MESOSCALE PRECIPITATION DISCUSSION 0170
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
909 PM EDT FRI MAY 18 2018

AREAS AFFECTED...WRN SC...WRN NC...FAR ERN TN...FAR SW VA

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 190109Z - 190339Z

SUMMARY...A FOCUSED THREAT OF FLASH FLOODING WILL AFFECT THE
UPSLOPE-PRECIPITATION-PRONE REGION FROM COLUMBUS NC...NORTHWARD.
THE THREAT MAY GRADUALLY END FROM SOUTH TO NORTH LATE THIS EVENING.

DISCUSSION...THE CONVECTIVE ENVIRONMENT HAD BECOME
FOCUSED...LEADING TO NEARLY STATIONARY THUNDERSTORMS WITH INTENSE
RAINFALL AROUND COLUMBUS NORTH CAROLINA AS OF 01Z. OTHER CELLS
WITH HEAVY RAIN WERE OCCURRING IN A REGION OF LESSER RADAR
COVERAGE FARTHER UP THE MOUNTAIN CHAIN TOWARD BOONE NC. GOES-16
INFRARED IMAGERY SHOWED CLOUD TOPS THAT HAD RAPIDLY COOLED AND
WERE PRODUCING A SMALL SCALE WEDGE SIGNATURE NEAR COLUMBUS...WHERE
DUAL-POL ESTIMATES FROM NEARBY KGSP RADAR WERE AROUND 2.5 INCHES
IN AN HOUR.

THE ENVIRONMENT WAS SUFFICIENTLY UNSTABLE AND LOCATED JUST
DOWNSTREAM OF A 250 MB TROUGH AXIS AND SMALL SPEED MAX OR
VORTICITY MAXIMUM IN NORTHEAST GEORGIA. THESE SYNOPTIC FEATURES
HAD ALSO CONTRIBUTED TO AN MCS AND ASSOCIATED OUTFLOW THAT WAS
PUSHING THROUGH SOUTH CAROLINA. RAIN-COOLED OUTFLOW HAS THE
POTENTIAL TO CAUSE CELLS AT THE SOUTHERN END OF THE MOUNTAIN-BASED
ACTIVITY TO WEAKEN IN THE LONG RUN...BUT MAY BRIEFLY BOOST
CONVERGENCE EVEN FURTHER.

INDEPENDENT OF THE OUTFLOW...THE SYNOPTIC FEATURES WILL SWEEP
NORTH TO NORTHEASTWARD...AND SHOULD SUPPORT DEEP CONVECTION AND
ASSOCIATED HEAVY RAINFALL IN THE UPSLOPE REGIONS...PERHAPS
REACHING UP INTO SOUTHWEST VIRGINIA WITH TIME. THE 12Z NSSL WRF
WAS PERFORMING VERY WELL OVER THE EASTERN U.S...AND ALONG WITH
SIGNALS FROM THE HRRR...IT APPEARS DEEP CONVECTION COULD PERSIST
THROUGH ABOUT 04Z. DURING THAT TIME THE RAINFALL INTENSITY MAY
WANE SOMEWHAT WITH THE LOSS OF HEATING AND AS THE ACTIVITY MOVES
DOWN GRADIENT TOWARD LESSER CAPE VALUES AT THE NORTH END OF THE
MPD.

UNFORTUNATELY...IMPACTS FROM FLASH FLOODING WERE LIKELY ALREADY
ONGOING...WITH SEVERAL INCHES OF RAIN ALREADY RECEIVED TODAY...AND
NEARLY STATIONARY STORMS EXPECTED IN THE NEAR-TERM. ADDITIONAL
FLASH FLOOD IMPACTS ARE LIKELY.

BURKE

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

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

LAT...LON   36768146 36558087 35078179 35148279 35678235