Mesoscale Precipitation Discussion 0642
NWS Weather Prediction Center College Park MD
1248 PM EDT Sun Aug 12 2018

Areas affected...Northeast Corridor including NJ, S NY, LI, CT,
RI, MA

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

Valid 121647Z - 122210Z

Summary...Thunderstorms are likely to develop this afternoon along
coastal fronts from New Jersey into southern New England, as well
as over interior southern New England. These storms are also
likely to be slow-moving, and this could lead to rain rates of 1-2
in/hr and flash flooding.

Discussion...GOES-16 visible satellite showed enhanced cumulus
fields and developing cumulonimbus along several coastal fronts --
(1) central and southern New Jersey, (2) Long Island, and (3) NYC
Metro into Connecticut and Rhode Island. Furthermore, it also
indicated a potential differential heating boundary that aligned
with a RAP analyzed low-level convergence maximum from central
Connecticut into eastern Massachusetts. These boundaries should
all play a role in thunderstorm development this afternoon.
Convection is likely to be slow-moving and may stay anchored to
some of these boundaries for a couple hours in some cases. An
isolated storm that developed earlier on the north side of Boston
produced over 5 inches of rain in a short period of time because
it was able to stay anchored along the coast in the E/NE low-level
flow to the north of a low that is situated just off Long Island.
Although the high-end rainfall totals may not be replicated, this
offers a preview of what may happen in a few areas of southern New
England and coastal New Jersey this afternoon. Precipitable water
values are still elevated, and the morning sounding from OKX
recorded 2 inches with a tall, narrow CAPE profile. This favors
efficient rainfall production, and the weaker updrafts may also
act to reduce the influence of some of the faster upper level flow
(leading to slower storm motions and anchoring near low-level
boundaries). Several hi-res models indicate 1-2 in/hr rain rates
into the afternoon hours with some of the thunderstorms that
develop, and that should be sufficient to produce some flash
flooding in a highly urbanized stretch of the Northeast.

Lamers

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

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

LAT...LON   42777148 42587053 41757062 41167140 40697264
            40517353 39827401 39357444 39347501 40057467
            40687475 41307446 41807389 42377259