Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
904 PM EDT Mon Aug 10 2026

Day 1
Valid 01Z Tue Aug 11 2026 - 12Z Tue Aug 11 2026

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
MIDWEST, OHIO VALLEY, & SOUTHERN ARIZONA...

01Z Update...

Hi-res models performed poorly through the day across the Midwest-Ohio
Valley, indicating convection would break out much earlier than
what has occurred. However, convection has finally started
organize, sparked by a mid-level shortwave moving from northern
Illinois into Indiana that is interacting with a moist (PWs 1.75-2
in) and very unstable airmass (MUCAPEs 4000-5000 J/kg). This should
continue to support storm development through the evening, with
the coverage of storms producing 2+ in/hr rates increasing. While
much of the 12-18Z guidance has verified poorly, the more recent of
runs of the HRRR/RRFS appear to have a better handle on the
current conditions and therefore, along with radar/satellite
trends, were the basis for much of the updates for the central to
eastern U.S. While some of the totals advertised by the daytime
guidance may be well-overdone and/or spatially misplaced, still
feel the Slight Risk centered over the Ohio Valley is warranted.
Heavy rates, along with areas of training are expected to support a
swath of heavy accumulations, with the heaviest amounts centering
along a northwest-southeast axis extending from northern Indiana
to West Virginia. Overnight localized totals of 3-5 inches can be
expected along this axis. Refer to WPC #999 for additional details
concerning the near-term heavy rainfall and flash flooding threat
across this region.

Across the Southwest, made some minor adjustments based on
observation trends, including a small expansion of the Slight Risk
centered over southeastern Arizona.

Pereira


Day 2
Valid 12Z Tue Aug 11 2026 - 12Z Wed Aug 12 2026

...THERE ARE A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY...

...Midwest and Ohio Valley...

20Z Update: Continued active setup across the Midwest to Ohio
Valley will lead to widespread heavy rainfall from multiple
convective complexes that will also exhibit potential for training
and back-building over areas of IN/OH during the period. A growing
consensus within both the CAMs and global deterministic allowed for
a higher confidence forecast across much of southern Ohio back into
eastern Indiana to connect with the already forecast MDT risk over
portions of West Virginia. Latest HREF blended mean QPF depicted a
long axis of 2-4" totals between areas just east of Indianapolis
all the way towards Charleston, WV with some of the local maxima
actually closer to 6-8" when assessing individual deterministic.
This setup is conducive for these totals with a classic summer
convective scenario of multiple MCS propagating through a
persistent northwest to southeast aligned mean flow thanks to a
squeeze player between the northern periphery of a broad ridge
across the Southern U.S. and a deep cyclonic gyre spinning up over
east-central Canada. The combined effort between the two distinct
upper features will create a mid to upper level "highway" for
several shortwave impulses to migrate out of the High Plains and
traverse the central Midwest into the Ohio Valley. The coupling of
favored jet dynamics and increased large scale forcing from the
prevalent mid-level vorticity maxima will interact with a suitable
thermodynamic environment positioned over the Ohio and Tennessee
Valley's as noted via some prolific theta_E advection occurring
during Tuesday afternoon.

Area PWAT anomalies on the order of 1-2 standard deviations above
normal will be present across much of the Ohio Valley back into the
central and southern Midwest providing a sufficient deep, moist
profile that will be conducive for efficient rainfall processes
necessary for more formidable heavy rain accumulations.
Increasingly backed flow in the lower boundary layer will occur
over portions of IN/OH thanks to the disturbance advecting into the
area by later Tuesday leading to a period of training and/or back-
building convection after initiation, carrying through at least the
first half of the evening hours. Remnant cold pool from today's
convection will be a focal point for the training potential and the
recent CAMs have been locking in on the area extending from eastern
IN into southwestern WV as the most likely orientation and location
of the surface feature in question. This correlated to some
significant rainfall forecasts from the 12z CAMs suite within that
zone leading to enhanced neighborhood probabilities for both the
>3" (50-80%) and >5" (30-50%) thresholds during the period. 24hr
10yr-ARI exceedance probs across the same areas are fairly robust
with a bullseye of 40-60% located between Muncie, IN to Wilmington,
OH with the same area already seeing 20-35% probs for even the
100yr-ARI exceedance threshold, a testament to the anticipated
magnitude of heavy rainfall for the coming setup.

In coordination with the OHRFC and local WFO's from IN/OH/WV, the
previous MDT risk was expanded across portions of eastern Indiana,
the southern-third of Ohio, into western side of WV, mainly along
and west of the I-79 corridor. Significant flash flooding is likely
in these areas with widespread flash flood warnings and rapid
riverine responses likely to cause issues lingering through the
rest of the week as the overall setup extends beyond the D2 period.

