the central/southern Appalachians

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

Valid 301651Z - 302250Z

Summary...Slow moving thunderstorms are expected to increase in
coverage across portions of the central and southern Appalachians
through the afternoon with rainfall rates of 1-2 in/hr and storm
total potential of 2-4 inches through 00Z. Given the sensitivity
of the terrain, flash flooding will be possible.

Discussion...Visible satellite imagery through 1630Z showed the
early development of thunderstorms south of a quasi-stationary
front extending from WV into central KY/TN. Ongoing activity was
over eastern KY into southern WV with observed storm motions of 10
kt or less, occurring just ahead of a 700 mb trough axis analyzed
with 12Z RAOB data. 12Z RAOBs and recent RAP analysis soundings
showed the 850-300 mb mean wind, a proxy for storm motions, was 5
to 15 kt, slower to the south across the entire Appalachian
region. Weak low level capping was already being overcome as
daytime heating has allowed temperatures to rise well into the 80s
with RAP forecast MLCAPE values in the 1000-2000 J/kg range by
21Z. Precipitable water values ranged anywhere between 1.5 and 1.9
inches, which corresponds to the 75th to 90th percentile,
sufficient to generate high rain rates when coupled with
increasing instability.

Given breaks in cloud cover, thunderstorm coverage is forecast to
increase through the afternoon with scattered areas of flash
flooding expected from eastern TN/western NC into western VA, WV
and eastern KY through late afternoon. 0-6 km bulk shear and
850-700 mb flow is generally 20 kt or less which should limit
storm organization. However, slow storm movement, terrain
sensitivity and high moisture with sufficient instability should
be enough to generate a few areas where storms temporarily
stall/hang up, producing locally 2-4 inches of rain in a 1-3 hour
time frame.

Otto/Weiss

ATTN...WFO...FFC...GSP...JKL...LWX...MRX...PBZ...RLX...RNK...

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

LAT...LON   39127983 39087941 39057869 38637833 38107879
            37437965 36708026 35898145 35238241 34868327
            34768402 35098437 35528412 35818342 36218343
            36258406 36098455 36298467 36658439 37038352
            37868259 38278213 38838096 39028044