Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Fri Sep 11 2026
Day 1
Valid 01Z Sat Sep 12 2026 - 12Z Sat Sep 12 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN FAR SOUTHERN
ARIZONA & NORTHERN MINNESOTA...
21Z Update: Made some modifications to the Marginal Risk across the
Lower MS Valley into the Southeast and Lower Mid Atlantic.
Adjustments were based on the latest observational and mesoanalysis
trends, along with the latest HRRR trends and 18Z HREF/REFS QPF
exceedance probabilities. One of the notable changes was to include
the coastal plain of GA and SC in the Marginal. For further
information, refer to the latest Mesoscale Precipitation Discussion
or MPD #1250.
Hurley
Previous Discussion...
...Southern Arizona...
The upper level ridge over southern AZ will direct embedded 500mb
vorticity maxima westward along the US/Mexico border on Friday.
Much of AZ, southern UT, and western CO will continue to contend
with anomalous moisture content with PWs >1.5" throughout much of
southern AZ. Latest 00Z REFS/HREF probabilistic guidance does show
a section of far southern Arizona particularly at-risk for flash
flooding. The latest 00Z HREF shows moderate-to-high chances
(50-70%) for 3-hr QPF surpassing 3-hr FFGs around Santa Cruz
county Friday afternoon. For these reasons, the Slight Risk remains
on track for far south-central AZ.
...Upper Midwest...
A 500mb shortwave trough and an accompanying frontal boundary will
track through the Upper Midwest at the same time a plume of >90th
climatological percentile PWs manifests over the region. Storm
motions should be progressive, although some training thunderstorm
activity closer to the warm front is becoming inc as low pressure
heads for the MN Arrowhead. Rainfall totals could range between
1-2" (locally higher amounts up to 3") in northern MN, while
rainfall rates could approach 1.5"/hr as storms track across the
region Friday afternoon and evening.
...Lower MS Valley, TN Valley, Central Gulf Coast, and the
Southern Appalachians...
A 500mb shortwave trough tracking towards the Mid-MS Valley and a
sharp 200mb trough axis aloft will generate healthy divergence
over the Mid-South, while copious amounts of moisture reside over
much of the Southeast. A nearby frontal boundary will trigger
widespread thunderstorms that are likely to contain >2"/hr rainfall
rates within the more intense storms. Model guidance remains in
fairly good agreement on the widely scattered t-storm activity
before storms dissipate for most areas by late evening. PWs >2.0"
will be common from the central Gulf Coast to the mid-MS Valley,
while PWs in the OH/TN Valleys range between 1.75-2.0", which are
still above the 90th climatological percentile. Storms are likely
to form as far east as the Southern Appalachians and Carolinas,
although the flash flood threat is less likely along the Southeast
coast. Localized flash flooding is possible, especially in poor
drainage areas and within more urbanized communities.
Mullinax
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Fri Sep 11 2026
Day 2
Valid 12Z Sat Sep 12 2026 - 12Z Sun Sep 13 2026
...THERE ARE SLIGHT RISKS IN PORTIONS OF THE OHIO VALLEY AND MID-
ATLANTIC...
21Z update... The QPF footprint saw a notable increase in areal
coverage and amounts for portions of the central Appalachian
spine/surrounding areas and for a few locations in the northern
Mid-Atlantic. This increase over already sensitive soil/steep
terrain/vulnerable areas necessitated the expansion of the Slight
Risk further north into Pennsylvania and east across West Virginia
and western Maryland in addition to a southward expansion of the
Slight over central/eastern Virginia. With the trough lifting out
portions of the Gulf Coast will see a reduction in QPF, thus a
small section of the Gulf Coast was trimmed from the Marginal Risk
area. A minor southward nudge was made across north-central
oklahoma to reflect the latest QPF guidance and WPC forecast.
Campbell
Previous discussion...
...Ohio Valley & Mid-Atlantic...
The broad 500mb troughing setup in the Mid-South will induce
healthy upper-level ascent over the Ohio Valley and Mid-Atlantic
on Saturday. A warm front is forecast to lift north through the
Mid-Atlantic on Saturday that will act as a trigger for storms,
while daytime heating and height falls support widespread showers
and thunderstorms from the Gulf Coast to the northern Mid-Atlantic.
PWs will rise above 2.0" as far north as the I-95 corridor from
Richmond and D.C. to Baltimore and Philly by Saturday afternoon
with increasing instability as the warm sector drifts north. Warm
cloud layers are likely to be at least 12,000ft, favoring highly
efficient warm rain processes within developing thunderstorms.
