Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
355 AM EDT Thu Aug 20 2026

Day 1
Valid 12Z Thu Aug 20 2026 - 12Z Fri Aug 21 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID ATLANTIC...

...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 2
Valid 12Z Fri Aug 21 2026 - 12Z Sat Aug 22 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
VIRGINIA AND THE CAROLINAS...

...Virginia to Carolinas...

Cold front will slowly advance south into the Carolinas during the
end of the D1 into D2 time frame with an eventual transition to a
quasi-stationary front as it reaches the latitude of north GA into
the Piedmont of SC. Moisture pooling along the front with continued
forcing driven by another shortwave advancing through the
Tennessee Valley will lead to another round of heavy convection in-
of the Southern Mid Atlantic with the primary axis likely to occur
from the southern Delmarva through the VA Tidewater into the
Carolina's. Environment across the region outlined will remain very
buoyant with PWATs between 2-2.3" for the entire area along and
south of the front. Models are consistent with their signature of
1-2" of rainfall on an areal average basis with some of the
deterministic outputs approaching upwards of 3-5" where storm
materialize and settle as the steering pattern weakens as we step
into the D2. Rainfall rates between 2-3"/hr with intra-hour rates
upwards of 4"/hr are forecast across the aforementioned area,
plenty aggressive enough to cause at least scattered flash flood
prospects, especially in more urbanized zones along I-95 and points
east. The most notable locations in the guidance are over the
Columbia, SC area up through Florence into east-central NC. 00z
HREF neighborhood probabilities for >3" within the 12z Fri to 00z
Sat period are running between 50-90% with the above zone with the
maxima focused between I-74/95 just southwest of Raleigh.
Instability across the area will be fairly robust with 1000-2000
J/kg of MLCAPE on average across NC with upwards of 2500-3500 J/kg
located down into SC. Deep, moist atmospheric profiles are
prevalent with the latest CAMs suite, as well relaying a textbook
heavy convective signature across much of the Carolina's,
especially along and east of the Piedmont.

With the signal maintaining consistent in the heavy QPF placement
and with a favorable environment for locally enhanced rainfall, the
SLGT risk was maintained from the previous forecast with an
addition of high-end SLGT wording as the threat for locally
significant rainfall >4" has increased steadily since the last few
updates.

...Southwest...

Monsoonal activity will continue for another day as the Southwest
CONUS lies underneath a broad ridge centered over the Four Corners
with multiple mid-level vorticity maxima pivoting under the ridge
axis. Seasonal to slightly elevated PWATs will extend throughout
the Southwest, even reaching a bit further north into the Great
Basin by Friday leading to some locally heavy rainfall even making
its presence into portions of NV and central UT while also
impacting those same areas that saw activity the prior period.
Local totals of 1-2" are forecast, but areal QPF average is pretty
much between 0.25-0.5" which is typical for these types of setups.
A broad MRGL remains in effect for much of the Desert Southwest up
into the Great Basin, extending east into the Southern Rockies
located over southwestern CO into NM.

Kleebauer


Day 3
Valid 12Z Sat Aug 22 2026 - 12Z Sun Aug 23 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG MUCH OF THE
EAST COAST, AND OVER THE WESTERN U.S...

...East Coast...

Large scale pattern will continue its evolution into a much more
amplified setup with a deepening mid-latitude trough extending from
Canada all the way into the Eastern CONUS, especially for areas
east of the Mississippi. Embedded shortwave disturbances will be
migrating through the flow with a few robust disturbances
anticipated to motion north-northeast around the front side of the
trough leading to heavy rainfall prospects up and down the eastern
seaboard. Current model consensus is a moderate to heavy QPF
footprint extending from coastal SC all the way up into ME with the
heaviest precip core located over Southern New England as of the
latest ensemble depictions. One of the positives in this setup is
the lack of a truly buoyant airmass with significant instability
maxima located within proximity of any of the disturbances. Despite
the lack of prime instability factors, enough mid and upper level
forcing coupled with some fairly rich tropical moisture being
pulled poleward will likely entice periods of heavy rainfall, both
stratiform and embedded convective pulses located within the main
precip shield. Ensemble mean QPF is between 1-2" for the D3 period
across coastal NJ up through southeastern MA into coastal ME with
0.5-1" located a bit further west with incrementally less the
further west you move away from the coastal plain and into the
Piedmont. Considering the above factors, the previous MRGL risk was
maintained with only some fine tuning across coastal ME, and closer
to the I-95 corridor from DC to NYC. Unless we see significant
changes in the timing of the disturbances and/or amplification
prospects that indicate more backed flow for coastal enhancements,
this could very well maintain a MRGL risk through the entire
series of forecasts.

...West...

Our persistent ridging across the Western U.S. will finally begin
to shift back to the east over the Plains as a robust trough over
the Pacific makes headway into the Pacific Northwest and begins
pushing the pattern downstream and flattening the ridge up top. The
progression of the trough will lead to an influx of both regional
moisture and ascent as the trough makes progress into OR/WA as we
move into the middle part of the weekend. PWAT anomalies are
anticipated to surge upwards of 2-3 standard deviations above
normal as far north as ID and western MT with the rest of the West
firmly entrenched within a +1 standard deviation PWAT anomaly, good
enough to maintain a persistent monsoonal type pattern over the
Desert Southwest and Great Basin, extending into the Central
Rockies. The previous periods have been slowly creeping further
north and northwest with the MRGL risk and this period continues
the pattern. A broad MRGL risk will encompass much of the Western
U.S, east of the Pacific coast states.

Kleebauer


Day 4 and Day 5

Valid 12Z Sun Aug 23 2026 - 12Z Tue Aug 25 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, INTERMOUNTAIN WEST, AND ROCKIES...

The overall pattern is expected to remain relatively unchanged
through the weekend into early next week, with an upper ridge
remaining anchored over the Southwest and southern Plains. Moisture
and energy moving along the western and northern periphery of the
ridge will contribute to daily shower and thunderstorm activity and
the potential for isolated flash flooding extending from the
Southwest to the northern Intermountain West and Rockies. However,
do expect a diminishing threat along the northwestern extent of
the Marginal Risk as drier air behind a cold front spreads into
parts of the northern Intermountain West/Great Basin Sunday into
the early part of next week.

Pereira


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt