FOUS30 KWBC 100653
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
253 AM EDT Mon Aug 10 2026

Day 1  Valid 12Z Mon Aug 10 2026 - 12Z Tue Aug 11 2026

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

...Midwest and Ohio Valley...
A deep layer cyclone across Hudson Bay rules the roost, keeping
the equatorward edge of the Westerlies from the Northern Plains
into portions of the Ohio Valley. This is near where a front and/or
resultant outflow boundaries from convection will align and where
an instability gradient is most likely to set up. Confluent low-
level westerly flow is forecast near this boundary. Two to three
rounds of convection are expected within an area of difluent
1000-500 hPa thickness, implying organized convection. The
ingredients available support hourly rain amounts to 3" where cells
merge/train or where mesocyclones form.  While the guidance is
showing an emergent signal for heavy rain across portions of SD and
IA, it is neither high enough nor agreeable enough for a Slight
Risk but do believe the threat is on the higher end of a Marginal
Risk. Farther to the east across portions of the Midwest/eastern
Corn Belt south of the Great Lakes into the Upper Ohio Valley,
a broad swath of rainfall is anticipated. Flash flood guidance is
modest to low here, so even though the mesoscale signal for 3"+ in
24 hours was modest and spotty, so saw no issues with the high end
Slight Risk inherited from continuity. Local totals to 6" are
possible on an isolated basis.

...Southeast...
Disorganized convection with brief, locally high rain rates are
expected during daytime heating. The more concentrated area of
heavy rainfall lies along portions of the Middle and Upper TX
Coasts during Monday morning as onshore flow brings a slug of 2"+
precipitable water values and instability off the Gulf of Mexico.
The Marginal Risk from continuity appears to cover the situation sufficiently.

...Southern Arizona...
Monsoonal moisture increases across southern Arizona on Monday.
Storms are expected to initiate along the Mogollon Rim, and the
numerous ranges of southeastern Arizona and terrain in NM during
the afternoon hours. Unlike prior days, enough easterly flow,
especially in the upper levels, will successfully push those
showers and storms off the mountains and towards the lower
elevations and valleys of southwestern Arizona and could lead to
some level of organization. Hourly rain amounts to 2" are possible
where cells sit, train, or backbuild. The inherited Slight Risk is
little changed -- southeast AZ is forecast to see the longest
period of heavy rainfall.

Roth
 

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

...THERE ARE A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR
PORTIONS OF OHIO AND WEST VIRGINIA...

...Midwest...
The general flow pattern is similar to Monday. The equatorward
edge of the Westerlies is forecast to lie from the Northern Plains
into portions of the Ohio Valley where there is also low-level
confluence, difluent 1000-500 hPa thickness, and an instability
gradient to focus organized convection. This led to similar risk
area placement. Hourly rain amounts to 3" and local totals to 6"
remain possible given the available ingredients. There is a decent
likelihood that prior days' rains over these same areas will have
saturated the soils further by Tuesday, and when combined with the
general very flashy nature of the rivers/terrain of West Virginia
and coordination with RLX/the Charleston WV forecast office, a
Moderate Risk area was introduced for portions of southern OH and western WV.

...Southern Arizona...
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, & ARIZONA...

...Portions of the Plains/Ohio Valley/Midwest/Corn Belt...
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/old
12z ECMWF the lowest and the 00z Canadian Regional/RDPM the
highest. Due to the uncertainty in the heavy rain signal, held off
on a Moderate Risk for the time being.

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

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
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