Where the heaviest thunderstorm activity unfolds remains unclear.
However, the preference was to favor a Slight Risk along the I-95
corridor largely due to the greater concentration of metropolitan
communities.
West of the Appalachians, soils throughout the OH Valley are
growing increasingly more sensitive with 1-hr FFGs hovering as low
as 1" over southeast OH. It will not take much from this favorable
atmospheric and hydrologic environment to support a concern for
scattered instance of flash flooding. For these reasons, a Slight
Risk was issued for portions of the Upper OH Valley for Saturday.
...Central Plains...
An approaching 500mb disturbance and a strengthening southerly
low-level jet will interact with a frontal boundary that results in
scattered thunderstorms from the central High Plains to the Mid-MS
Valley. PWs ranging between 1.5-1.75" are generally around the 90th
climatological percentile and ample instability will be supportive
of strong thunderstorms producing >2"/hr rainfall rates.
Guidance has trended lower in QPF in recent forecast cycles,
although the reduction in QPF is largely because model spread in
where the axis of heavier storms occur remains unclear. A Marginal
Risk remains in place given the favorable atmospheric setup with
localized flash flooding possible.
...Florida...
Much of the Sunshine State will remain mired in an environment
containing copious amounts of moisture that will continue to
support scattered thunderstorms containing torrential downpours.
PWs between 2.0-2.25" will be common and storms will be capable of
causing localized flash flooding, especially in poor drainage areas
and in the more urbanized sections of Florida.
...Rockies & Southwest...
A strong 500mb closed-low with heights below the 10th
climatological percentile will work with an increasingly anomalous
PW field to produce scattered thunderstorms with efficient rainfall
over the Northern Rockies. Farther south, the pool of anomalous PWs
that supported daily rounds of excessive rainfall in the Southwest
through the week will stick around and cause additional
thunderstorms that produce locally excessive rainfall rates.
Instability will be lacking over the northern Rockies, but soils
are more saturated following recent rainfall and may provide a
slightly more favorable environment for localized flash flooding.
Mullinax
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Fri Sep 11 2026
Day 3
Valid 12Z Sun Sep 13 2026 - 12Z Mon Sep 14 2026
...THERE IS A SLIGHT RISK OVER PORTIONS OF THE MIDWEST...
21Z update... The NW-SE orientated QPF axis over the
Northern/Central Plains and Mississippi Valley had a southward
trend amongst a majority of the guidance for this time period. With
this shift portions of southern Wisconsin and eastern Illinois was
trimmed out of the Marginal while the southern boundary of the
Marginal now includes more of southern Missouri and southern
Illinois. The entire Slight Risk area was moved 1-2 tier of
counties southward as well.
Campbell
Previous discussion...
...Midwest...
To start Day 3 (Sunday) a frontal passage that is tied to a fast
moving 500mb shortwave trough heading for the Great Lakes will
trigger rounds of showers and thunderstorms from South Dakota and
the Nebraska Sand Hills to as far south as northern KS/MO and as
far east as the MS River. As the first shortwave trough exits
east, a strong 500mb closed low will head through the Northern
Rockies, prompting the development of low pressure in the northern
High Plains. This area of low pressure will work in tandem with
high pressure over the Great Lakes Sunday night to generate a 30-40kt
LLJ over the Central Plains. The LLJ will act to advect rich
moisture and warmer theta-e air into the Midwest with the frontal
boundary that passed through early Sunday morning lifting as a warm
front Sunday night and into early Monday morning.
Latest ECMWF guidance shows a pool of anomalous PWs (above the
97.5 climatological percentile) sitting over the Central Plains on
Sunday that lifts north into the Midwest Sunday night. With the LLJ
intersecting the frontal boundary, and elevated instability
available, additional strong thunderstorms are likely to form late
Sunday night and into early Monday morning. While exact placement
of the heaviest rainfall still differs, most guidance is in
agreement that the Midwest will be the focus for potential flash
flooding on Sunday and into Sunday night. The latest WPC QPF calls
for 1-2" of rainfall in central IA with locally higher amounts
possible.
...Southern Rockies & Four Corners...
The troughing pattern in the Northwest and the mid-upper level
ridge axis over the Southern Plains will direct additional
monsoonal moisture into the Southwest and Southern Rockies. ECMWF
percentiles suggest PWs will rise to above the 97.5 climatological
percentile Sunday evening as IVT values >300 kg/m/s direct moisture
into the regions. Some locations continue to sport saturated soils,
thus making these areas more prone to potential flash flooding.
