Mesoscale Precipitation Discussion 0784
NWS Weather Prediction Center College Park MD
452 PM EDT Thu Sep 06 2018

Areas affected...Northeast Corridor from eastern PA into
Massachusetts

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

Valid 062050Z - 070130Z

Summary...A squall line will progress across the Northeast
Corridor, and will bring heavy rain to most of the region. Some
segments of the line may briefly hang up and produce locally
greater rainfall and the potential for flash flooding,
particularly if that occurs over urban areas. Rain rates of up to
2 in/hr will be possible with the strongest storms.

Discussion...Regional radars indicated a prominent and
nearly-continuous squall line just ahead of a synoptic cold front
stretching from near Boston, to just south of Hartford, to just
north of Allentown. In general, the squall line was progressing
steadily to the southeast, and should clear the Northeast Corridor
in the next several hours. However, some of the segments in the
line were oriented in a more west-east, or even west-northwest to
east-northeast, fashion and this could produce localized pockets
of an hour or two of training. Radar rainfall estimates from MRMS
were peaking in the 1.5 to 2.0 in/hr range where this sort of
storm morphology was occurring, and that could produce flash
flooding, particularly if aligned with urban areas.

Precipitable water values were around 1.8 inches per multiple GPS
observations, and this was very close to model analyzed values.
1.8 inch PWs for this time of year are above the 90th percentile
in this region, and should support heavy rain rates. The upper end
of rainfall potential may be capped somewhat by a few factors: (1)
the relatively progressive motion of the overall squall line; (2)
strong instability supporting greater amount of hail; (3) some
slight drying in the mid-upper levels of the troposphere, evident
in the GOES-16 Air Mass RGB.  Nevertheless, with the heavy rain
rates and potential for transient pockets of training convection,
flash flooding does seem possible in the region.

A favored area for flooding issues may be from southern New York
into northern New Jersey and the NYC Metro area. GOES-16 visible
satellite showed a second band of developing convection ahead of
the main squall line near I-80 in northern New Jersey, and this
could eventually lead to a wider band of thunderstorms in the
region (thus prolonging the period of heavy rain), or perhaps
providing some antecedent rainfall to saturate the top layers of
the ground.

Lamers

ATTN...WFO...ALY...BGM...BOX...CTP...OKX...PHI...

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

LAT...LON   42387072 41767044 41027138 40277354 39997498
            40427642 41237543 41937307