374
FOUS30 KWBC 170725
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
325 AM EDT Thu Sep 17 2026

Day 1
Valid 12Z Thu Sep 17 2026 - 12Z Fri Sep 18 2026

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTH-CENTRAL PLAINS, MIDWEST, & SOUTHWESTERN U.S...

...Northern Plains & Midwest...

Current UA analysis over the CONUS notes a strong 600dm ridge
positioned over the Lower Mississippi Valley with the northern
fringes of the height field extending into the Upper Midwest and
expanding as far west as the Central High Plains. A robust
shortwave centered over central AZ with origins from the tropics,
will eject out of the Southwestern U.S through the course of the D1,
pivoting up through the Central Rockies and rounding the
northwestern flank of the ridge, entering into the Northern Plains
as we head into the following evening. This shortwave will also
provide a significant PWAT increase as noted via both the the
ensemble anomaly plots and IVT climatological percentiles
approaching the 95-99th percentile up across South Dakota into
southern portions of MN by the end of the forecast period. There
are even some ensemble members pushing for PWAT indices up towards
not just the daily max, but monthly max for the September time
frame over places like Rapid City in SD, a testament to the depth
of the moisture field, as well as the origin of the deep layer
moisture from the tropics. Given the strength of the forecast
shortwave ejection, regional forcing will increase significantly as
the disturbance approaches leading to more organized convective
clusters to materialize by later this afternoon and evening,
lingering through the overnight hours to Friday morning as the
disturbance migrates across SD. Current indications are for heavy
convection to develop over the WY Front Range, eventually migrating
northeast into SD with a blossoming of the precip field as we move
into the evening thanks to the additional aid from a budding LLJ
over the High Plains.

Model outputs continue to be bullish on a swath of 2-4", locally
higher occurring within the confines of the shortwave impulse as
the environmental conditioning and ascent pattern remain favorable
within proximity to the disturbance. Assessment of local forecast
bufr soundings indicate a traditional tall, skinny CAPE signature
with a saturated column typical of tropical-like air being pulled
poleward. These types of environments become more privy to enhanced
rainfall rates and more efficient warm rain processes that can
cause issues for areas further north. This setup is more favorable
for at least scattered flash flood prospects over the Northern
Plains into the Upper Midwest with the primary threat likely
occurring over the terrain areas of SD, extending east-northeast
into the area of I-90 and north, especially within close proximity
of the center of the disturbance. Latest HREF neighborhood probs
for >2" are between 50-80% across a large portion of the High
Plains with the western extension back into northwestern NE
rotating back into southwestern SD and encompassing much of central
and southern SD. Modest probs for >3" also exist with the centroid
positioned just east and southeast of Rapid City placing the threat
firmly into Badlands National Park.

A high- end SLGT remains the forecast across much of SD with a
recent expansion into eastern zones of the state given the current
trends within guidance for heavier rain to spread towards the Sioux
Falls area and surrounds the final 6-8hrs of the forecast period.
The SLGT extends as far east as southwestern MN and northwest IA
and as far west-southwest as the Cheyenne, WY region just east of
the Larime Range.

...Southwest U.S...

Prevailing southerly flow will maintain a steady plume of elevated
moisture across the Southwestern CONUS, but the shortwave
disturbance associated with the proliferation of the previous
period`s convection will vacate to the northeast allowing for a
gradual de-escalation of the convective magnitude over the region
for Thursday. That said, lingering PWAT anomalies ranging from the
90-99th percentile of climo will allow for another busy afternoon
of convective potential over much of NM and eastern AZ, extending
into eastern UT and western CO, leading to an elevated flash flood
concern, once again. Precip totals between 1-3" are forecast for
the Sangre de Cristos and San Juan`s down through much of the
terrain focused in the interior areas of NM. Latest HREF EAS
probabilities for at least 1" remain substantial (60-90%) for the
San Juan`s in CO with a solid 30-60% located over the southern
portion of the Sangre de Cristos, as well as the Sacramento
Mountains. Despite the environment not as prolific as Wednesday,
sufficiently buoyant air and upslope flow within these ranges will
still lead to enhanced convective development during the
traditional diurnal heat flux leading to heavy rainfall likely in
these particular zones. The Sacramento Mountains are still dealing
with the sensitive burn scar remnants in the area and those have
been tested already with the onslaught of convection from the
previous period. Heightened sensitivity for this region leads to a
potential for locally Moderate level impacts embedded within the
broad higher-end SLGT risk forecast.

The previously inherited SLGT was generally maintained with an
extension of a higher-end SLGT further north into western CO to
cover for the latest trends of heavy rainfall into areas that were
recently impacted today and will still be recovering. SLGT risk
extends into northeast UT with the MRGL expanded through northern
UT to cover the SLC area up into southeastern ID as some guidance
maintains isolated thunderstorm concerns within the urban corridor
during the first half of the forecast. Additional influence of an
upper low entering into the Pacific Northwest will add a broad
diffluent area downstream with expectation for scattered convection
to occur over much of eastern ID into western WY and southwest MT
late-afternoon and evening. Less of a threat for flash flooding
with this setup, however there`s indications that a few cells could
be robust enough to bring a threat of flash flooding to these
areas.

