Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Fri Sep 18 2026
Day 1
Valid 16Z 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 IN SOUTHWEST MONTANA...
...Upper Mississippi Valley into Midwest...
16Z update... Latest guidance depicted higher QPF reaching
southeast Wisconsin and northeast Illinois, areas that have had
recent heavy rainfall and are sensitive to additional rainfall. The
eastern boundary of the Slight Risk and the high-end Slight Risk
were expanded about 1-2 tier of counties to the east. The highest
amounts are still expected to focus over southeast Minnesota and
west-central Wisconsin during this period.
Campbell
A disturbance over the Northern Plains will continue to track
eastward from the Dakotas across southern Minnesota and into
Wisconsin by late tonight. A moisture-laden atmosphere is already
in place across much of the Upper Midwest will be supportive of
local downpours as the disturbance approaches. The development of a low
level jet will enhance the convective footprint across the Upper
Midwest with much of the guidance showing a blossoming area of
heavy precipitation across southern Minnesota, advancing eastward
with the mean flow. Model consensus QPF remained in the 2 to 4
inch range...and the previously issued high-end Slight Risk area
tended to cover the spread of higher QPF so only minor adjustments
were needed.
...South Florida and Keys...
16Z update... A small northward nudge of the Marginal Risk area was
made to reflect the latest model trends for South Florida.
Campbell
A Marginal Risk was maintained due to high rainfall rates being
possible across the southern tip of the peninsula southward 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.
...Western U.S...
16Z update... Current radar and observations have organized
convection lifting through portions of Utah, Nevada and Idaho that
were either partially covered the Marginal Risk area or well
outside of it. Some of these cells are producing locally heavy
rainfall therefore the western bounds of the Marginal Risk was
expanded westward to now include all of central/southern Idaho,
eastern Oregon, northeast Nevada and northwest/west Utah.
Additionally, with the Slight was adjusted a bit further east
across south-central Montana.
Campbell
An upper low over Idaho will make its way into the interior
Northwest and 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 situates itself over the northern Great Basin into the
northern Rockies. Based on the cell behavior on Thursday and the
deterministic QPF and spaghetti plots of 2 inch QPF from the GEFS
and EC Ensemble member...introduced a Slight risk area in southwest
Montana. The previously expressed concern about progressive cell
motion working against the flash flooding potential still exists
but the rates could still prove problematic. This area is
consistent with a forecast of 0.5-1.5 inches.
A Marginal Risk area also remained in place for a corridor from
New Mexico and parts of Arizona northward into the Northern Rockies
as lingering moisture and a diurnal instability maximum will lead
to widely scattered thunderstorms for another afternoon leading to
isolated flash flood concerns for yet another day, mainly in the
terrain.
Bann
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Fri Sep 18 2026
Day 2
Valid 12Z Sat Sep 19 2026 - 12Z Sun Sep 20 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST INTO PARTS OF THE SOUTHERN GREAT LAKES AND IN
SOUTHERN MONTANA...
...Midwest and Ohio Valley...
The environment will remain conducive for heavy rainfall with
precipitable water values firmly at or above 1.8" across the entire
region, locally exceeding 2 inches 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. In particular, there
is potential for overlap between the rainfall on Saturday with a
west-northwest to east- southeast stripe of low Flash Flood
Guidance in northern Illinois which resulted from heavy rainfall 36
to 48 hours earlier. The RRFS was most supportive of this scenario
but it has a bias of high QPF and higher probability of
exceedance. Even so...finer scale models also show some overlap
with locally heavy amounts so the Slight Risk was expanded in areal
coverage and deemed a higher-end Slight Risk...especially if there
are multiple rounds of convection.
...Western and Central U.S...
Latest deterministic model and WPC trended upward across the
Northern Rockies on Saturday as an upper low over the Northern
Rockies slowly pivots eastward providing continued/prolonged
diffluent flow. The flow aloft ahead of the upper low creating a
threat for pockets of heavier convective potential over the Front
Range of Colorado/Wyoming while also still delivering heavier
precip to Montana as ascent remains strong under the closed low. As
a result...there was enough support to upgrade a portion of
southern Montana to a Slight Risk given the expect amounts on
Saturday that would be in addition to what falls today.
Over the Central Plains, scattered thunderstorms will be possible
along and just ahead of an associated cold 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.
...South Florida and Keys...
A Marginal Risk was continued this period as a continuation from
the Day 1 period. High rainfall rates will be possible across the
southern tip of the peninsula southward across the Keys with
flooding a concern due to weak steering flow.
Bann
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Fri Sep 18 2026
Day 3
Valid 12Z Sun Sep 20 2026 - 12Z Mon Sep 21 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY AND ACROSS A SMALL PORTION OF THE NORTHEAST US...
...Ohio Valley/Southern Great Lakes...
Surface low pressure moving eastward across the Mid-Mississippi
Valley will help draw moisture northward into the Ohio Valley
leading to a continued chance of showers and thunderstorms from
Saturday into early Sunday. The rainfall amounts do not look to be
blockbuster in magnitude but Flash Flood Guidance in the area is
suppressed from several round of heavy to excessive rainfall. As a
result...expanded the previously issued Slight Risk focused along
the lower 1- and 3-hourly flash flood guidance values.
Farther to the east...trimmed the areal coverage of a previously
issued Slight Risk along parts of the Interstate 95 corridor from
northern New Jersey into Connecticut based on shrinking
deterministic QPF footprint and waning ensemble support.
