Mesoscale Precipitation Discussion 0855
NWS Weather Prediction Center College Park MD
521 AM EDT Mon Sep 17 2018

Areas affected...eastern WV into western VA

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

Valid 170920Z - 171515Z

Summary...The threat for flash flooding will increase across
eastern WV into western VA through the remainder of the morning.
Rainfall rates of 0.5 to 1.0 in/hr will be possible with 6 hour
totals of 1-3 inches through 15Z.

Discussion...09Z infrared satellite imagery and area VAD wind
plots placed the 850 mb circulation associated with Tropical
Depression Florence over eastern KY. The core of the strongest 850
mb winds, which were 40-50 kt, was located along the central WV/VA
border, oriented roughly 90 degrees to the spine of the
Appalachians. However, largely stratiform rain and a lack of
higher reflectivity values on area radars suggests that
instability is limited. The latest SPC mesoanalysis page suggests
MUCAPE is 250 J/kg or less across the WV/VA border and observed
rain rates have been between 0.5 and 1.0 in/hr over the last 3
hours through 09Z.

A slow but steady translation of the 850 mb flow toward the NNE is
expected through 15Z which will cause a veering of 850 mb flow
toward the SSW across WV/VA through 15Z. Wind speeds are also
forecast to weaken slightly to below 40 kt across western VA as
the height gradient loosens up a bit. However, the southerly flow
will advect deeper 850-700 mb moisture toward the north, which
when combined with the onset of daytime heating will allow MLCAPE
values to rise beginning near 15Z. The increased CAPE may allow
rainfall rates to increase to near 1 in/hr. While this region of
the Mid-Atlantic has not seen much rainfall from Florence over the
past few days, pockets of lower Flash Flood Guidance of 1.5 inches
in 3 hours may be exceeded with maximum 6 hour rainfall totals of
1-3 inches expected through 15Z.

Otto

ATTN...WFO...CTP...LWX...PBZ...RLX...RNK...

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

LAT...LON   39767860 39167804 38347830 37527936 37328025
            37778062 38388077 39028034 39727924