Mesoscale Precipitation Discussion 0863
NWS Weather Prediction Center College Park MD
1248 PM EDT Tue Sep 18 2018

Areas affected...Central/Eastern VA...Southern/Eastern MD...Far
Southeast PA...DE..NJ...Southeast NY/Long Island...CT...RI and
Southeast MA
Concerning...Heavy rainfall...Flash flooding likely

Valid 181645Z - 182245Z

Summary...Heavy showers and thunderstorms with extremely heavy
rainfall rates will tend to become better organized and expand in
coverage going through the afternoon hours. Some flash flooding is
likely.

Discussion...The latest GOES-16 1-minute visible satellite imagery
shows a highly elongated surface trough extending
southeast/northeast from central VA up across northern NJ and
through southern New England as the Post-Tropical low center of
Florence quickly traverses eastern MA. Diurnal heating via solar
insolation is destabilizing the boundary layer and this is evident
with an agitated CU/TCU field and cloud streets setting up within
the low level southwest flow ahead of the trough. In fact, the
latest RAP analysis already depicts a well-defined instability
axis along and just ahead of the surface trough with MLCAPE values
locally exceeding 1500 j/kg.

PWATS across the Mid-Atlantic region and southern New England and
generally running between 1.9 and 2.2 inches based on the latest
GPS-derived data, and again consistent with the tropical airmass
brought up to the north over the last couple of days with
Florence. Given the depth of tropical moisture, and with stronger
instability profiles setting up this afternoon, expect a more
expansive and organized axis of showers and thunderstorms with
extremely heavy rainfall rates that may approach 3 inches/hr.

The latest 12Z hires model consensus, including the recent HRRR
guidance supports a southeast advance of the surface trough down
across the I-95 corridor and farther east across the adjacent
coastal plain this afternoon. Very heavy rainfall totals of as
much as 3 to 4 inches with locally heavier amounts will be
possible. This will likely cause some flash flooding and
especially for any urbanized areas and also any areas that have
seen heavy rainfall over the last day or so.

Orrison

ATTN...WFO...AKQ...ALY...BOX...LWX...OKX...PHI...

ATTN...RFC...MARFC...NERFC...SERFC...

LAT...LON   42257132 42257040 42007000 41587004 41327090
            40997182 40577293 39977388 38867475 37757553
            37007671 36947737 37227780 38057773 38927686
            39397642 40057552 40917450 41417369 41917273