While confidence in higher rainfall rates is lower at the moment,
the rgion's sensitive soils and potential for excessive rainfall
rates >1/0"/hr support a Marginal Risk as of this forecast cycle.
Mullinax
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Fri Sep 11 2026
Day 4 and Day 5
Valid 12Z Mon Sep 14 2026 - 12Z Wed Sep 16 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN PORTIONS
OF THE MISSOURI VALLEY INTO PARTS OF THE CORN BELT AND UPPER MIDWEST...
1930Z Update: Few changes were made to the Days 4/5 EROs based on
the 12Z model suite -- both deterministic and probabilistic. Based
on the guidance trends, we did trim western areas of the Slight
and especially Marginal Risk areas across the Upper MS Valley on
Day 4. The expectation here is that any lingering rainfall (along
the occluded front/TROWAL) will likely be more light-moderate in
intensity (i.e. rainfall rates below 1/3/6 FFG) given the lack of
deep- layer instability.
Hurley
...Day 4...
Early next week will likely see a growing concern for heavy
rainfall and widespread convective episodes beginning across the
Central and North-Central U.S. with a longwave trough/ridge
evolution likely to induce a prolonged threat for flash flooding.
The initial setup begins with two distinct features of interest,
one of which will become an indirect player in the anticipated
threat. The first feature is a strong mid-level trough ejecting
through the Northern Plains, making headway into the Northern Great
Lakes and adjacent Ontario Province with a subsequent surface low
and cold front materializing as a result. The cold front will be
the surface forcing mechanism that will aid in convective foci, as
well as a delineation point for where the heavier precipitation
prospects will evolve. The cold front will lose latitude back over
the Plains with the current evolution likely to take the frontal
boundary down to at least Kansas latitude before incurring some
resistance due to the evolution occurring downstream. That of which
is our second feature of note; a well-defined ridge axis that will
control the ambient weather pattern for the Southeastern U.S. up
through the Southern Mid Atlantic and Tennessee Valley for at least
the first half of next week. This ridge will be a road block for
any frontal progression upstream as the main ascent driver in the
shortwave trough will continuing ejecting northeast, leaving behind
the front and the primary driving mechanism for the surface feature
in question.
Monday afternoon and evening will see an uptick in convective
activity from the Southwestern U.S. up through the Central Plains
to Upper Midwest as a steady moisture advection pattern
materializes around the western and northern periphery of our broad
upper ridge field centered east of the Continental Divide.
Anticipating PWAT anomalies between +1 to +2 deviations, at
minimum, to be positioned over the aforementioned areas with
embedded shortwave perturbations rounding the western flank of the
ridge, eventually into the northern tier of the CONUS. Mean winds
aloft will become more parallel with the flow occurring in the
boundary layer leading to a heightened threat of training
convection across the Plains to Great Lakes area with the primary
concern materializing over the Missouri Valley into the Central
Midwest where the additional forcing brought upon by the stronger
mid-level shortwave ejection will aid in the heavier convective
footprint. PQPF from the current forecast and 90th percentile QPF
outputs are right within the 2-3" marker at this current lead, a
testament to the potential as historical precedence indicates
these types of outputs at lead tend to trend closer to more
impactful scenarios, so long as the synoptic evolution doesn't
deviate from consensus. Latest First Guess Fields are keen on a
broad SLGT risk across the Midwestern U.S. with an extension back
into IA and points northeast into the U.P. ML models and a few
global outputs extend that threat back further to the west to
include more of the Missouri Valley through eastern NE, perhaps
even into northern KS. Utilizing some of the recent probs >50% for
at least 1" precip on ensembles, the SLGT risk from previous
forecast was extended back into that above zone with the SLGT now
situated from eastern NE up through the Corn Belt, extending
northeast through portions of MN/WI and the Michigan U.P.
This threat will flow into the following day, but this is the
beginning of an active period of heavy rainfall potential with
prospects for future upgrades, pending the behavior of the evolving
synoptic pattern, both aloft (trough/ridge strength and
orientation) and at the surface (low pressure and cold front
positioning). MRGL risk extends back into the Southwest as
scattered thunderstorm activity will likely maintain a foothold on
the terrain of AZ/CO/NM thanks to the continued moisture advection
regime and suitable thermodynamic environment in place.
