Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
325 AM EDT Thu Sep 17 2026
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 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt