Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 PM EDT Fri Sep 18 2026
Day 1
Valid 01Z Sat Sep 19 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...
01Z Update...
Utilizing recent hires guidance and radar trends, adjustments
concentrated mostly on narrowing the focus for where the threat for
heavy to excessive rain is expected to linger or develop late in
the evening and overnight hours -- trimming away some of the
outlook areas where the threat has ended.
Areas where the threat is expected to continue include the Slight
Risk, which was maintained over portions of Idaho and Montana, as
well as a 'high-end Slight' extending from southeastern Minnesota
and northeastern Iowa to southern Wisconsin and far-northern
Illinois. Models continue to indicate that the heaviest rains
overnight are expected to develop along an axis extending from near
Rochester, MN to Milwaukee, WI, with HREF and REFS probabilities
indicating localized amounts exceeding 3 inches are likely along
this line.
Pereira
Previous Discussion...
...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
429 AM EDT Sat Sep 19 2026
Day 1
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...
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN MONTANA TODAY INTO TONIGHT...
...Midwest and Ohio Valley...
Maintained the Moderate Risk area that was coordinated on Friday
for portions of the Midwest and Ohio Valley. The model signal
remained strong for a concentrated corridor of heavy rainfall from
multiple rounds of training convection. One round will be on-going
over parts of southwestern/southern Wisconsin at the start of the
period after leaving a stripe of excessive rainfall that began over
far southeast Minnesota and made its way into southwest Wisconsin.
Questions remain about the ramification lingering rainfall and
cloud cover have on convection evolution later in the period. With
1.5 to 3inches of rainfall that has already fallen as of 0730Z in
parts of southwestern Wisconsin and more expected...the Moderate
risk area was expanded northward.
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.
Maintained the placement of the Moderate Risk area primarily on
the southern periphery of global model consensus to account for a
typical bias (subsequent rounds of convection are often further
into the inflow of unstable air) than guidance would indicate. It
still lines up well with the depiction shown by the RRFS, REFS, AI-
GFS. and ECMWF- AIFS. Where the heavy rainfall footprints overlap,
the potential for locally significant rainfall totals with
considerable impacts are possible. 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 Ohio, far western West
Virginia, and far southwest Pennsylvania, 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...
Upgraded to a Moderate Risk in parts of southern Montana as
deterministic and ensemble guidance QPF continued to increase.
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.
The primary uncertainty continued to be with 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 WPC
deterministic QPF increased to nearly 3.5 inches while the 00Z
HRRR and RRFS continued to 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. They also depicted
neighborhood probabilities in excess of 70 percent at the 100-yr
ARI threshold for 24 hours in southeast Montana. If the
instability is not realized, this could be high, but the trend has
been toward the higher values over the past few outlook cycles. As
a result...a targeted Moderate Risk was hoisted.
...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 remained
low. Additionally, the Marginal Risk areas were maintained in West
Texas and the Texas Big Bend, as well as South Florida.
Bann
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Sat Sep 19 2026
Day 2
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 THE OHIO VALLEY...
...Ohio Valley/Southern Great Lakes...
Very few changes were made to the placement of the Day 3 Excessive
Rainfall Outlook as it propagated into the Day 2 period. There was
a tightening of the east-west length and a subtle southward shift
to match the latest guidance but maintained the focus for heavy
rainfall in the Ohio Valley, There remained some potential for
backbuilding and training convective bands affecting those areas
from late tonight into Sunday. 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 removed as model and
ensemble QPF dwindled. Cool, stable air being fed out of Canada
by a sprawling area of high pressure will overspread the area and
limit rainfall rates over an increasing amount of New England and
the Northeast states. A boundary separating this airmass with
moister and more unstable conditions (still fairly modest magnitude
of instability) to the south may still prove to be the focus for
convection capable of producing moderate to heavy rainfall rates
but the signals for heavy rainfall have become weaker and more
isolated in nature.
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.
