Mesoscale Precipitation Discussion 0563
NWS Weather Prediction Center College Park MD
1120 AM EDT Mon Jul 30 2018

Areas affected...WV...WRN MD...PA

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

Valid 301519Z - 302004Z

Summary...A well defined shortwave trough will produce streaks of
heavy rain through mid afternoon. Given mountainous terrain in
addition to wet soils, flash flooding is possible.

Discussion...At 15Z thunderstorms in West Virginia had prompted a
couple of flash flood warnings. These were occurring in the
mid-level frontogenetical zone ahead of a well defined shortwave
per both upper air data and a baroclinic leaf structure seen in
GOES-16 water vapor loops. As with this and other cells forming
farther downstream where deep layer ascent had begun, the
convection is likely to be slightly elevated, occurring north of a
surface front. The storms were, however, possibly feeling
southerly inflow of up to 20 knots at 850 mb based on VAD Wind
Profile data from KRLX (even prior to thunderstorms in that
vicinity) and RAP forecasts which develop a corridor of 20 knot
southerlies into Pennsylvania late this morning.

Instability, with mixed-layer CAPE values 500-1000 J/kg, was
increasing northward through a combination of warm advection, lift
to cool the mid levels, and partial insolation. Models such as the
HRRR and 12Z WRF-ARW, in particular, indicate potential for
multiple swaths of convection within the MPD area, training from
SW to NE, following the mean deep layer flow. Given some low level
veering and any local terrain effects, some slower moving
supercell structures are possible as well.

Migration of the shortwave and increasing cloud cover should put a
cap on any rainfall extremes, but much of the hi-res guidance
indicates spotty 1 to 3 inch totals through 20Z, much like what
was already observed in WV. The very wet July in parts of this
region had lowered guidance values, and flash flooding is possible.

Burke

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

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

LAT...LON   41937735 41647662 40567711 39737780 38757880
            37828108 38328200 38668128 39228005 40397968
            41517929 41907859