Mesoscale Precipitation Discussion 0562
NWS Weather Prediction Center College Park MD
755 AM EDT Mon Jul 30 2018

Areas affected...Eastern Mid-Atlantic Region

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

Valid 301154Z - 301754Z

Summary...A band of showers and thunderstorms is expected to
remain quasi-stationary over the next several hours.  Hourly
totals to 2.5" with local amounts of 5" are possible.

Discussion...A band of thunderstorms has been moving northward
into southern and southeast MD within a warm advection pattern
east of a weak surface low near the Mouth of the Potomac. 
Divergence aloft is being sponsored by an incoming upper trough
across the Ohio Valley.  Precipitable water values of 1.75-2.2"
lie here per GPS data.  Inflow at 850 hPa is 15-20 kts out of the
southwest, though effective bulk shear of 25-30 kts has organized
the band.  ML CAPE of 500-1500 J/kg exists in this area per SPC
mesoanalyses.

The guidance shows some additional northward motion before halting
in the very near future.  It appears that RAP mass fields keep the
surface boundary pretty much where it is now for the next several
hours.  The problematic mesoscale wave has been moving eastward at
15 kts over the past couple hours, which the RAP guidance in
particular may not be handling well.  The area has received
100-600% of the average rainfall over the past couple weeks, so
soils should be partially saturated.  The available moisture and
instability should allow for hourly rain totals to 2.5".  The
mesoscale guidance has a strong signal for local amounts in the 5"
range through 18z.  This would be problematic in any urban areas
or where soils have been partially saturated by the wetness seen
during the last half of July.

Roth

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

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

LAT...LON   39147476 38697494 38247504 37757532 37937616
            38237707 38807664 39107579