Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
357 PM EDT Sat Aug 8 2026

Day 1
Valid 16Z Sat Aug 08 2026 - 12Z Sun Aug 09 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE LOW COUNTRY
OF SOUTH CAROLINA AND GEORGIA...

...Low Country...

16Z Update: Weak disturbance is analyzed off the coast of GA with
the northern periphery of the inverted trough nosing into southern
SC with the radar depiction over Charleston finally showing some
activity sneaking ashore. The setup remains conducive for locally
heavy rainfall as PWATs remains ~2 standard deviations above normal
within the immediate coastal plain of GA and southern SC. The
cutoff on the northern edge of the disturbance is sharp, however,
as noted via a solid ~0.7" difference in area PWATs between the
12z KJAX (2.42") and KCHS (1.74") soundings this morning associated
with the disturbance. Mesoanalysis does show the more prolific
tongue of higher PWATs slowly advecting northward into the
Charleston area, but the greater PWAT anomalies are still forecast
to be closer to Hilton Head down into coastal GA where the heaviest
rain is more likely to occur.

Latest HREF probs remain more prolific for localized 3+ inch totals
between Brunswick up through Savannah and nudging into Hilton Head.
Probs fall off as you move north, so it could be a case where
Charleston proper misses out on the worst, but it'll still be very
close, so maintained the city in the SLGT risk, but just barely as
of this latest update. Expectation is for the heaviest rain to
occur now until 21z as the disturbance is forecast to shift
northeast and the rain chances dwindle heading into the evening. As
a result, expectation is for the SLGT risk to likely fall off to a
nil during the 01z update, so the primary period of concern will be
over the next 4-6 hrs.

Kleebauer

...Other Marginals across the Country...

16Z Update: The broad MRGL risk extending from New England into the
Lower Mississippi Valley was relatively unchanged as a prominent
deep moist airmass with widespread elevated instability will
maintain an environment conducive for locally heavy rainfall with
rates between 1-2"/hr pretty much anywhere within the
aforementioned area. The most notable adjustments were made over
southern VA where the MRGL was expanded to include a bit more of
the I-95 corridor between Fredericksburg through Richmond, down
into Emporia. Heaviest rainfall threat will likely situate west of
I-85 in VA, but some of the CAMs are a bit more aggressive towards
Richmond, so wanted to include the urban center as this area of VA
is more susceptible to flash flood concerns due to the urban
footprint. New England remains generally unchanged as well,
although did trim a touch more of southeastern MA out of the MRGL
given the latest trends of convection expected to be a bit more
northwest than originally progged.

The Southwest U.S. will continue to see Monsoonal moisture play a
role in the daily convective regime with the expansion of higher
moisture northward into the Great Basin for today. Latest CAMs are
privy for a bit more thunderstorm coverage into NV for this
afternoon and early evening before waning after sunset. The MRGL
risk was expanded a bit further west to encompass the trend,
however, did maintain the nil into Vegas proper as the threat looks
to avoid the main urban area, but will be something to watch if
any outflows can buck the trend and sneak into the area from the
east. Otherwise, the heat remains the talking point for the area
with the ridge overhead. Primary concerns will be those areas in
southwestern UT, northern AZ, and the Mogollon Rim due to the slot
canyons and susceptible grounds for run off.

Kleebauer

..Previous Discussion..

Marginal Risks cover nearly the entire Eastern Seaboard, much of
the Southeast, and relatively small areas of the Upper Midwest and
Southwest. For the East, the same pocket of deep tropical moisture
hosting the aforementioned area of disturbed weather off the
Atlantic coast of the Southeast will support widely scattered
showers and storms from the lower Mississippi Valley up the length
of the Appalachians and into the Northeast. This will occur this
afternoon at peak heating. Multiple rounds of storms may impact
portions of Arkansas and Mississippi as well as into New Hampshire
and western Maine today. In both these areas, slightly better
forcing along the moisture gradient will support additional storms
impacting these areas compared to the rest of the Eastern Seaboard.
Both areas also feature drier than normal soils, which should help
to temper any flooding to isolated instances. Thus, while Slights
were considered, neither were upgraded for this reason.

