Mesoscale Precipitation Discussion 0783
NWS Weather Prediction Center College Park MD
428 PM EDT Thu Sep 06 2018

Areas affected...Nrn West Virginia, Far Srn Pennsylvania, Wrn
Maryland

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

Valid 062025Z - 070015Z

Summary...Thunderstorms were slowing down over the central
Appalachians and beginning to produce locally heavy rainfall from
northern West Virginia into far southern Pennsylvania. Rain rates
of around 2 in/hr, or locally higher, were being estimated by
radar, and this could lead to localized flash flooding in the
region.

Discussion...Recent radar trends from KPBZ and KRLX show
convective cells and small clusters beginning to hang up in
northern West Virginia and adjacent portions of Maryland and
Pennsylvania. Average observed storm motions in this region were
rather slow, and the region appeared to be south of a belt of
stronger mid-upper level flow, as well as just downstream of an
upper level trough. This was likely acting to reduce the westerly
component of the deep layer mean wind, and overall storm motions.
Additionally, backbuilding storm motion speeds were less than ten
knots. All of these factors confirm broader atmospheric support
for the observed slow storm motions. RAP analyzed precipitable
waters were generally above 1.5 inches near the WV-PA border, but
were lower to the south near the WV-VA border. This is generally
consistent with GPS-PW observations, and GOES-16 Simple Water
Vapor RGB which show a pocket of drier air further to the south.
Therefore, the southward extent of convection and the threat of
heavy rainfall may be somewhat limited. Nevertheless, where
convection is able to develop, it should move very slowly, have a
tendency to backbuild slightly to the west-southwest, and the rain
rates should be relatively high. The strong instability in place
(MLCAPE 1500-2000 j/kg) should allow 2+ inch per hour rain rates
to be realized with the PWs near the WV-VA border region above the
90th percentile for early-to-mid September. The convective
intensity and coverage is expected to be tied closely to the
diurnal cycle, so there should be a diminishing trend closer to
00Z.

Lamers

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

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

LAT...LON   40468067 39887966 40187762 39487745 38437968
            39058139 39738141