Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 PM EDT Mon Sep 14 2026
Day 1
Valid 16Z Mon Sep 14 2026 - 12Z Tue Sep 15 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
IOWA...
16Z Update...
...Midwest...
Low pressure over the Dakotas/eastern Montana tracks to Manitoba
today as an upper high remains over the Mid-South. An associated
cold front crosses the northern Plains today and slowly progresses
over eastern Nebraska/Iowa overnight. The warm sector ahead of this
front is feeding elevated activity over Iowa this morning with
additional rounds this evening overnight ahead of the cold front.
A deep, moist conditions in this warm sector drive the PW anomaly
to 2 to 3 standard deviations above normal with a 2" PW contour
extending from Iowa through central Wisconsin tonight. Low level
flow should be nearly parallel to the approaching front, so
repeating activity is likely and the central Iowa Moderate Risk
should be maintained. 12Z REFS/HREF guidance is less focused in
central Iowa, but more east-west oriented.
12Z CAMs increased in coverage over north-central WI this
evening/overnight where the Slight Risk remains, but should be
considered more enhanced than before.
A swath of 2-4" QPF are now favored for both central Iowa and
southeast MN through n-central WI in the 12Z REFS and HREF.
The main uncertainty at this point is the eastern extend of the QPF
from ongoing activity over IA and how much repeating occurs ahead
of the front this evening/tonight.
...Southwest...
Monsoonal moisture advection pattern over the area with PWAT
anomalies in the +2 to +3 standard deviations range, coincident
with PWATs encroaching on the 95-99th percentile over eastern AZ up
through much of NM into southern CO. Coupled with a suitable
diurnal heat flux leading to SBCAPE signatures between 1000-2500
J/kg over the area, scattered to widespread showers and
thunderstorms will materialize over the above area leading to
scattered flash flood concerns for the afternoon and evening hours.
A strong 60-90% neighborhood probability for >1" exists over a
large portion of NM into eastern AZ and southwestern CO, coinciding
with an array of complex terrain features and remnant burn scars
noted for flash flooding in any stronger convective environments.
The signal for >2" maintains a modest signature as well, with the
greatest signal residing in southeastern AZ within the forecast
thermodynamic max (SBCAPE >2000 J/kg). The environmental factors
and widespread nature of the convection are suitable for a broad
SLGT risk over the region with the primary target currently aimed
west of the I-25 corridor. 12Z REFS/HREF focus a bit more QPF over
southeast AZ, so the Slight Risk there can be considered more
enhanced than before.
Adjusted the southern end of the Marginal Risk over west Texas with
a separate Marginal now over the Davis Mtns in southerly warm
advective flow.
Jackson
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 PM EDT Mon Sep 14 2026
Day 2
Valid 12Z Tue Sep 15 2026 - 12Z Wed Sep 16 2026
...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS, MISSOURI VALLEY, AND SOUTHWEST U.S...
...Central U.S...
The heavy rain focus over the central states will align with the
slow moving cold front over Kansas, northern Missouri, and western
Illinois where a Slight Risk persists. The Slight was trimmed from
the north, but kept for far southeast Nebraska and southern Iowa
given wet antecedent conditions despite less QPF there. Deep layer
SWly flow will remain parallel to the cold front, promoting
training convection. 12Z guidance favors this repeating rain threat
area to continue farther southwest to the Texas Panhandle where the
Slight Risk was expanded into. PW anomalies of 2 to 3 deviations
above normal can be expected with a swath of actual values around
2". QPF in the 2-4" range are in both the 12Z REFS and HREF with
the HREF favoring northeast KS while the REFS is broader across
northern MO. This area can be considered to have a more enhanced
risk than a general Slight. Scattered flash flooding is considered
likely, especially for overlap from the several inches of rain that
fell north of the KC metro this morning.
The overall risk area continues east along the front across
northern Indiana to Cleveland where the Marginal was expanded.
...Southwest U.S...
The Southwest Monsoon continues in earnest at least through
midweek as an upper high persists over the Mid-South and low
pressure tracks down the West Coast to Northern California.
Moisture anomalies of 2 to 2.5 standard deviations are expected
Tuesday for southern/central Arizona, much of New Mexico, and
over southern Utah through central Colorado. This will be the 4th
day in a row with this pattern leading to increasingly vulnerable
areas/wet antecedent surface. Expanded the Slight areas to include
all of the Slot Canyon Region of southern Utah east to the San
Juans in Colorado as well as more of the Mogollon Rim area in
central Arizona. PW anomalies are less over northern Arizona for
now, but southerly flow is across the region, so further expansion
may be eventually warranted.
Jackson
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 PM EDT Mon Sep 14 2026
Day 3
Valid 12Z Wed Sep 16 2026 - 12Z Thu Sep 17 2026
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDWEST, CENTRAL PLAINS & SOUTHWESTERN U.S...
...Central Plains & Midwest...
High pressure in the wake of the low tracking across the Canadian
Prairies today shifts over the northern Great Lakes Wednesday.