The Marginal in the Midwest was trimmed from the southwest, an
unusual direction. Guidance continues to trend drier and further
north and east, so the Marginal was adjusted accordingly. A Slight
risk upgrade in this area is no longer being considered.

Monsoonal moisture will continue to impact portions of the
Southwest, following the Mogollon Rim, with broader southwesterly
flow allowing a few showers and storms to impact portions of
northeastern New Mexico. Any showers and storms will be few and far
between, so the flash flooding threat there will be isolated.

Wegman


Day 2
Valid 12Z Sun Aug 09 2026 - 12Z Mon Aug 10 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
UPPER GREAT LAKES AND UPPER MISSISSIPPI VALLEY...

...Upper Great Lakes & Mississippi Valley...

20Z Update: The overall pattern remains favorable for a multi-
round convective episode during the D2 period with the area of
greatest focus likely to materialize over northern IL and southern
WI and points east with the Chicago area the most notable area of
potential impacts. There's a growing concern for a more locally
significant flash flood prospect for the Chicago metro, including
adjacent suburbs with the nocturnal convective maturation that's
expected on Sunday night into early Monday morning. Warm front
positioning and a strengthening 850mb LLJ will be key components in
the threat over the area, one that is more susceptible to flash
flooding thanks to the broad urban sprawl that extends from
southern Lake Michigan and encompasses the entire area spanning
northeast IL through northwestern IN. 12z HREF neighborhood
probabilities for >3" are upwards of 60-80% currently for the above
region, including the center of Chicago proper where high run off
capabilities are notorious for local flooding hazards. There are
even some notable >5" probs of 40-60% within the area spanning the
western and northern Chicago suburbs down through the center of
Chicago proper, a testament to the potential in this setup, and a
general agreement from the CAMs on the positioning of the heavier
rainfall.

Ensemble mean QPF is greater than 2" over much of the
Chicago metro, a signal that typically favors at least scattered
flash flooding with perhaps widespread flash flood concerns if
everything comes together perfectly. The back-building scenario
with the setup is the reason for such a strong mean QPF signature,
correlating well with the strong low-level convergence regime and
locally backed flow between sfc-700mb positioned near the warm
front. Ultimately, that frontal positioning will be pivotal in
where the heaviest rainfall will materialize, and current
indications is it will be within that zone bisecting northern IL
into northern IN by the second half of the period. In this case,
there is enough of a signal and general consensus on a high-end
SLGT risk forecast for that zone with emphasis on northeast IL and
northwest IN given the elevated probabilities for >3" of rainfall.

Kleebauer

..Previous Discussion..

A series of disturbances moving eastward along the southern
periphery of a large upper low well into Canada will result in
multiple rounds of storms on Sunday across the Slight Risk area.
The first will be the remnants of an MCS that will start the day in
far southeastern Minnesota, and will then track south and east
along the Wisconsin/Illinois border. The storms in general should
be pretty fast moving, so it's unlikely too many flooding impacts
will be seen from this feature alone. However, what it will do is
prime the soils by moistening them up for the later round.

The second round will form at the nose of a developing low level
jet Sunday night. The storms will follow the greatest instability,
which will mean the storms will track southward. The should be
significantly slower moving, and much more widespread compared to
the first round earlier in the day. The widely scattered instances
of flash flooding associated with the Slight will be mostly
confined to this Sunday night period. Urban and small stream
impacts in and around Chicago and possibly Milwaukee are the most
likely impacts.

Wegman

...Marginals Elsewhere...

