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AWUS01 KWNH 230749
FFGMPD
MAZ000-RIZ000-CTZ000-NYZ000-NJZ000-PAZ000-DEZ000-231400-

Mesoscale Precipitation Discussion 0665
NWS Weather Prediction Center College Park MD
349 AM EDT Tue Jul 23 2019

Areas affected...I-95 corridor from Philadelphia, PA to Boston, MA

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

Valid 230800Z - 231400Z

Summary...Increasing showers and thunderstorms developing along a
cold front and in response to a shortwave lifting northward will
produce renewed rounds of heavy rain. Rainfall of 1-2" is likely,
which on top of saturated soils may produce flash flooding through
the morning.

Discussion...GOES-16 WV imagery this morning shows a pronounced
plume of moisture streaming overhead from the Gulf Coast into New
England. On the northern edge of this moisture axis, a strong jet
streak is lifting through southeast Canada, and is expected to
pivot to the SW as the upper trough sharpens, placing the region
in an increasingly favorable position beneath the diffluent RRQ.
At the same time, a subtle shortwave near Delaware will lift
northeastward in the large scale SW-NE flow, interacting with a
baroclinic boundary along a stalled cold front. This large scale
ascent will lead to an increase in convection, which may impact
areas that have pre-conditioned soils after already receiving
heavy rainfall over the past 12 hours.

Despite the front being along the coast, the thermodynamic
environment remains at least marginally favorable for heavy rain
rates. MUCape of 250-500 J/kg still remains, and may get tugged
back to the north in response to the large scale lift and a weak
surface low developing along the stalled boundary. Additionally,
PWATs at 00Z/23 were near or above daily records at IAD/OKX, and
near the 90th percentile at CHH/WAL. While some of this may have
been convectively enhanced, PWATs are still analyzed to be around
2 inches, and should climb above 2 inches as the front wavers
slightly to the north through the morning. Weak mid-level lapse
rates and deep warm cloud depths in this environment suggest
efficient rain processes as well. Even if instability remains
somewhat limited, re-supply of moisture and CAPE on an increasing
850mb LLJ this morning which may approach 40 kts should suffice
for heavy rain in the strongly forced environment.

High res CAMs are in good agreement that a swath of 1-2" of
rainfall will occur, and recent WoFS runs indicate a modest
probability for these same amounts. Although rain rates may not be
extreme, likely struggling to exceed 1"/hr as noted by low/spotty
HREF neighborhood probabilities, FFG in some areas is less than
1"/hr due to high soil moisture. In these areas, or across urban
low-permeability locations, flash flooding will be possible as
training will lead to rounds of heavy rain potentially impacting
the same areas multiple times.

Weiss

...Please see www.wpc.ncep.noaa.gov for graphic product...

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

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

LAT...LON   42377090 42187033 41997007 41806992 41567008
            41297066 41077122 40907189 40757239 40627296
            40457333 40297365 40097402 40007435 39897491
            39817533 39787597 39817618 39937637 40207615
            40547567 41117477 41597373 41927309 42187250
            42277211 42307183 42357121


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