Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 PM EDT Thu Aug 20 2026
Day 1
Valid 16Z Thu Aug 20 2026 - 12Z Fri Aug 21 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF FAR EASTERN PA, CENTRAL/NORTHERN NJ, NYC, AND FAR WESTERN LONG
ISLAND...

16Z Update...

Primary change with the Day 1 ERO was an introduction of a MDT Risk
across parts of the Mid-Atlantic. The approaching shortwave over
the Upper Ohio Valley and associated showers have been exhibiting
warm rain processes and intense rainfall rates even without ample
instability due to thick cloud cover. Flash Flood Warnings extend
from southwest to central PA as of 16Z. Rainfall rates around 2"/hr
without sufficient instability are possible in combination with a
favorable right- entrance region of a potent jet streak over the
Northeast and New England, as well as a very high melting layer
(IAD sounding WBZ height over 13k feet and over the 90th
climatological percentile per SPC's sounding climo). Additionally,
two frontal boundaries are in play across the Mid- Atlantic to
create a focus for additional thunderstorm development as well as
an anchor for thunderstorms to remain near and create the duration
aspect necessary for higher-end rainfall totals and related flash
flooding impacts. The first quasi- stationary boundary was analyzed
at 14Z extending across southern PA before lifting northeast into
central NJ and following I-95 into southern CT. This boundary and
areas to the north will be the focus for 2-4" rainfall amounts,
locally higher, due to the repeating nature of thunderstorms ahead
of the approaching MCV. Once the MCV exits off the NJ coastline by
around 00Z, lingering rainfall is possible to the north and could
be moderately heavy given the increasing easterly flow overrunning
the lingering boundaries and coastal influence. This may add onto
any ongoing flash flooding concerns.

12z HREF/REFS neighborhood probabilities for at least 3" per
6-hours ending at 03Z Fri are 40-80% from eastern PA through
central/northern NJ and into the NYC metro region as well as
western Long Island. This is a strong signal for an area that is
prone to flash flooding due to the urban/impervious ground
conditions, which was the primary reason for the upgrade after
consulting local forecast offices (OKX and PHI). For NYC
specifically, the URRD (Urban Rain Rate Dashboard) from the 06Z run
shows over 60% chances for more than 3", with several members over
4" and maximum amounts up to 7". The MDT Risk area highlights not
only where numerous instances of flash flooding are likely, but
also significant impacts are possible. See MPD 1098 for more
information regarding the heavy rainfall potential through 22Z. Be
sure to have numerous ways of receiving warnings, follow advice
from local officials, and avoid driving through flood roadways.

Snell

Previous Discussion...

...Mid Atlantic...

Shortwave over the Ohio Valley will continue its progression to
east, then northeast as it migrates into the Central and Northern
Mid Atlantic by later this afternoon. At the surface, a cold front
will remain steadfast on its advancement through the Piedmont and
the eastern sections of PA by late morning leading to a favorable
scenario for a line of thunderstorms to develop along and east of
the front itself. Increased dynamical forcing and favorable
diffluent flow within the RER of a 130kt jet over the Northeast
U.S. will lead to a blossoming of afternoon convection within
proximity of the front itself. Latest CAMs are quite aggressive on
their interpretation of both the magnitude and coverage of the
heavy convective signals, especially for areas extending from
southern CT down through the NYC metro, Northern NJ, eastern PA,
into the Delmarva and the VA Tidewater. Thunderstorm initiation
will be very close to the metro areas of DC-Balt- Philadelphia with
the majority of CAMs developing cells almost overhead before
advancing steadily eastward, away from the I-95 corridor. The
latest CAMs have really provided a more significant heavy rainfall
footprint across the NYC metro and surrounds with several of the
CAMs members within the latest HREF depicting localized maxima
between 3-5" across NYC proper, neighboring areas of Northern NJ,
western LI, and the southwest reaches of CT.