20Z Update: Only minor adjustments made to the remaining MRGL risks
across both the Eastern and Southwestern U.S. Primary change was a
narrow expansion of the MRGL to include the VA Tidewater and
southern VA as guidance insists on another round of convection with
rates between 1-2"/hr impacting these areas by Sunday afternoon and
early evening before dwindling with the loss of diurnal
instability after sunset. Southwestern U.S. remains on track for
prevalent monsoonal moisture making its presence known across AZ,
extending northward through the Great Basin.

Kleebauer

..Previous Discussion..

As on previous days, deep tropical moisture across the Southeast
will favor another round of afternoon showers and storms from
northern Mississippi into the southern Appalachians. Flash flooding
will likely be confined to areas where the storms are most
persistent (northern Mississippi), or where terrain helps funnel
the rainwater (southern Appalachians). Generally the storms, while
scattered, should be disorganized, so a Marginal was maintained for
now, though targeted Slights are certainly possible in the coming
days.

The monsoon will continue across the 4 Corners region on Sunday.
While a bit more widespread over the Southwest, most organized
convection will be very limited, so few changes to the inherited
Marginal Risk were needed.

Wegman


Day 3
Valid 12Z Mon Aug 10 2026 - 12Z Tue Aug 11 2026

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE GREAT LAKES AND SOUTHEASTERN ARIZONA...

...Great Lakes...

20Z Update: The setup for D3 remains pretty consistent in run-to-
run continuity with the highlight of heavy rainfall extending over
the southern Great Lakes and northern Ohio Valley into western PA
and the adjacent WV Highlands. A lot of the setup is contingent on
the behavior of any complexes that materialize upstream over the
Midwest and migrate east through the forecast. Looks like activity
will be prone to producing rates upwards of 2"/hr, locally higher
given a robust moist and unstable airmass present on the northern
periphery of a broad ridge to the south, and the southern
inflection of a sprawling ULL over Canada. In any case, the setup
is textbook for an area of above normal convective chances that
will take advantage of both a favorable thermodynamic and kinematic
environment as the positioning of the jet will favor large scale
ascent across much of the Great Lakes and Ohio Valley into the
Appalachian front. As of now, the only change from the previous
forecast was to expand the SLGT a bit further south into WV and
east to bank up against the western slopes of the Appalachians in
PA. Will be monitoring this setup closely as a high-end SLGT is not
out of the realm of possibility considering all of the above
factors, as well as the addition of terrain influences on the
eastern extent of the current SLGT.

Kleebauer

..Previous Discussion..

A large upper level low well over Canada will spin a strong
shortwave across the Great Lakes on Monday. Deep tropical moisture
will be drawn northward from the reservoir of moisture over the
Southeast, allowing for widespread showers and thunderstorms,
generally from Chicago east to portions of the Central
Appalachians. The inherited Slight was expanded to the east, to
includes much of eastern Ohio, western Pennsylvania, including
Pittsburgh, and far western New York, including Buffalo with this
update.

Wegman

...Southeastern Arizona...

20Z Update: A favorable setup for heavy rainfall across
southeastern AZ exists within a majority of guidance with ensemble
mean QPF pushing >1" over much of the terrain focused within
eastern Pima, Santa Cruz, and Cochise counties along and south of
I-10. The previous high-end SLGT forecast continues to have merit
with some of the deterministic outputs pushing local totals up near
3", a threshold that would undoubtedly cause locally significant
flash flood concerns within the aforementioned areas. Areas like
the Huachuca mountains over to I-19 are the most likely areas to
see significant rainfall in the setup. As we move closer to CAMs
range, it will be an area to monitor for potential further upgrades
if the signal becomes more pronounced in the general magnitude of
the rainfall forecast. For now, the SLGT remains with the
highlighted area across the southern tier the noted high-end SLGT
risk.

Kleebauer

..Previous Discussion..