...Day 5...
The forecast synoptic pattern heading into Tuesday is trending to
continue a very active stretch for the Central U.S. as a frontal
boundary will struggle to lose much latitude over the course of the
first half of next week leading to what could be a focal point for
convection for multiple days beginning with the previous D4 time
frame. The longwave pattern will remain relatively stable east of
the Mississippi with a strong mid and upper ridge axis located
across the southeastern quarter of the U.S. with a formidable area
of surface high pressure dominating the weather pattern for these
areas. Across the Western CONUS, multiple shortwave troughs will
lead to a general trough axis and prevailing southwesterly flow
extending from the Southwest U.S. through the center of the nation
with an expected quasi-stationary frontal placement bisecting areas
from the Central High Plains to points east-northeast through the
Missouri Valley and Corn Belt with the eastern inflection back
into the Lower Great Lakes. This setup will be coincident with a
bit of a Maddox signature which typically precludes more
appreciable heavy rain prospects given the limited forward
propagation expected of a surface front with mean flow generally
parallel to the surface feature leading to training and back-
building scenarios within proximity of the front. Global
deterministic implies multiple smaller shortwave perturbations
ejecting northeast through the main axis of diffluence situated
over the above regions. This will only act as additional regional
ascent to aid in convective genesis, and creating a repeated
pattern of heavy rain prospects as the setup evolves. Latest
ensemble mean QPF is relatively robust at 5-day leads with 2-3"
mean QPF with some members already depicting closer to 4-6" over
areas of northeastern KS up through northwest MO and IA. These are
noteworthy as they reflect the potential for a setup of this
nature, one that tends to infer a more enhanced impact where
repeated convection occurs.
Given the latest signals in the means and the associated probs from
the latest ECAIFS-ENS and ECENS, a broad SLGT risk was added to the
new D5 encompassing the Central Plains and points northeast into
the Lower Great Lakes (southern WI and northern IL). This is a
setup that will need to be monitored closely given the potential
for significant rainfall totals over a multi-day period where
antecedent soils may become primed and then impacted shortly after
leading to a rising flash flood threat and potential hydrologic
responses.
Another period of scattered convective potential across the
Southwest U.S. will lead to yet another MRGL risk in place for the
D5, but will also have to keep an eye for any short term upgrades
given the propensity for repeated action, as well as the
introduction of better kinematics as these smaller shortwaves make
their way around the western periphery of the ridge.
Kleebauer
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Fri Sep 11 2026
Day 4 and Day 5
Valid 12Z Mon Sep 14 2026 - 12Z Wed Sep 16 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN PORTIONS
OF THE MISSOURI VALLEY INTO PARTS OF THE CORN BELT AND UPPER MIDWEST...
1930Z Update: Few changes were made to the Days 4/5 EROs based on
the 12Z model suite -- both deterministic and probabilistic. Based
on the guidance trends, we did trim western areas of the Slight
and especially Marginal Risk areas across the Upper MS Valley on
Day 4. The expectation here is that any lingering rainfall (along
the occluded front/TROWAL) will likely be more light-moderate in
intensity (i.e. rainfall rates below 1/3/6 FFG) given the lack of
deep- layer instability.
Hurley
...Day 4...
Early next week will likely see a growing concern for heavy
rainfall and widespread convective episodes beginning across the
Central and North-Central U.S. with a longwave trough/ridge
evolution likely to induce a prolonged threat for flash flooding.
The initial setup begins with two distinct features of interest,
one of which will become an indirect player in the anticipated
threat. The first feature is a strong mid-level trough ejecting
through the Northern Plains, making headway into the Northern Great
Lakes and adjacent Ontario Province with a subsequent surface low
and cold front materializing as a result. The cold front will be
the surface forcing mechanism that will aid in convective foci, as
well as a delineation point for where the heavier precipitation
prospects will evolve. The cold front will lose latitude back over
the Plains with the current evolution likely to take the frontal
boundary down to at least Kansas latitude before incurring some
resistance due to the evolution occurring downstream. That of which
is our second feature of note; a well-defined ridge axis that will
control the ambient weather pattern for the Southeastern U.S. up
through the Southern Mid Atlantic and Tennessee Valley for at least
the first half of next week. This ridge will be a road block for
any frontal progression upstream as the main ascent driver in the
shortwave trough will continuing ejecting northeast, leaving behind
the front and the primary driving mechanism for the surface feature
in question.