Kleebauer


Day 2
Valid 12Z Fri Sep 18 2026 - 12Z Sat Sep 19 2026

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

...Upper Mississippi Valley into Midwest...

Our disturbance over the Northern Plains will continue to traverse
the northern tier of the CONUS with a trajectory eastward into
south- central MN, eventually into WI by Friday late-Friday
afternoon. Environment will still favor heavy rainfall prospects
as deep, tropical origin moisture will accompany the disturbance
along with rich theta_E air that will translate to better regional
instability to enhance rate potential. Current model consensus is
for a wave of heavy rainfall to protrude into the area later Friday
morning and continue for several hours into the evening, ending
west to east as it migrates out of the Northern Plains into the
Upper Midwest/western Great Lakes. Further north over Ontario,
surface high pressure will situate itself over the northern half of
the Upper Midwest leading to an increased low- level convergence
regime that will work in tandem with the disturbance located
further south. There`s a sharp delineation in where the heaviest
rainfall will occur on some of the guidance, and the pattern
evolution makes sense given the positioning of the high pressure,
and upper pattern over Canada would likely exhibit a decent area of
confluence around the latitude of the Arrowhead meaning the precip
field will only be able to push so far north. By Friday evening, a
stout LLJ development over the Midwest will enhance the convective
footprint over the region with much of the guidance showing a
blossoming area of heavy precipitation across southern MN,
advancing eastward with the mean flow.

There is some discrepancy in the global deterministic with the
vigor of the LLJ and potential for backed flow over the course of
the evening in proximity to southeastern MN and western WI. The
ECMWF continues to be the most potent with the development of a
weak 850mb low across southern MN that would enhance the prospects
for significant rainfall and back-building precip potential in the
aforementioned area. The GFS is not as aggressive and provides a
setup that`s a touch more progressive and could limit the upper
threshold of heavy rain potential. ML outputs from both the EC-AIFS
and AIGFS signal the 850mb low development, although timing of
when it occurs is where they differ leading to the AIGFS showing a
heavier QPF signal a bit downstream into WI and less so in MN. In
any case, the ML and ensemble means lean towards the more
aggressive solution in the case of the evolution with this setup,
so tended to lean closer to that consensus with a stronger QPF
footprint still positioned over southern MN into western WI.

At this juncture, the area with highest probability for seeing
heavy rain is likely south of I-94 with the forecast of 2-4", on
areal average, locally higher, located from Minneapolis down
through southeastern MN into WI. It will be something we`ll be
monitoring when it comes to the area of greatest impact potential
as the setup is very much conducive for significant rains with
prospects for flash flooding in those urban settings located along
and south of I-94. A broad high-end SLGT risk remains forecast for
those areas above with the SLGT extending eastward into the
Milwaukee metro and all of southern WI. This setup could very well
see a higher risk forecast as we move closer and fine tune details
considering the environmental conditioning and forecast synoptic
evolution.

...Western U.S...

Upper low is progged to be over eastern OR while slowly migrating
through the interior Northwest, maneuvering into the area of the Northern
Rockies as we head into early Saturday. Ascent under the influence
of the upper low will likely aid in scattered convective potential
with locally heavy rainfall occurring under the low center, and
within the broad diffluent axis that will situate itself over the
northern Great Basin into WY/MT. Precip will be relatively
progressive considering the nature of the flow ahead of the
disturbance, so the prospects for more appreciable flash flooding
remains low. Still, some of these cells could produce heavy enough
rain to have some isolated flash flood concerns during Friday
afternoon and evening through the overnight hours as the setup
evolves. Best chance for convection with slower overall motions
will be under the center of the upper low which is generally
textbook for these types of patterns. A MRGL risk is in effect
across the above areas with the best chance located over MT down
into western WY. This area is consistent with a forecast of
0.5-1.5" with some local amounts of 2-3" plausible in the hardest
hit areas. Rainfall rates will be between 0.5-1"/hr in the
strongest cells which would preclude the necessity for a higher
risk, however if the signal comes up even a little bit further, a
targeted upgrade is not out of the question.

A MRGL risk also exists for NM, eastern UT into CO as lingering
moisture and a diurnal instability max will lead to widely
scattered thunderstorms for another afternoon leading to isolated
flash flood concerns for yet another day, mainly in the terrain.

Kleebauer


Day 3
Valid 12Z Sat Sep 19 2026 - 12Z Sun Sep 20 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDWEST, GREAT LAKES, AND OHIO VALLEY...

...Midwest and Ohio Valley...

