Mesoscale Precipitation Discussion 0584
NWS Weather Prediction Center College Park MD
500 PM EDT Thu Aug 02 2018

Areas affected...Piedmont Region including parts of GA, SC, NC, VA

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

Valid 022100Z - 030300Z

Summary...Thunderstorms were becoming increasingly widespread
across portions of the western Carolinas and into south-central
Virginia. This should continue into the evening, with an
increasing threat of heavy rainfall and flash flooding. Rain rates
of 1-2 in/hr will be more common, with 2-3 in/hr possible in
localized areas.

Discussion...Visible satellite, radar, and surface observation
trends show several boundary collisions either ongoing or likely
to happen in the near future, and these were focusing increasing
convective activity. The first was taking place in west-central
North Carolina, from near Rocky Mount, VA south to Winston Salem,
NC, and then to an area just east of Charlotte, NC. This should
continue to focus a band of thunderstorms for at least another
hour or two with the potential for heavy rainfall and flash
flooding. This also overlapped with some areas that received heavy
rainfall (and some reported flooding) earlier this morning, so
additional rainfall in these areas would be likely to cause flash
flooding.

Another boundary collision appeared likely to take place closer to
Upstate South Carolina. A composite of the Atlantic sea breeze and
thunderstorm outflow was pushing quickly to the WNW into western
parts of the Carolinas, while another band of convection was
beginning to bend to the east closer to the GSP area. This may
eventually lead to enhanced low-level convergence and a focused
band of thunderstorm development from near Aiken, SC north to an
area just west of Charlotte, NC. Surface observations reported
wind speeds of around 10 knots (from opposite directions) behind
both of the approaching boundaries, so low-level convergence would
be expected to be relatively strong. Hi-res models such as recent
runs of the HRRR, and the 12Z HREF, also focus some of the heavier
QPF in the region in this same area through 03Z. Therefore, flash
flooding will become possible in some of these areas too.

Zooming back out to the overall mesoscale environment: generally
confluent low-mid level flow and PWs over 2 inches should support
an increasingly focused area of heavy rainfall into the early
evening hours. In the outlined area, flash flood guidance is most
reduced in parts of northwest North Carolina and into southern
Virginia, so this may be where flash flooding would be favored.
However, flash flooding cannot be ruled out further south into
South Carolina and far eastern Georgia as well.

Lamers

ATTN...WFO...CAE...FFC...GSP...RAH...RNK...

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

LAT...LON   37507938 36977908 35697949 34498036 33748097
            33028181 33868285 35478181 37128014