Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
325 AM EDT Thu Sep 17 2026
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 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt