Mesoscale Precipitation Discussion 0814
NWS Weather Prediction Center College Park MD
124 PM EDT Sun Sep 09 2018

Areas affected...Wrn/Ctrl Pennsylvania, SE Ohio, Nrn West
Virginia, NW Maryland

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

Valid 091723Z - 092315Z

Summary...Steady moderate, to occasionally heavy, rain is expected
into the afternoon across a sizable portion of the Mid-Atlantic
region. Soil moisture content across the region is also quite
high, so flooding should still result from even modest rain rates.
Widespread flooding issues are expected from the gradual
accumulation of rain across the region, with localized flash
flooding possible, particularly from northern West Virginia into
far southwest Pennsylvania.

Discussion...Rain continued to spread over most of the
Mid-Atlantic region at 17Z, with some more enhanced radar
reflectivity streaming into southwest Pennsylvania along the
ascending warm conveyor belt as it lifts north of the surface
front. Precipitable water values across the region will remain
above the 90th percentile for September, and the abundant deep
moisture combined with relatively strong forcing will be a recipe
for widespread rain and areal flooding. Over southern PA, western
MD, and northeastern WV, streamflow was running above the 98th
percentile for numerous streams and rivers, indicative of the
relatively high soil moisture content. Therefore, any additional
rain should sustain or exacerbate existing flooding, and could
lead to increasingly widespread flooding into the afternoon hours.
Rain gauge observations in the vicinity of banded precip in
southwest Pennsylvania have occasionally reported rates in excess
of 0.5 in/hr, well above radar estimates, suggesting that
precipitation efficiency is relatively high. Given the favorable
location relative to the warm conveyor belt this afternoon, and
ongoing rain bands, areal flooding could be particularly
widespread in the terrain of SW PA and in the general vicinity of
Pittsburgh.

Some flash flooding cannot be ruled out, and this would be most
likely from northern West Virginia, into perhaps far southwest
Pennsylvania. The RAP model analysis places a nose of neutral to
negative most-unstable LI values right along the warm conveyor
belt, with the 0C contour right near the southwest tip of
Pennsylvania. Therefore, some convective bands with higher rain
rates close to 1 in/hr cannot be ruled out in those areas.
Additionally, the 0.5 to 0.6 in/hr rain rates being occasionally
observed from the stratiform rain bands near Pittsburgh could
trigger flash flooding in urban areas if sustained for a
sufficiently long period of time.

Lamers

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

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

LAT...LON   41397947 40837706 39327672 39327893 39138084
            39458211 40718109