Mesoscale Precipitation Discussion 0824
NWS Weather Prediction Center College Park MD
147 PM EDT Tue Sep 11 2018

Areas affected...Eastern MD...DE...Extreme SE PA...Southern NJ

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

Valid 111706Z - 112300Z

Summary...Showers and thunderstorms developing in a region of
clear sky and enhanced instability will increase in coverage and
train NE through the afternoon. 2-4" of rainfall in a short period
of time will have the potential to produce flash flooding.

Discussion...A shortwave evident on GOES-16 WV imagery moving
across WV will continue to progress NE towards Long Island into
this evening. This feature will combine with the approach of a
50kt 300mb jet lifting into New England to enhance diffluence atop
the Mid-Atlantic. A region of clear sky noted on GOES-16 visible
is coincident with RAP analyzed MLCape of 1500-2000 J/kg, and this
will expand on deep southerly flow, eroding the stationary
boundary to the north.

The increasing warm sector will be favorable for convective
development as surface dewpoints in the low 70s combined with
PWATs over 2 inches, near record values for the date at OKX and
WAL, create an environment favorable for heavy rainfall. The mean
0-6km flow on 12Z OKX sounding was from the SW at 20 kts, parallel
to the cold front approaching from the west. Boundary parallel
flow and RAP forecast corfidi vectors aligned just left of this
mean flow suggest training of thunderstorms is likely, capable of
producing flash flooding despite rapid storm motion. Additionally,
0-6km shear of 25-30 kts is favorable for storm organization in
the form of multicell clusters, increasing the likelihood of
training convection.

High-res guidance is in good agreement with a signal for 2-4" of
rainfall across the threat area. 3-hr FFG guidance in this region
is 2-3", and with rainfall rates potentially reaching 1.5-2" per
hour, flash flooding is possible, especially in urban areas.

Weiss

ATTN...WFO...AKQ...LWX...PHI...

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

LAT...LON   40417452 40347432 39927418 39657419 39287447
            38867467 38447510 38147547 38007573 37947610
            38077636 38327665 38907660 39327633 39857591
            40227548 40407493