Kleebauer

...Southern Arizona...

20Z Update: Latest CAMs and associated HREF continue to pin another
round of heavy rainfall across the southern and southeastern
portion of AZ thanks to elevated PWATs between 2-3 deviations above
normal and a very buoyant environment conducive for heavy rainfall
development. Prob fields for >1" remain very high (70-90%) over the
area extending from Cochise county over into the eastern half of
Pima, extending north towards the I-10 corridor in Tuscon. I-19
remains the focal point for heavier rainfall, however the ridge
positioning would invite more of southeast AZ into the picture for
locally heavy rains, including the Huachuca Mountains that will
likely be another focus for development as we move into Tuesday
afternoon. The previous SLGT risk forecast was maintained as the
signal remains consistent with little run-to-run variability.

Kleebauer

..Previous Discussion..

Southeast AZ remains most at risk for more persistent heavy
rainfall, so left the Slight Risk intact. Across the Great Basin,
the Marginal Risk was broadened as moisture subtly increases from
Monday to Tuesday as an upper level trough settles in to the CA
Coast and ridging near the Four Corners weakens, which should lead
to less mid-level capping and broader convective coverage.

Roth


Day 3
Valid 12Z Wed Aug 12 2026 - 12Z Thu Aug 13 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE OHIO VALLEY, MIDWEST, & CENTRAL HIGH PLAINS...

...Portions of the Plains/Ohio Valley/Midwest/Corn Belt...

20Z Update: The previous SLGT risk with high-end SLGT risk
potential remains across portions of the Ohio Valley as the signal
for heavy rainfall continues over the region, despite continued
inconsistencies within the global deterministic. The setup on
exactly where the heaviest rainfall will focus is still on the
tricky side as the location of the remnant cold pool from
Tuesday/Wednesday AM convection will be a focal point for where the
next round of convection will transpire. There is a growing
likelihood that the area extending from northern KY through
southern OH will be the primary area of interest when you couple
with the expected heavy rains the prior periods leading to priming,
or even already flooded areas before the next round occurs. The
global deterministic is all over the place with where the heavier
rains will occur, but one consistency is the areal expanse of heavy
convection is broader than the previous periods leading to a much
larger SLGT risk situated from the Midwest through much of the Ohio
Valley.

The prospects for a risk upgrade where any overlap of heavy
rainfall is high, so the expectation is for an addition of a higher
risk probable at this point in time. That said, we need a bit more
consensus from ensembles and deterministic, alike to prompt an
upgrade over a particular area. The most notable area for a
consideration to an upgrade is likely the southwestern quadrant of
WV into southeast OH where complex terrain and funneling within
the valleys west of I-79 and within the bounds of I-64 have the
greatest potential just based off the National Blend and modest
probs for >1" and >2" thresholds. For now, felt it was prudent to
provide another round or two of model runs before inserting any
upgrades, but will make mention that the opportunity in the future
is higher than normal for continued significant flash flooding and
an expected upgrade.

Kleebauer

..Previous Discussion..

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 that
has shifted into Quebec. A frontal boundary will lie along the
southern periphery of this broad trough from portions of the
Northern and Central Plains into portions of the Ohio Valley and
Appalachians. Embedded shortwaves in this broad cyclonic flow will
accentuate lift in the vicinity of this front where PW values are
expected to rise to ~2" ahead of each set of shortwaves and low-
level confluence is expected.

The guidance is showing a growing signal for heavy rainfall that at
the moment is most centered over the NE Sand Hills. Due to the
hydrology there, thought it prudent to maintain the inherited
Marginal Risk.  When looking at the ingredients, organized
convection looks most likely near portions of the Ohio Valley and
the Corn Belt Wednesday into Thursday morning, where the better
1000-500 hPa thickness difluence is forecast to remain. Hourly
rain amounts to 3" and local totals to 6" remain possible. A high
end Slight Risk lies across portions of the region due to previous
days of rains further saturating soils. There is a bit of spread of
the heavy rain signal seen in the guidance, with the 00z UKMET/00z
ECMWF the lowest and the 00z Canadian Regional/RDPS the highest.
Due to the uncertainty in the heavy rain signal, held off on a
Moderate Risk for the time being.

Roth

..The West/Central & Northern Plains...

