Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
332 PM 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
332 PM EDT Fri Sep 18 2026
Day 2
Valid 12Z Sat Sep 19 2026 - 12Z Sun Sep 20 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN PORTIONS OF
THE MIDWEST...PARTICULARLY EASTERN IOWA, NORTHERN ILLINOIS, AND
NORTHERN INDIANA...
...Midwest and Ohio Valley...
A Moderate Risk upgrade was coordinated with local WFOs primarily
across portions of E IA, N IL, far S WI, far SW MI, and N IN. There
is growing model signal for a concentrated corridor of heavy
rainfall from multiple rounds of training convection. One round may
be ongoing at the start of the period, a continuation from the
current Day 1 period. Although hi-res guidance varies in the vigor
and placement of the early morning convection, EAS probabilities
from the HREF and REFS suggest it would be most likely around 12Z
Saturday from southern Wisconsin into southwest Michigan. This may
have implications for future rounds of convection if denser and
more persistent cloud cover exists in those areas.
In other words, what happens Saturday morning will have a
significant role in the placement of the heavy rainfall maxima for
the rest of the forecast period. However, there is reasonably high
confidence in additional rounds of convection that should develop
near the effective front as low-level flow veers and strengthens to
around 30 knots. This configuration, and low-level inflow being
close to parallel with the deep layer mean flow, should favor
backbuilding and training. The overall risk area will be embedded
within an anomalous deep moisture plume (PWs around or in excess of
2 inches, above the 99th percentile for mid- September). Combined
with strong instability in areas that experience more sunshine and
heating, this would be supportive of high hourly rain rates,
potentially exceeding 2 inches at times.
The Moderate Risk was drawn on the southern periphery of global
model consensus to account for a typical bias (subsequent rounds of
convection are often further into the inflow -- in this case to the
WSW -- than guidance would indicate), and potential for morning
cloud cover in the northern portions of the risk area. This places
it closer to what the RRFS, REFS, AI-GFS and ECMWF-AIFS are
depicting. Where the heavy rainfall footprints from the multiple
rounds of convection overlap, there will be the potential for
locally significant rainfall totals. These areas, or where intense
rain rates intersect urban areas or saturated soils, significant
flash flood impacts will also be possible.
The Slight Risk was also expanded east to OH, far western WV, and
far southwest PA, primarily for later in the forecast period as
low-level westerly flow increases with PW values also on the
increase. There is some potential for backbuilding and training
convective bands (shown by some hi-res model reflectivity
forecasts) in the vicinity of the hill country and western upslope
regions of the Central Appalachian region by Saturday
evening/night.
...Southern Montana...
Anomalously high moisture (PWs near the mid-late September maximum)
and a slow-moving mid-upper level low will provide the focus for a
concentrated area of heavy rainfall in southern Montana. The
primary uncertainty will be the degree of instability. Greater
instability in combination with the available moisture could yield
small bands of training convection with heavy rain rates on the
north side of the upper low. The HRRR and RRFS depict some of the
high end scenarios that could be realized with more instability,
with localized 24-hour rainfall maxima as high as 7-8 inches. This
would be well in excess of the 100-yr ARI thresholds, and both hi-
res ensemble systems have very high neighborhood probabilities of
reaching the 24-hour 100-yr ARI in southeast Montana (60-80
percent). If the instability is not realized, this could be high,
but the trend has been toward the higher end of the Slight Risk
probability range today, and a targeted Moderate Risk could need to
be considered in future updates if guidance continues to increase.
...Elsewhere...
Existing Marginal Risk areas were largely maintained with minimal
edits. Scattered thunderstorms within a broader plume of
anomalously high moisture could produce locally heavy rainfall and
flash flooding from the Rockies to the Central Plains and as far
east as the Central Appalachians. However, confidence in placement
of localized heavy rainfall within the Marginal Risk area is low.
Additionally, the Marginal Risk areas were maintained in West Texas
and the Texas Big Bend, as well as South Florida.
Lamers
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
332 PM 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 MIDWEST AND OHIO VALLEY, AND ACROSS A SMALL PORTION OF THE
NORTHEAST US...
...Ohio Valley/Southern Great Lakes...
