873
AWUS01 KWNH 202155
FFGMPD
CTZ000-NYZ000-NJZ000-PAZ000-DEZ000-MDZ000-VAZ000-DCZ000-210330-

Mesoscale Precipitation Discussion 1100
NWS Weather Prediction Center College Park MD
555 PM EDT Thu Aug 20 2026

Areas affected...Northern Mid-Atlantic

Concerning...Heavy rainfall...Flash flooding likely

Valid 202200Z - 210330Z

SUMMARY...A strengthening low-level cyclone will tap into copious
amounts of atmospheric moisture and increasing instability to
produce excessive rainfall rates up to 3"/hr. Flash flooding is
likely this evening in parts of the heavily urbanized I-95
corridor. Locally significant and life-threatening flash flooding
is possible, especially north of the storm track.

DISCUSSION...GOES East satellite imagery shows a blossoming area
of strong-to-severe storms over southeast PA that are racing
towards the Delaware Valley, with additional storms heading for
the NYC metro. These storms are a byproduct of a volatile
atmospheric setup: a potent 500mb shortwave trough and a 250mb jet
streak`s diffluent right-entrance region are providing excellent
upper-level ascent, while a strengthening low-level cyclone in
eastern PA happens to coincide within a highly unstable and
tropical air-mass.

PWs will range between 2.0-2.25" from the I-95 corridor on east
along the NJ Shore and as far north as the Tri-State area. Dew
points in southern NJ and the northern DelMarVa are in the low-mid
70s as well. MUCAPE is highest over the DelMarVa and southern NJ,
but a narrow area of 500-1,000 J/kg MUCAPE is available from
Trenton on north and east to Long Island. Effective bulk shear is
>40kts and effective SRH is >100 m2/s2, giving storms the
potential to develop mesocyclones within them. 20Z HRRR area
averaged soundings show warm cloud layers at least 13,000ft deep
and low-mid level RH values >90%. These parameters are indicative
of an atmosphere primed to produce thunderstorms with highly
efficient warm rain processes, which when occurring over a region
that is highly urbanized, is ripe for flash flooding.

Over the next hour, storms will continue to move east through the
I-95 corridor and towards NJ. As the 850mb low moves east and
strengthens, WAA on its eastern flank will lead to wrap-around
anomalous moisture on its northern flank, placing parts of
northern NJ and east towards the NYC metro under a deformation
axis of 925-850mb FGEN aloft. This area of low-level FGEN is
already evident on SPC RAP mesoanalysis, and will continue to move
east this evening. Beneath the 925-850mb FGEN, a narrow corridor
of intense rainfall rates will unfold with hourly rates around
3"/hr likely. New 18Z HREF guidance suggests as high as 30-50%
chances for 1-hr and 3-hr FFG exceedance in northern NJ on east
through the NYC metro this evening.

Portions of northern DE and southern NJ already dealt with storms
over the past couple of hours that have saturated their soils.
Additional storms moving through will contain 2-3"/hr rates and
could cause additional flash flooding, especially in southern NJ.
Farther north over northern NJ, the NYC metro, and eastward to
Long Island, there is increasing concern for at least a couple
hours of 2-3"/hr rainfall rates before finally tapering off close
to midnight. These areas are most at-risk for locally significant
and life-threatening flash flooding with rapid water rises
possible on roads. This could in turn lead to worsening impacts to
infrastructure that makes commuting challenging through the
evening hours.

Mullinax

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

ATTN...WFO...AKQ...ALY...BGM...CTP...LWX...OKX...PHI...

ATTN...RFC...RHA...TAR...NWC...

LAT...LON   41667357 41587303 41407282 41077270 40677273
            40067320 39317360 38867413 38537468 38177552
            38137625 38527733 38947704 39527620 40407602
            40667596 41017559 41127488 41327443 41657388