Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1134 AM 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 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1134 AM EDT Sat Aug 8 2026
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...
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 and Detroit
are the most likely impacts.
...Marginals Elsewhere...
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 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1134 AM EDT Sat Aug 8 2026
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...
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.
...Southeastern Arizona...
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 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1134 AM EDT Sat Aug 8 2026
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
EASTERN CORN BELT & UPPER OHIO VALLEY...
...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
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1134 AM EDT Sat Aug 8 2026
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
EASTERN CORN BELT & UPPER OHIO VALLEY...
...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