Deep tropical moisture moving up the Gulf of California will
intrude into southeastern Arizona on Monday, with PWATs exceeding
1.5 inches in many areas. A thermal low due to the strong high
aloft will support widespread shower and thunderstorm activity,
particularly along the Mexican border and up I-19 from Nogales
through Tucson. This corridor is both highly flood prone, has seen
numerous days of widely scattered flash flooding in recent days,
and is right in line for additional widespread convection Monday
afternoon. There is considerable uncertainty moving west up I-10
beyond Tucson, so the introduced Slight is primarily for this most
vulnerable corridor, where a higher-end Slight in in place.

Wegman


Day 4 and Day 5

Valid 12Z Tue Aug 11 2026 - 12Z Thu Aug 13 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
OHIO VALLEY...

20z Update: Still on track for an excessive rainfall threat
Tuesday from portions of IL/IN into WV. Some uncertainty on the
exact latitude of the threat, and not seeing any strong trend in
todays models. Thus only made a small southward expansion of the
Slight risk to account for the consistent AIFS, which has been on
the southern end of the model envelope. We were able to cut back
on the Marginal risk over the Northeast. Only the GFS keeps
robust convection across this region...with the
ECMWF/ECENS/CMC/AIGFS all showing drier air over the northeast and
the better convective threat farther southwest.

On day 5 (Wednesday) a heavy rainfall threat will continue on the
periphery of the southern U.S. ridge, with convection from the
Southwest into the central/northern Plains, and then extending
southeastward into portions of the OH and TN valley. The inherited
Slight risk over WV was made a bit larger to include some of
eastern KY and southern OH. Overall confidence in this area is low,
as seeing divergence in model QPF solutions. Think there will be
an excessive rainfall threat, but could easily see this Slight
risk area shift in location and/or size over the coming days.
Still seeing a model signal for organized convection over portions
of SD/NE, but enough uncertainty on those details to keep a
Marginal for now. The Southwest still look to be more active
Wednesday, but spatial confidence too low to introduce any slight
risk area at this time.

Chenard

...Previous Discussion...

...Great Lakes/Northern and Central Plains/Midwest/Upper Ohio
Valley...
Early to mid next week, broad cyclonic flow expected to
persist across the northern tier of the Lower 48 from the Northern
Plains/Upper MS Valley, eastward through the Midwest/Great Lakes
and into the Northeast, to the south of a closed mid-level low
moving from Hudson Bay across Quebec. A frontal boundary will lie
along the southern periphery of this broad trough. Embedded
shortwaves in this broad cyclonic flow will accentuate lift in the
vicinity of this front where PW values are expected to rise to
~1.75" ahead of each set of shortwaves and low- level confluence is
expected. When looking at the ingredients, organized convection
looks most likely south of the Great Lakes within eastern portions
of the Corn Belt Tuesday into Tuesday night and across the Upper
Ohio Valley Wednesday into Wednesday night, where hourly rain
amounts to 2.5" are possible. There is significant disagreement on
Wednesday/Wednesday night on where a heavy rain axis would be
across the eastern Corn Belt/Midwest, so for the time being, no
Slight Risk has been drawn/assigned. Recent guidance nudged north,
so the Slight Risk area on Tuesday was shifted similarly. For
Wednesday/Wednesday night, a Slight Risk area was introduced for WV
where the guidance has relatively good agreement on the potential
for heavy rainfall within areas of significant relief.

..Southwest/Colorado...
Broad Marginal Risk areas exist early to mid next week across
portions of the Southwest, Rockies, and Great Basin. Ridging/warm
700 hPa temperatures shifts north and weakens/lowers somewhat
across the Intermountain West, which opens the door to broader
convective development in the Southwest at some point, potentially
next Wednesday, where above average precipitable water values
persist over the next five days with minimal day to day variation.
While there continues to be low confidence with the details,
locally heavy rains with the potential of 2" an hour amounts could
become more expansive Wednesday. A Slight Risk cannot be ruled out
across portions of this area as we get closer to the days in
question. As usual, urban areas, arroyos/dry washes, box/slot
canyons, and burn scars would be most at risk for flash flooding.

Roth


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