Mesoscale Precipitation Discussion 0599
NWS Weather Prediction Center College Park MD
805 AM EDT Sat Aug 04 2018

Areas affected...Enter areas affected

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

Valid 041204Z - 041534Z

Summary...Ongoing convective heavy rainfall will likely persist
over southern New England this morning. Flash flooding is possible.

Discussion...Convective vigor had survived the diurnal minimum to
some degree. At 1145Z showers and perhaps a few rumbles of thunder
extended from near Philadelphia to Hartford, CT. With deep layer
southwesterly flow favoring training, there was little separation
between the convective line / cold pool and the deep layer trough
that will continue to produce height falls as it migrates east
along the coast this morning.

The SPC mesoanalysis indicated mixed layer CAPE of at least
500-1000 J/kg and, more importantly - no inhibition - throughout
the coastal Mid-Atlantic states and southern New England, where
surface dewpoints were in the lower 70s. Precipitable water was
roughly near 1.70 inches in the vicinity of the stronger updrafts,
but may increase as a plume of even greater moisture up to 2.10
inches is transported from coastal New Jersey up into CT/RI, New
York and Long Island. RAP forecast soundings over CT indicate
steep low level lapse rates despite near saturation, and plenty of
CAPE, owing to the low level warmth. Little lightning was
detected, but with echo tops sampled around 35,000 feet, there is
likely a robust warm rain process occurring, and rainfall could be
deceivingly heavy.

GOES-16 water vapor loops reveal a convective wedge structure just
north of Philadelphia, with convection fanning out to the
northeast. This point will likely slide ENE...into southern New
England where the hi-res model consensus is for 1 to 2 inch
rainfall, just as we have seen in recent hours per dual-pol
estimates. Much of this will fall in a training fashion and fall
quickly enough to induce flash flooding for any sensitive basins
or urban locations.

Burke

ATTN...WFO...ALY...BOX...OKX...PHI...

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

LAT...LON   42697179 42687076 41707117 41187193 40557380
            40367508 42077336