Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
825 PM EDT Thu Sep 17 2026
Day 1
Valid 01Z Fri Sep 18 2026 - 12Z Fri Sep 18 2026
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHERN PLAINS, OHIO VALLEY, & SOUTHWESTERN U.S...
0100Z Update: Main changes to the Day 1 ERO were to curtail the
outlook areas over most areas, given the observational and
mesoanalysis trends. Also adjusted the Slight Risk area over the
OH Valley a bit to the south and east, which too corresponded to
the observational and current CAM trends, including recent HRRR
guidance and HREF and REFS rainfall exceedance probabilities. Both
of these Slight Risk areas align along the northern portion of the
max PW axis and the tight PW gradient just to the north, where ML
CAPE trends have been on the uptick over the past 3 hours per the
SPC mesoanalysis.
Hurley
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
825 PM 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...
21Z update... Overall the environment and setup described below is
still expected, with the greatest amounts focusing over the
northern Illinois and Indiana. Minor reshaping of the Marginal and
Slight Risk areas were made across Minnesota, Wisconsin and Iowa to
reflect the latest guidance and WPC QPF footprint.
Campbell
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.
...South Florida and Keys...
A Marginal Risk was raised for this period as a continuation from
the Day 1 period. High rainfall rates will be possible across the
southern tip of the state and across the Keys. A conveyor of deep
tropical moisture will keep pumping into the region to help
enhance rainfall. With the ridge in place, weak low level winds
will lead to slow storm motion thus increasing the threat for local
flash flooding concerns.
Campbell
...Western U.S...
21Z update... The guidance continues to show convection tracking
further west across portions of Wyoming, Montana and Idaho
therefore a minor westward nudge of the Marginal Risk area was made
for this update.
Campbell
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
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
825 PM 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...
21Z update...
Campbell
Previous Discussion...
...Midwest and Ohio Valley...
21Z update... A minor reshaping of the western bounds of the
Marginal and Slight Risk areas over Iowa was made to reflect the
latest guidance trends and overlap areas of heavy rainfall on Day
2.
Campbell
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.
...South Florida and Keys...
A Marginal Risk was raised for this period as a continuation from
the Day 2 period. The environment previously described will still
be present with convection likely ongoing over recent wet areas.
The threat for localized flash flooding will persist across the tip
of Florida and the Keys.
Campbell
...Western and Central U.S...
21Z update... Convection over southwest Montana and into Idaho as
depicted in the current suite of guidance was not covered by the
inherited Marginal Risk. A westward expansion was made to encompass
sensitive areas to potentially heavy rainfall and/or rain rates
greater than 0.5 inch/hour.
Campbell
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
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
825 PM EDT Thu Sep 17 2026
Day 4 and Day 5
Valid 12Z Sun Sep 20 2026 - 12Z Tue Sep 22 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE
MIDWEST AND NORTHEAST SUNDAY/SUNDAY NIGHT, AND ACROSS PARTS OF THE
LOWER MID ATLANTIC MONDAY/MONDAY NIGHT...
2030Z Update: On day 4 (Sun-Sun night), based on the latest
guidance trends (including GEFS/ECENS/CMC QPF exceedance
probabilities), have nudged the Slight Risk a little farther north
across the Midwest, to encompass much of eastern IA, northern IL,
and northwest IN. Have also added a Slight across eastern PA-much
of NJ into southern New England.
On Day 5, while not an overwhelming consensus from the guidance,
there are growing signals for at least a low-end Slight across
parts of the Lower Mid Atlantic (central to SE VA and parts of
eastern NC), ahead of the front and within the 2-2.25" PW plume.
This also follows the uptick in QPF from the NBM (both
deterministic and probabilistic), along with some of the AI models
(AIGFS especially).
Hurley
Previous discussion...
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
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
825 PM EDT Thu Sep 17 2026
Day 4 and Day 5
Valid 12Z Sun Sep 20 2026 - 12Z Tue Sep 22 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE
MIDWEST AND NORTHEAST SUNDAY/SUNDAY NIGHT, AND ACROSS PARTS OF THE
LOWER MID ATLANTIC MONDAY/MONDAY NIGHT...
2030Z Update: On day 4 (Sun-Sun night), based on the latest
guidance trends (including GEFS/ECENS/CMC QPF exceedance
probabilities), have nudged the Slight Risk a little farther north
across the Midwest, to encompass much of eastern IA, northern IL,
and northwest IN. Have also added a Slight across eastern PA-much
of NJ into southern New England.
On Day 5, while not an overwhelming consensus from the guidance,
there are growing signals for at least a low-end Slight across
parts of the Lower Mid Atlantic (central to SE VA and parts of
eastern NC), ahead of the front and within the 2-2.25" PW plume.
This also follows the uptick in QPF from the NBM (both
deterministic and probabilistic), along with some of the AI models
(AIGFS especially).
Hurley
Previous discussion...
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