Very few changes were made to the existing Day 3 ERO. The primary
adjustment was to expand the Slight Risk a bit further into the
Upper Ohio Valley and Central Appalachians (SE OH, SW PA, WV). As
described in the Day 2 ERO, there is some potential for
backbuilding and training convective bands affecting those areas
from late Saturday Night into Sunday as low-level westerly flow
increases and PWs increase close to 2 inches in some spots, which
is above the 99th percentile for mid-late September. Further west,
the Slight Risk was mostly kept the same, and it still extends from
E IA into N IL and N IN. Some of the threat may be due to a
continuation of convection from late in the Day 2 period (late
Saturday Night) into Sunday morning. As certainty grows in where
the heaviest rain will fall Saturday and Saturday evening, it's
possible the risk level could need to be increased if it seems
likely repeated rounds of heavy rain will affect saturated soils
into Sunday. However, confidence is low on placement or convective
evolution beyond the first 36 hours of the forecast, so this is
being noted as a possibility for now. Anyone within the Slight Risk
area should be monitoring the potential for flash flooding,
however, given just how anomalously high the moisture levels are.
Farther to the east, the Slight Risk was maintained, although the
ensemble support continues to wane a bit for very heavy rain rates.
There is good potential for rain in portions of the Northeast, and
there should be good convergence along a well defined front, but
available instability may be limited. Therefore, although the
chances of rain are high, this may be distributed more evenly
through the forecast period rather than in intense bursts. Guidance
will continue to be evaluated as to whether to maintain the Slight
Risk, trim the area, or convert it to a Marginal Risk.
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.
...Elsewhere...
A large Marginal Risk area was maintained from the northern Rockies
through much of the Plains and into western Texas. Scattered
thunderstorms embedded within a large plume of anomalously high
moisture will pose the threat for localized heavy rainfall and high
instantaneous rain rates. However, confidence is low on placement,
so a Marginal Risk seems appropriate. The one area that may be
most likely to see a risk upgrade in the future would be in the
vicinity of the upper low progressing from Montana and Wyoming into
the western Dakotas. A Marginal Risk was also maintained in south
Florida where high rainfall rates are also expected.
Lamers
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
332 PM 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...
20Z Update...
For Day 4, the biggest adjustment was the removal of the Slight
Risk from southern Virginia and north-central North Carolina. The
overall signal for heavy amounts has diminished, while model spread
has increased. This increased uncertainty, along with relatively
dry antecedent conditions, supported the removal of the Slight Risk
for now.
For Day 5, with a little more model support and relatively greater
forecast confidence, maintained the Slight Risk in the Mid
Atlantic with some relatively minor adjustments. A bigger change
was the addition of a Slight Risk centered over the upper Ohio
Valley from southwestern Pennsylvania to southern West Virginia.
There is a decent signal for heavier QPF supported by deepening
moisture along an inverted trough in this area. In addition to the
growing signal for heavy rain, relatively low FFGs indicate this
area is more susceptible to flash flooding.
Elsewhere on Day 5, broadened the Slight Risk in New Mexico to
extend into southwestern New Mexico -- where there is fairly good
signal for organized amounts, supported in part my mid-level
energy lifting north into that part of the state.
Pereira
Previous Discussion...
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
332 PM 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...
20Z Update...
For Day 4, the biggest adjustment was the removal of the Slight
Risk from southern Virginia and north-central North Carolina. The
overall signal for heavy amounts has diminished, while model spread
has increased. This increased uncertainty, along with relatively
dry antecedent conditions, supported the removal of the Slight Risk
for now.
For Day 5, with a little more model support and relatively greater
forecast confidence, maintained the Slight Risk in the Mid
Atlantic with some relatively minor adjustments. A bigger change
was the addition of a Slight Risk centered over the upper Ohio
Valley from southwestern Pennsylvania to southern West Virginia.
There is a decent signal for heavier QPF supported by deepening
moisture along an inverted trough in this area. In addition to the
growing signal for heavy rain, relatively low FFGs indicate this
area is more susceptible to flash flooding.
Elsewhere on Day 5, broadened the Slight Risk in New Mexico to
extend into southwestern New Mexico -- where there is fairly good
signal for organized amounts, supported in part my mid-level
energy lifting north into that part of the state.
Pereira
Previous Discussion...
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