20Z Update: The previous SLGT risk forecast was generally unchanged
from the overnight issuance as models remains steadfast in their
presentation of heavy rainfall situated across southeastern AZ.
This area will have seen multiple days of heavy rains leading into
the period, so the prospects for scattered flash flood concerns are
elevated relative to normal. Totals of 1-2", locally higher are
forecast across the area with multiple areas across the Interior
West and the Northern Plains also likely to see similar precip
outputs thanks to a favorable setup across the western half of the
CONUS. FFG's are much higher outside the southern deserts, so the
threat for more significant impacts are lower, so a broad MRGL
exists elsewhere to account for the threat.

Kleebauer

..Previous Discussion..

A broadening upper level trough is forecast across portions of the
West and Great Basin Wednesday into Thursday morning. Broad
Marginal Risk areas exist near and ahead of this upper level trough
for portions of the Southwest and Great Basin. Increased upper
level troughing opens the door to strengthening low-level level
flow and broader convective development/coverage across the West
and Plains when compared to previous days as the mid-level cap
further erodes. Above average precipitable water values slowly
shift northward through the region. While there continues to be low
confidence with the details, locally heavy rains with the
potential of 2.5" an hour amounts could be problematic.  Urban
areas, arroyos/dry washes, box/slot canyons, and burn scars would
be most at risk for flash flooding. A Slight Risk was introduced
across southeast AZ.

Roth


Day 4 and Day 5

Valid 12Z Thu Aug 13 2026 - 12Z Sat Aug 15 2026

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FROM THE MID-
MISSOURI VALLEY THROUGH THE OHIO VALLEY, PORTIONS OF THE HIGH
PLAINS, AND SOUTHEASTERN ARIZONA TO WESTERN NEW MEXICO ON
THURSDAY...

20Z Update...
The 12Z guidance did not present any reason to make significant
changes to the previous outlook. Therefore, made only minor
adjustments.

Pereira

Previous Discussion...
Out west, the monsoon will have one final
day where moisture across Arizona and New Mexico will be well above
normal. Flattening upper level troughing that lifts northeastward
away from Arizona and southern New Mexico should allow the showers
and storms that form in that region to reduce the total atmospheric
moisture quite a bit, with no replenishment overnight Thursday
night. The Slight risk in this region follows where numerous prior
days heavy rains impacted portions of southeastern Arizona and
western New Mexico. Low FFGs still support widely scattered
instances of flash flooding due to highly favorable hydrology
(hydrophobic soils) in the flood prone areas.

A large Slight Risk was introduced for much of the Front Ranges and
High Plains from the Sangre de Cristos of northern New Mexico north
through southeastern Montana. The aforementioned lifting upper
level trough will have the opposite effect as further south, namely
greatly increasing convective coverage and intensity in this
generally flood prone region. A potent upper level shortwave riding
northeastward along the eastern periphery of the positively tilted
longwave trough will locally greatly increase convective coverage
across the Plains of Colorado in particular. With a potent easterly
low level jet advecting deep tropical moisture that has been in
place across the eastern half of the country westward, the storms
will run into an increasingly moisture rich and unstable
environment. This will support backbuilding and training storms as
the fuel for the storms is continually replenished. Further
upgrades to the risk outlook are possible in the coming days,
especially should urban areas such as Denver remain in the forecast
for multiple inches of rain Thursday afternoon and evening.

Finally, the Slight across portions of the Midwest and lower Ohio
Valley was expanded east through much of West Virginia. With
continued rain for a third consecutive day expected into West
Virginia, despite somewhat lower total expected rainfall, the flood
prone nature of the terrain there supports a higher risk for
flooding. Downstream along the Ohio River, they will also be
enduring the latest of a multi-day rainfall event on Thursday, also
suggesting that FFGs by Thursday will be much lower across much of
the Ohio Valley and supporting the Slight Risk upgrade. A plume of
deep tropical moisture will bank itself along a stationary front
suck just south of the southern ends of Lakes Michigan and Erie.
This will support backbuilding and training of convection from Iowa
straight through to West Virginia in the form of numerous rounds of
storms that each follow a similar track, resulting in repeating
storms. Some areas in this zone too may require risk upgrades as
the event draws closer and moves into CAMS range.

By Day 5/Friday, the signals for heavy rain become much more
diffuse and spread out, with a few areas still highlighted,
including northeastern Colorado, much of Iowa, and southern West
Virginia. All 3 of these areas may have been hit hard with heavy
rains on Day 4/Thursday, but since it's highly likely there will be
large changes in both the placement and intensity of the heavy rain
signals in each of these areas, have opted to keep all 3 in a
Marginal risk for now, albeit a higher end one. Slight Risks will
definitely be considered should these signals remain in future
forecasts.

Wegman


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