FOUS30 KWBC 110714
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
314 AM EDT Tue Aug 11 2026

Day 1  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/Ohio Valley/Mid-Atlantic States...
The general flow pattern remains fairly static since yesterday.
The equatorward edge of the Westerlies is forecast to lie from the
Northern Plains into portions of the Ohio Valley and Mid- Atlantic
States where there is also low- level confluence, difluent 1000-500
hPa thickness, and an instability gradient to focus organized
convection. Precipitable water values are forecast to be 2"+ in
spots. Hourly rain amounts to 3" and local totals to 6" remain
possible given the available ingredients. Previous days' rains over
these areas have saturated the soils further across portions of
northern IL/northern IN/OH/WV/southwest PA. When combined with the
general very flashy nature of the rivers/terrain of West Virginia,
the Moderate Risk level continues to make sense. Recent guidance
led to some southward shift/expansion in the Slight/Moderate Risk
areas. An eastward expansion of the Slight Risk area into Richmond/
Baltimore/Washington DC was made as several members of the global
guidance had a heavy rain signal over or very near the region which
received heavy rainfall on Monday.

...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 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 2  Valid 12Z Wed Aug 12 2026 - 12Z Thu Aug 13 2026

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR PORTIONS
OF THE OHIO VALLEY AND SOUTHEAST ARIZONA...

...Portions of the Plains/Ohio Valley/Midwest/Corn Belt...
On Wednesday, 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 continues to show a 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. Spread remains in the guidance
concerning QPF magnitude, so allowed to existing risk levels to
ride, though there was some southward nudging to the Slight Risk
positioning based on the recent guidance.

..The West/Central & Northern High 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 High Plains when compared to previous days as the mid-level
cap further erodes, with upslope flow developing across portions of
the High Plains. Above average precipitable water values slowly
shift northward through the Intermountain West. While there
continues to be low confidence with the details, locally heavy
rains with the potential of 2.5" an hour amounts (particularly
across southeast AZ) 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 from continuity remains
across southeast AZ.

Roth
 

Day 3  Valid 12Z Thu Aug 13 2026 - 12Z Fri Aug 14 2026

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

Midwest/Ohio Valley...
A sharpening upper level trough extending south of a deep layer
cyclone in Quebec forces the leading edge of the Westerlies to
extend from the Central/Northern Plains through the Ohio Valley
into the southern Mid-Atlantic States. Enough moisture,
instability, and effective bulk shear should exist for cell
organization. Given the moisture available (precipitable water
values of 2"+), hourly rain amounts to 3" with local totals to 6"
would be possible. Continuity was fairly well maintained in this region.

High Plains...
Upslope flow behind a front and ahead of a surface low combined
with a broadening Western trough leads to increased moisture
pooling and instability across the region and leads to minimal, if
any, mid-level capping. The guidance brings 1.25-1.75"
precipitable water values into portions of eastern MT, eastern WY,
and eastern CO which would approach record levels for any time of
the year. ML CAPE could rise above 2000 J/kg. There's an
antiparallel flow relationship between the low-level upslope and
expected convective propagation, which should lead to high
precipitation efficiency wherever storms can cluster/have greater
coverage. There are a couple areas near the WY/CO/NE border
junction and irregular WY/MT/SD/ND border junction where the
guidance has higher amounts. Given the favorable ingredients for
heavy rainfall, believe the Slight Risk is higher end in those
areas. Risk areas were generally maintained from continuity.
Hourly rain amounts to 3" with local totals to 6" can't be ruled
out where cell stall, backbuild, merge, or train.

Intermountain West...
A broadening upper level trough brings an increasing south-
southwest low-level flow to portions of the region and cooling
aloft. Precipitable water values exceed 1" in spots, with 1.5-2"
values across southern AZ. Some areas could see ML CAPE rise above
1000 J/kg. Enough effective bulk shear could exist for cell
organization/training but it's unclear if enough cell coverage will
exist, outside of Southeast AZ. Risk areas were generally
maintained from continuity. Hourly rain amounts to 2" are possible
where cells train and/or merge. This would be most problematic in
burn scars, urban areas, box/slot canyons, and arroyos/dry washes.

Roth
 

Day 4 and Day 5  Valid 12Z Fri Aug 14 2026 - 12Z Sun Aug 16 2026

...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE FRONT RANGES AND PLAINS OF COLORADO AS WELL AS FOR PORTIONS OF
THE UPPER MIDWEST ON FRIDAY...

...Colorado and Adjacent Areas...
An easterly low level jet branching off of broad southerly flow up
the Plains will support a combination of upslope and convective 
rainfall across much of northern Colorado on Friday. PWATs near the
Kansas border will approach 1.75 inches, lowering to around 1 inch
into the Rockies behind the Front Range. This ample moisture will 
funnel north and west ahead of a large upper level trough over the 
West Coast. Upper level shortwaves will also support more 
widespread convective development across much of Colorado. A Slight
Risk area was introduced for the potential for flooding from these
storms, since many of them will be tied to the local terrain, but 
still being refueled by the easterly low level jet of moisture into
the Front Ranges.

...Upper Midwest...
A small Slight Risk area was introduced for portions of eastern 
Iowa, far southeastern Minnesota, southwest Wisconsin, and 
northwestern Illinois with this update. A strong warm front with 
PWATs over 2 inches in many areas will support plentiful convection
across this region. FFGs in the area are generally around 2 inches
per hour, lower in any urban areas. Thus, confidence has increased
given these very favorable factors to introduce a Slight at this 
time. The 00Z EC favors this area for heavier rain, which will 
ultimately be dependent on how fast the warm front pushes 
northeastward into Wisconsin.

...Day 5/Saturday...
The warm front pushing northeast into Wisconsin will favor 
continued heavy rainfall into this region from the ongoing 
storms/MCS that begins Friday. Given how far out this is, have 
opted to hold at just a Marginal for now, as the track and 
intensity of MCSs at this range are frequently far off from what 
ends up happening. That said, the abundant moisture, strong 
forcing, and low FFGs into Wisconsin would support Slight Risk 
level impacts. Any southward shift of the heaviest rains may impact
the Chicagoland area more significantly, so that will have to be 
monitored in the coming days.

Wegman
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