Bann
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Sat Sep 19 2026
Day 3
Valid 12Z Mon Sep 21 2026 - 12Z Tue Sep 22 2026
...THERE IS A BROAD MARGINAL RISK AREA FROM PARTS OF THE SOUTHERN
MID-ATLANTIC STATES AND NEW MEXICO...
Southern Mid-Atlantic States... An upper level low/trough moving
from the Upper Midwest into the Great Lakes while an upper low in
the eastern Gulf shears northward ahead of the closed low will set
the stage for showers and thunderstorms on Monday. Model spread
increased while the overall signal for heavy amounts diminished
during the past 24 hours. Given the increased uncertainty combined
with relatively dry antecedent conditions, was not inclined to
reintroduce a Slight Risk area. However...isolated instances of
excessive rainfall are possible
Southern High Plains...
Increasing low-level Gulf moisture with time could lead to an
increasing threat for excessive rainfall in and near the Southern
High Plains as mid-level flow increases over the central US and
starts to push a surface front eastward. Precipitable water values
should generally be around 1.75 inches from Texas northward into
the Mid-Mississippi Valley with enough effective bulk shear and
instability to result in organized convection capable of producing
heavy rainfall with the associated risk of flash flooding.
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 the expected rainfall in the Day 1 period and
how much of that lingers into Day 2.
Bann
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Sat Sep 19 2026
Day 4 and Day 5
Valid 12Z Tue Sep 22 2026 - 12Z Thu Sep 24 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE
APPALACHIANS AND NEW MEXICO...
...Ohio Valley and Mid-Atlantic...
Mid level troughing with multiple embedded shortwaves will slowly
track eastward into the OH Valley and Mid-Atlantic, interacting
with a backdoor front. Should be plenty of moisture overrunning
this front, resulting in what should be a broad area of steady
rainfall. However, quite a bit of uncertainty with the details, as
a coastal low should eventually develop offshore, helping advect
drier air in from the north over portions of the Mid-Atlantic.
This trend has resulted in a southward trend in the rainfall axis
on day 4. Also of question is the degree of instability, with the
backdoor front likely making a push all the way into the Carolinas,
leaving a more stable airmass to its north. Given these
uncertainties we opted to drop the central/southeast VA Slight risk
with this update. Overall just enough uncertainty on placement,
and questionable peak rainfall rates given the limited instability,
to be confident in a scattered flash flood risk. We will keep a
Marginal though, as plenty of moisture is in place, so even just a
bit of CAPE could drive some low topped convection and locally
higher rates. We will continue to monitor and possible a Slight
risk is reintroduced somewhere over the region as we get closer and
confidence increases.
The Slight risk over PA and WV on day 4 (Tue) was maintained. An
inverted trough should help focus convection here, and while still
weak, up to ~500-1000 j/kg of CAPE could develop. In addition, this
area has lower FFG than the coastal plain, and should see
additional rain in the days leading up to Tuesday, potentially
lowering FFG further.
Similar setup on day 5 (Wed), just with even more uncertainty as
model spread increases. The degree of model spread supports keeping
the risk level at Marginal, with the understanding an embedded
Slight risk is possible as we get closer. The best chance appears
to be from TN/KY into OH and WV as the upper low moves overhead and
decent instability is forecast. If the 00z EC and AIFS are
correct, a Slight risk will be needed on future updates. However,
model spread is high and run to run continuity is low. The GFS is
more aggressive along the coastal plain with an offshore low
bringing heavy rain inland. However, it generally seems like an
outlier at this time, with both the GEFS mean and AIGFS more
offshore (although still more aggressive than the non-ncep
guidance).
...New Mexico...
