FOUS30 KWBC 171245
QPFERD

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

Day 1  Valid 1235Z 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...

13Z update... Heavy rainfall from the past couple of days has lead
to some areas to reach or exceed soil saturation and there are
locations with ongoing flash flooding. A current line of storms
tracking over the area may aggrevate the flooding through the mid-
morning hours before fizzling out before early afternoon. There is
another signal for additional showers and thunderstorms to possibly develop
late tonight across Illinois and Indiana that may track over
vulnerable locations.

Campbell

...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
 
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