Monday afternoon and evening will see an uptick in convective
activity from the Southwestern U.S. up through the Central Plains
to Upper Midwest as a steady moisture advection pattern
materializes around the western and northern periphery of our broad
upper ridge field centered east of the Continental Divide.
Anticipating PWAT anomalies between +1 to +2 deviations, at
minimum, to be positioned over the aforementioned areas with
embedded shortwave perturbations rounding the western flank of the
ridge, eventually into the northern tier of the CONUS. Mean winds
aloft will become more parallel with the flow occurring in the
boundary layer leading to a heightened threat of training
convection across the Plains to Great Lakes area with the primary
concern materializing over the Missouri Valley into the Central
Midwest where the additional forcing brought upon by the stronger
mid-level shortwave ejection will aid in the heavier convective
footprint. PQPF from the current forecast and 90th percentile QPF
outputs are right within the 2-3" marker at this current lead, a
testament to the potential as historical precedence indicates
these types of outputs at lead tend to trend closer to more
impactful scenarios, so long as the synoptic evolution doesn't
deviate from consensus. Latest First Guess Fields are keen on a
broad SLGT risk across the Midwestern U.S. with an extension back
into IA and points northeast into the U.P. ML models and a few
global outputs extend that threat back further to the west to
include more of the Missouri Valley through eastern NE, perhaps
even into northern KS. Utilizing some of the recent probs >50% for
at least 1" precip on ensembles, the SLGT risk from previous
forecast was extended back into that above zone with the SLGT now
situated from eastern NE up through the Corn Belt, extending
northeast through portions of MN/WI and the Michigan U.P.
This threat will flow into the following day, but this is the
beginning of an active period of heavy rainfall potential with
prospects for future upgrades, pending the behavior of the evolving
synoptic pattern, both aloft (trough/ridge strength and
orientation) and at the surface (low pressure and cold front
positioning). MRGL risk extends back into the Southwest as
scattered thunderstorm activity will likely maintain a foothold on
the terrain of AZ/CO/NM thanks to the continued moisture advection
regime and suitable thermodynamic environment in place.
...Day 5...
The forecast synoptic pattern heading into Tuesday is trending to
continue a very active stretch for the Central U.S. as a frontal
boundary will struggle to lose much latitude over the course of the
first half of next week leading to what could be a focal point for
convection for multiple days beginning with the previous D4 time
frame. The longwave pattern will remain relatively stable east of
the Mississippi with a strong mid and upper ridge axis located
across the southeastern quarter of the U.S. with a formidable area
of surface high pressure dominating the weather pattern for these
areas. Across the Western CONUS, multiple shortwave troughs will
lead to a general trough axis and prevailing southwesterly flow
extending from the Southwest U.S. through the center of the nation
with an expected quasi-stationary frontal placement bisecting areas
from the Central High Plains to points east-northeast through the
Missouri Valley and Corn Belt with the eastern inflection back
into the Lower Great Lakes. This setup will be coincident with a
bit of a Maddox signature which typically precludes more
appreciable heavy rain prospects given the limited forward
propagation expected of a surface front with mean flow generally
parallel to the surface feature leading to training and back-
building scenarios within proximity of the front. Global
deterministic implies multiple smaller shortwave perturbations
ejecting northeast through the main axis of diffluence situated
over the above regions. This will only act as additional regional
ascent to aid in convective genesis, and creating a repeated
pattern of heavy rain prospects as the setup evolves. Latest
ensemble mean QPF is relatively robust at 5-day leads with 2-3"
mean QPF with some members already depicting closer to 4-6" over
areas of northeastern KS up through northwest MO and IA. These are
noteworthy as they reflect the potential for a setup of this
nature, one that tends to infer a more enhanced impact where
repeated convection occurs.
Given the latest signals in the means and the associated probs from
the latest ECAIFS-ENS and ECENS, a broad SLGT risk was added to the
new D5 encompassing the Central Plains and points northeast into
the Lower Great Lakes (southern WI and northern IL). This is a
setup that will need to be monitored closely given the potential
for significant rainfall totals over a multi-day period where
antecedent soils may become primed and then impacted shortly after
leading to a rising flash flood threat and potential hydrologic
responses.
Another period of scattered convective potential across the
Southwest U.S. will lead to yet another MRGL risk in place for the
D5, but will also have to keep an eye for any short term upgrades
given the propensity for repeated action, as well as the
introduction of better kinematics as these smaller shortwaves make
their way around the western periphery of the ridge.
Kleebauer