A favorable setup for heavy rain and scattered to numerous
instances of flash flooding appears to be setting up for New
Mexico on days 4 and 5, and probably continuing into days 6 and 7
as well. Conditions will likely get more sensitive with time as we
see multiple days in a row with areas of heavy rainfall, and this
may very well end up being a MDT risk event at some point during
the Tue-Fri timeframe. Broad troughing in the west will place NM
persistently within the right entrance region of a strong upper
jet. One stronger wave will move to the north Tuesday, helping
enhance lift, with a few more subtle waves on Wednesday providing
ascent. We will also have easterly upslope flow in the low levels,
sufficient instability, and eventually a possible PW connection to
an eastern Pacific tropical system. This combination of forcing,
instability and well over 90th percentile PWs for September,
suggest heavy rainfall and a growing flash flood risk. We will
carry Slight risks for now given we are several days out and some
uncertainties continue with the details, but overall this seems to
be an above average confidence forecast for this lead time, and
would expect to see some flash flood issues arise. Can not rule out
the potential for locally significant flash flooding in that Tue-
Fri timeframe given the setup, so we will need to closely monitor
trends.
Chenard
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Sat Sep 19 2026
Day 4 and Day 5
Valid 12Z Tue Sep 22 2026 - 12Z Thu Sep 24 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE
APPALACHIANS AND NEW MEXICO...
...Ohio Valley and Mid-Atlantic...
Mid level troughing with multiple embedded shortwaves will slowly
track eastward into the OH Valley and Mid-Atlantic, interacting
with a backdoor front. Should be plenty of moisture overrunning
this front, resulting in what should be a broad area of steady
rainfall. However, quite a bit of uncertainty with the details, as
a coastal low should eventually develop offshore, helping advect
drier air in from the north over portions of the Mid-Atlantic.
This trend has resulted in a southward trend in the rainfall axis
on day 4. Also of question is the degree of instability, with the
backdoor front likely making a push all the way into the Carolinas,
leaving a more stable airmass to its north. Given these
uncertainties we opted to drop the central/southeast VA Slight risk
with this update. Overall just enough uncertainty on placement,
and questionable peak rainfall rates given the limited instability,
to be confident in a scattered flash flood risk. We will keep a
Marginal though, as plenty of moisture is in place, so even just a
bit of CAPE could drive some low topped convection and locally
higher rates. We will continue to monitor and possible a Slight
risk is reintroduced somewhere over the region as we get closer and
confidence increases.
The Slight risk over PA and WV on day 4 (Tue) was maintained. An
inverted trough should help focus convection here, and while still
weak, up to ~500-1000 j/kg of CAPE could develop. In addition, this
area has lower FFG than the coastal plain, and should see
additional rain in the days leading up to Tuesday, potentially
lowering FFG further.
Similar setup on day 5 (Wed), just with even more uncertainty as
model spread increases. The degree of model spread supports keeping
the risk level at Marginal, with the understanding an embedded
Slight risk is possible as we get closer. The best chance appears
to be from TN/KY into OH and WV as the upper low moves overhead and
decent instability is forecast. If the 00z EC and AIFS are
correct, a Slight risk will be needed on future updates. However,
model spread is high and run to run continuity is low. The GFS is
more aggressive along the coastal plain with an offshore low
bringing heavy rain inland. However, it generally seems like an
outlier at this time, with both the GEFS mean and AIGFS more
offshore (although still more aggressive than the non-ncep
guidance).
...New Mexico...
A favorable setup for heavy rain and scattered to numerous
instances of flash flooding appears to be setting up for New
Mexico on days 4 and 5, and probably continuing into days 6 and 7
as well. Conditions will likely get more sensitive with time as we
see multiple days in a row with areas of heavy rainfall, and this
may very well end up being a MDT risk event at some point during
the Tue-Fri timeframe. Broad troughing in the west will place NM
persistently within the right entrance region of a strong upper
jet. One stronger wave will move to the north Tuesday, helping
enhance lift, with a few more subtle waves on Wednesday providing
ascent. We will also have easterly upslope flow in the low levels,
sufficient instability, and eventually a possible PW connection to
an eastern Pacific tropical system. This combination of forcing,
instability and well over 90th percentile PWs for September,
suggest heavy rainfall and a growing flash flood risk. We will
carry Slight risks for now given we are several days out and some
uncertainties continue with the details, but overall this seems to
be an above average confidence forecast for this lead time, and
would expect to see some flash flood issues arise. Can not rule out
the potential for locally significant flash flooding in that Tue-
Fri timeframe given the setup, so we will need to closely monitor
trends.
Chenard
