Mesoscale Precipitation Discussion 0650
NWS Weather Prediction Center College Park MD
1226 PM EDT Mon Aug 13 2018

Areas affected...Southwestern PA...Western MD...Northern and
Eastern WV...Northwestern VA

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

Valid 131625Z - 132030Z

SUMMARY...Ascent, Moderate Lapse Rates, Sufficient Moisture to
Yield Quick Burst 1-2"/hr Totals in Complex Terrain/Low FFG May
Pose Localized Flash Flooding Concern...

DISCUSSION...Goes-16 Meso 1-minute imagery depicts a cluster of
developing TCU and CBs across the highest ridges of S Central PA,
Western MD, and Northeast WV, as well as forming in the Allegheny
River Valley in W PA.  Sufficient early morning heating along the
ridges supported development along the eastern periphery of the
deep upper level closed low.  This cold pool aloft, supported
lapse rates to 7.5/km and SBCAPEs to 1500 J/kg.  Moisture filtered
around the western side of the lower level cyclones/pooled within
the valleys with Tds in the mid-60s, provide solid moisture with
total PWATs in the rates of 1.2 in PBZ sounding to 1.5" toward
HGR.  Weak flow in the mid-level will not support cells, with
strong quick burst wet downdrafts capable of 1-2" in less than 1
hour.  This is near exceeds the FFG values in the region for
isolated possible flash flooding.  

Hi-res CAMs depict the cold pool from initial cells propagating in
all directions to seek out other wells of instability, further
west stronger updrafts but lower moisture may lead to
shorter/stronger wet downdrafts, while cells further east with
less instability/lower mid-level lapse rates may be more efficient
but a bit weaker/narrower in updraft/downdraft.  Mean flow on the
southern periphery of the upper low should support faster
propagation off the terrain to the west in MD/E WV... but may be
counter-acted in Northern WV, allowing for a bit longer duration
or even secondary development repeating in spots.  Trends
currently favor 12z ARW, ARW2 and NAM-Conest.  While the HRRR
agrees in evolution, it is already about 1-2hours slow in
organized development, making it less favorable of a solution, but
adding some confidence in the overall evolution and potential
flash flooding rain rates.  

Gallina

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

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

LAT...LON   40938000 40897918 40417894 40367832 40547760
            40237735 39797752 39087790 38287844 38107858
            38347986 38728122 40418041