The remnants of the convective conglomeration over the Upper
Midwest will translate eastward through the southern half of the
Great Lakes with heavy rainfall likely in-of western Lake Michigan
areas, including Milwaukee to Chicago, extending east into lower
Michigan through the first half of Saturday. Environment will
remain conducive for heavy rainfall with PWAT firmly above 1.8"
across the entire region, even breaching 2" for those areas west of
Lake Michigan. This will provide efficient rainfall types, even in
convective elements that could put down 2-4" of rainfall over the
entire area leading to at least scattered flash flood potential
over the impacted area(s). The highest potential for flash flooding
will reside in those urban corridors littered across southern WI,
northern IL, and southern MI where urban sprawl is a common
occurrence in these locales. Latest ECENS probabilities for >1"
are between 50-90% for the entire stretch from Madison, WI through
Milwaukee into the area of Grand Rapids, extending into the Detroit
metro. Considering the conservative probs typically shown on the
ECENS, this signal is fairly robust and follows the general theme
of the heaviest QPF axis bisecting this area.

By Saturday afternoon. a strong surface ridge over northern
Saskatchewan will make a quick advancement to the southeast,
strengthening in the process as it moves into southern Manitoba by
Saturday evening. This will press a cold front through the region
and finally disperse the warm, moist environment further south into
the northern and central Ohio Valley. Increasing convergence will
occur within proximity to the front leading to a narrow corridor of
convection to materialize Saturday evening over southern MI into
the northern reaches of IN/OH which could enhance the flash flood
threat further south with lingering elements over the southern
peninsula of MI. Pending the latitudinal push of the front, it`s
plausible that flash flood concerns will continue for southern MI
which would likely factor into a higher-end SLGT potential, or
perhaps an upgrade in the risk if the threat is appreciable, and/or
overlaps with areas hit hard earlier Saturday from the first wave
of heavy rainfall. It will be something to monitor in the coming
days as guidance gets a strong handle on the setup.

...Western and Central U.S...

Our upper low over the Northern Rockies will slowly pivot east
providing continued diffluent flow and sustained convective
footprint across the interior west and High Plains of MT/WY.
Several smaller mid-level perturbations around the base of the mean
trough will eject east-northeast within the flow ahead of the upper
low creating a threat for pockets of heavier convective potential
over the Front Range of CO/WY while also still delivering heavier
precip to MT as ascent remains strong under the closed low, as well
as just to the north. Models are indicating 1-2", locally higher
possible in both these regions with the area over the High Plains
the most likely to see a stronger convective footprint with
supercells and multi-cell clustering the probable convective modes
given the increasing shear and forcing within modest instability. A
broad MRGL is in place currently for both areas, encompassing a
large area due to the influence from our slowly migrating upper
low.

Over the Central Plains, surface low will develop over the Corn
Belt thanks to the pattern downstream, advancing east while
dragging a cold front through the region. Scattered thunderstorms
will be possible along and just ahead of the front as it advances
southeastward leading to perhaps an isolated flash flood threat for
any stronger storms forecast. The MRGL risk extends throughout much
of the Central Plains and Missouri Valley as a result.

Kleebauer


Day 4 and Day 5

Valid 12Z Sun Sep 20 2026 - 12Z Tue Sep 22 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR
NORTHERN MO SUNDAY/SUNDAY NIGHT...

By Sunday/Sunday night, the upper trough makes its way east
through the Northern Plains towards the Upper Midwest. A frontal
boundary will be the focus for thunderstorms with heavy rainfall
from West TX into the Midwest the OH Valley, then into the Mid-
Atlantic States/Southern New England as a cold front approaches.
Both NAEFS and ECMWF SATs show at least 90th percentile PWs in
place from the MS Valley to the Mid- Atlantic. Since there is
reasonable agreement, and it has rained heavily as of late, added a
Slight Risk in and around Northern MO, figuring the ground won`t
fully dry out by then. Given the parameters present, hourly rain
amounts to 2.5" with local totals to 4" are possible in the
environment expected anywhere within the risk areas wherever
thunderstorms merge or manage to train.

For Monday/Monday night, an upper level low/trough moves towards
Chicago. High enough moisture, instability, and effective bulk
shear for organized thunderstorms with heavy rainfall is expected
to stretch from portions of the OH Valley across the Mid-Atlantic
States and southern New England within an area of decent 850 hPa
confluence near and ahead of a front. Portions of the Southeast
should have enough moisture/instability for heavy rain concerns
around the northeast periphery of an upper level low, but it`s
unclear if enough effective bulk shear will be present to organize
convection. A broad Marginal Risk has been introduced; there`s not
enough model agreement on a heavy rain axis to depict a Slight Risk
at this time in and near OH/WV, but an upgrade can`t be ruled out
as we move closer in time to the possible impact, which bears
watching. Hourly rain amounts to 2.5" with local totals to 4" would
be possible, given the available parameters, anywhere within the
Marginal Risk area wherever thunderstorms merge or manage to train.

Roth


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt



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