00z HREF neighborhood and EAS probabilities have increased further
since the last few forecast cycles with the signal now firmly
greater than 50% (60-80%) for both the neighborhood >3" prob and
>1" EAS probability over the corridor extending from Trenton up
through the NYC metro area. Further south towards the Baltimore to
Philadelphia area, the signals are not as prolific as the above
areas further up I-95, but they are still pretty robust compared to
where they were last evening and even a bit higher than the 12z
probabilities. The question becomes, "Where does the line
initiate?", and, "Will the storms have enough vigor behind their
development near the Balt/Phi metro corridor to entice more flash
flooding potential than recently thought?"

The answer to both questions haven't been thoroughly resolved for
this particular zone, however the environmental conditions in place
would absolutely favor the threat if storms were to materialize a
bit upstream to provide a more substantial impact to that
particular area of the Mid Atlantic. PWATs between 2-2.3" coupled
with a strong theta_E ridge in place ahead of the front promotes a
formidable environment capable for producing significant rainfall
rates in any convective core that materializes (1-3"/hr likely
according to the latest HREF hourly rate probabilities). Analysis
of multiple soundings from both the global and hi- res suite at 00z
indicated a textbook signal for heavy rainfall with a tall, skinny
CAPE signature with moisture layers present from surface to the
tropopause. Typically these types of environments favor, at least
scattered flash flood prospects in areas where thunderstorms reach
their mature phases, which seems to align with areas along and east
of I-95.

The NYC area has trended to a more formidable signature in part to
the region potentially situated in an area that sees a more
prolonged heavy rain potential as flow becomes backed locally along
the northern periphery of what would be a bookend vortex that
materializes from an organized MCS structure that is anticipated
for areas of eastern PA into NJ. This has been a signature within
multiple CAMs, mainly the RRFS/HRRR outputs that place Northern NJ
into NYC within a position that favors back-building potential for
a period of time early this evening that would ultimately lead to
significant rainfall across the metro corridor. The caveat behind
this is these signals can be hard to parse out perfectly at range.
These types of scenarios are best defined as we move closer to the
initiation phase and as the organization begins for the anticipated
complex. As of now, the indications are the metro area between
Philadelphia to NYC into all of Northern NJ is the best target for
this evolution. This signal provided enough confidence to warrant
an increase in the magnitude of rainfall potential and more
significant language to the incoming threat. That said, we still
have some uncertainty on the exact positioning of the organized
complex and eventual bookend vorticity. Unless we get overwhelming
consensus, felt the maintenance of the SLGT risk with a high-end
SLGT positioned on a line from Balt-Phi-NYC along I-95 and all the
areas in-between. This is a delicate situation considering the
urban footprint providing ample run-off opportunities that tend to
loom large when dealing with expected rainfall rates of 1-3"/hr
with intra-hour rates very likely to exceed this locally. We will
continue to monitor the trends closely over the next several hours
leading into the event.

...Southwest U.S. into the Southern Rockies...

Relatively textbook monsoonal pattern across the west will lead to
scattered thunderstorm development across much of the Desert
Southwest extending north into parts of the Great Basin and through
the Southern Rockies. As of now, the signal is mainly seasonal with
regards to the PWAT anomalies and general expectation as the most
notable areas will likely be in those terrain focused zones across
AZ (Mogollon Rim), NM (Sangre de Cristos and Sacramento Mtns.), and
UT (Zion National Park). Low probs for >1" on the latest
ECENS/GEFS suites and 01z NBM allow for a standard MRGL risk
issuance for the area as the setup is genuinely favored for pulse
convection and slow cell motions tied to the terrain as a broad
597dm ridge centers over the Four Corners and makes little
progression in the D1 window. No changes were necessary to the
previous forecast given the consistency within guidance over the
last series of model runs. Most favorable areas of impact will
include the more flashy streams and arroyos across the Southwest,
as well as the slot canyon areas of southwest UT and the burn scars
across each of NM and AZ.

Kleebauer


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt