Mesoscale Precipitation Discussion 0576
NWS Weather Prediction Center College Park MD
457 PM EDT Wed Aug 01 2018

Areas affected...Ern WV, Wrn MD, NWrn VA, and CTRL PA

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

Valid 012057Z - 020200Z

Summary...Thunderstorms will continue to develop late this
afternoon from near the West Virginia and Virginia border region,
northeast into central Pennsylvania. Some of these storms may
repeatedly affect the same areas and produce heavy rainfall. The
rain rates could reach 1-2 in/hr, and could lead to flash flooding.

Discussion...Between 20-21Z, regional radars showed a marked
increase in convection between Altoona, PA and Lewisburg, VA. The
cooling cloud tops and increasing radar reflectivity with the
developing lines of storms was occurring on the western periphery
of a pool of stronger instability in the relatively sunny warm
sector east of the Appalachian Mountains. Hi-res models suggest
that thunderstorms will continue to develop in the instability
gradient, and then will likely propagate to the northeast with the
deep layer mean wind. Given the current orientation of some of the
line segments (SW-NE), this may favor training of thunderstorms
and localized swaths of heavy rainfall. RAP analysis places the
PWs just shy of 2 inches, which is close to the 90th percentile
for the more mountainous regions of the Mid Atlantic. The
combination of deep moisture and moderate to strong instability
should support organized convection with heavy rain rates, likely
reaching the 1-2 in/hr range in some places. This could lead to
some flash flooding.

Antecedent conditions are also favorable for flash flooding, with
streamflow over much of central Pennsylvania and western Maryland
over the 90th percentile as well, and very heavy rainfall in
recent weeks. This should limit infiltration of rainwater and lead
to increased runoff. The primary source of uncertainty will be
related to the relatively fast storm motions with the deep layer
mean wind speeds reaching around 40 knots. If the rain bands are
not oriented parallel to the storm motion vectors, then there may
be enough drift to limit focused bands of heavier rainfall.

Lamers

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

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

LAT...LON   41497638 40877586 39707719 38567823 38097899
            38407963 38987958 40057885 40877781