However...there should be good convergence along a well defined
front separating strong/cool high pressure over Canada from
unstable airmass to the south. Latest runs suggest that the threat
of excessive rainfall should occur in the early part of the period
in this area then drop as an airmass with lower precipitable water
sweeps in.
Rockies to the Central Plains...
There continues to be a Marginal Risk of excessive rainfall from
the Southern High Plains northward to the Western High Plains
adjacent to the Northern Rockies as deep moisture remains in place
with weak shortwave energy embedded helping to fuel scattered
convection.
...South Florida and Keys...
A Marginal Risk was maintained given a continuation from the Day 2
period. High rainfall rates will be possible across the southern
tip of the peninsula southward across the Keys.
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Fri Sep 18 2026
Day 4 and Day 5
Valid 12Z Mon Sep 21 2026 - 12Z Wed Sep 23 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE
SOUTHERN MID-ATLANTIC STATES AND NEW MEXICO...
Southern Mid-Atlantic States...
An upper level low/trough moves across the Upper Midwest into the
Great Lakes while an upper low in the eastern Gulf shears northward
ahead of the closed low and a surface high pressure system on the
strong side of September climatology wedges in east of the
Appalachians. Precipitable water values are forecast to be
1.75-2.25" across the Southern Mid- Atlantic States and Carolinas
in the vicinity of where a confluence zone at 850 hPa will exist.
Enough instability and effective bulk shear should exist for
thunderstorms with heavy rainfall. Hourly rain amounts to 2.5" with
local totals to 4" would be possible, given the available
parameters wherever thunderstorms merge or manage to train. The
best agreed upon area for Monday into Tuesday night for heavy
rainfall/more numerous convection is the area in and around south
to southeast VA, so a Slight Risk exists both days there. There's a
bit of uncertainty concerning a heavy rain signal near WV and
eastern OH and on either side of southeast VA, so left the a decent
radius of a Marginal Risk around the Slight Risk.
Southern High Plains...
Increasing low-level Gulf moisture with time coupled with upper
level Pacific moisture which may have a connection with a future
northeast Pacific tropical cyclone should lead to an increasing
threat for excessive rainfall with time in and near the Southern
High Plains. Tuesday/Tuesday night looks better than Monday/Monday
night, particularly across eastern NM where there's fairly good
agreement on 2-4" areal average rain, which led to the introduction
a Slight Risk area (day 5). On Tuesday and Tuesday night, enough
effective bulk shear and instability should be available when
combined with high moisture (1.25-1.5" precipitable water values)
to allow for organized convection. Hourly rain amounts to 2.5" with
local totals to 4" are easily supported in the forecast
environment.
Midwest/Southern Appalachians...
The Midwest isn't showing a unified/cohesive signal for heavy
rainfall Monday/Monday night but enough effective bulk shear,
moisture, and instability should exist near an incoming upper
low/trough for organized thunderstorms. The area around IN/OH/WV
needs to be watched for a possibly upgrade to the Slight Risk
should the guidance show a more cohesive/agreeable heavy rainfall
signal considering recent ground saturation and the usual
sensitivity in the Appalachians.
Roth
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Fri Sep 18 2026
Day 4 and Day 5
Valid 12Z Mon Sep 21 2026 - 12Z Wed Sep 23 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE
SOUTHERN MID-ATLANTIC STATES AND NEW MEXICO...
Southern Mid-Atlantic States...
An upper level low/trough moves across the Upper Midwest into the
Great Lakes while an upper low in the eastern Gulf shears northward
ahead of the closed low and a surface high pressure system on the
strong side of September climatology wedges in east of the
Appalachians. Precipitable water values are forecast to be
1.75-2.25" across the Southern Mid- Atlantic States and Carolinas
in the vicinity of where a confluence zone at 850 hPa will exist.
Enough instability and effective bulk shear should exist for
thunderstorms with heavy rainfall. Hourly rain amounts to 2.5" with
local totals to 4" would be possible, given the available
parameters wherever thunderstorms merge or manage to train. The
best agreed upon area for Monday into Tuesday night for heavy
rainfall/more numerous convection is the area in and around south
to southeast VA, so a Slight Risk exists both days there. There's a
bit of uncertainty concerning a heavy rain signal near WV and
eastern OH and on either side of southeast VA, so left the a decent
radius of a Marginal Risk around the Slight Risk.
Southern High Plains...
Increasing low-level Gulf moisture with time coupled with upper
level Pacific moisture which may have a connection with a future
northeast Pacific tropical cyclone should lead to an increasing
threat for excessive rainfall with time in and near the Southern
High Plains. Tuesday/Tuesday night looks better than Monday/Monday
night, particularly across eastern NM where there's fairly good
agreement on 2-4" areal average rain, which led to the introduction
a Slight Risk area (day 5). On Tuesday and Tuesday night, enough
effective bulk shear and instability should be available when
combined with high moisture (1.25-1.5" precipitable water values)
to allow for organized convection. Hourly rain amounts to 2.5" with
local totals to 4" are easily supported in the forecast
environment.
Midwest/Southern Appalachians...
The Midwest isn't showing a unified/cohesive signal for heavy
rainfall Monday/Monday night but enough effective bulk shear,
moisture, and instability should exist near an incoming upper
low/trough for organized thunderstorms. The area around IN/OH/WV
needs to be watched for a possibly upgrade to the Slight Risk
should the guidance show a more cohesive/agreeable heavy rainfall
signal considering recent ground saturation and the usual
sensitivity in the Appalachians.
Roth