Return flow west of the high brings the frontal zone and moisture
back north into vulnerable areas of southeast Nebraska, northeast
Kansas, northern Missouri, southern into central Iowa and northern
Illinois. Right entrance region jet dynamics are also in play from
the Upper Midwest. Expect more convective development near the
front on the strong LLJ. The higher-res 12Z RRFS/Canadian Regional
are farther south, but the 12Z ECMWF/EC-AIFS are a tick farther
north of Iowa. Some trim to the north side of the Slight Risk was
made with areas of central/southern Iowa still considered more of
an enhanced Slight. Swaths of 2-5" are given in these dynamic
models. Depending on what happens tonight and where guidance favors
the maximum going forward, further upgrades are possible.
...Southwest U.S...
Vigorous Southwest Monsoon activity on tap for Wednesday as the
West Coast low reaches far northern California with downstream
troughing pumping stronger southwest flow over the Desert
Southwest. Moisture looks to peak for the week from southern AZ
through much of NM and southern CO with PW anomalies of 3 standard
deviations above normal. 12Z RRFS and Canadian Regional (two of the
few higher res models for Day 3) are aggressive with 2-4" over much
of southeast AZ up the Mogollon Rim and western NM and southern CO.
The Slight Risk is expanded farther up the Mogollon Rim and down
over the Sacramento Mountains, with a larger area of more enhanced
Slight Risk areas over much of western NM.
Jackson
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 PM EDT Mon Sep 14 2026
Day 4 and Day 5
Valid 12Z Thu Sep 17 2026 - 12Z Sat Sep 19 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST U.S, CENTRAL PLAINS, MISSOURI VALLEY, AND INTO THE UPPER
MIDWEST...
20Z Update...
...Southwest...
Overall, no large-scale changes were made to the previous outlook.
For Day 4, confidence is increasing that parts of the Southwest --
notably southwestern to north-central New Mexico -- may be
categorized as a "high-end Slight." Following what is expected to
be a very active Wednesday, areal averages are expected to drop
off some on Thursday, but the coverage of impactful amounts still
looks to be significant. PW anomalies on Thursday are forecast to
remain 2-3+ standard deviations above normal across much of New
Mexico and eastern Arizona on Thursday. Meanwhile, mid-to-upper
level energy embedded within southerly flow between an approaching
low/trough and a downstream ridge will help produce areas of
enhanced ascent. One limiting factor may cloud cover from the
previous day's activity and reduced instability. However, even with
reduced rainfall rates, soils are likely to be remain vulnerable
to excessive runoff given the previous days' rainfall.
...Upper Midwest...
Made mostly minor adjustments here as well. While the 12Z guidance
continued to show a good signal for heavy amounts extending east
across parts of the northern Plains toward the upper Mississippi
Valley on Day 4, and then from the upper Mississippi Valley toward
the Great Lakes on Day 5, they also showed notable latitudinal
spread regarding its axis. The Slight Risk areas remains near the
middle of the envelope of solutions.
Pereira
Previous Discussion...
The Days 4-5 period (Thu. Sept 17 - Fri. Sept 18) can be
summarized by an expansive and highly anomalous upper- level ridge
of high pressure taking hold over much of the Southern U.S., while
an upper-level trough in the West makes its way from the West Coast
towards the Northern Rockies. There is also a deep upper- trough
tracking through eastern Canada, which in contrast with the upper-
ridge to the south will create tight thermal and moisture gradient
from the Great Lakes to the Northeast. The 850mb ridge will remain
positioned over the Mid- South and is ideally located to produce an
extended fetch of southerly flow over the Southern Plains. This
results in unusually strong IVTs over the Great Plains both Days 4
and 5. The ECMWF SATs show >90th climatological percentile IVTs
aimed at the Great Lakes and Northeast on Day 4, which combined
with increasing instability and a nearby frontal boundary present,
could lead to scattered thunderstorms capable of producing
localized flash flooding.
By Day 5, the Midwest becomes the focus for flash flood potential
as a pair of jet streaks; one over the Great Lakes and another
emerging from the Rockies, place their divergent jet streak
quadrants overhead. Combined with PWs >1.75", which is above the
99th climatological percentile, this serves as an ideal setup for
additional rounds of excessive rainfall. The Midwest will have also
dealt with heavy rainfall the first half of the week, making
overly saturated soils a key player in potential flash flooding
late week. A pair of Slight Risks are in place over parts of the
North Central U.S. for both Thursday and Friday to cover this
prolonged window of excessive rainfall.
Lastly, the Southwest remains mired in a steadfast monsoonal flow
regime that is pumping copious amounts of moisture into the region.
Day after day, scattered thunderstorms will produce locally
excessive rainfall rates in excess of 1"/hr, keeping the threat for
flash flooding in place through the end of the week. A Slight Risk
remains in place for Day 4 (Thursday) while a large Marginal
encompasses an area that extends from southern NM and western TX,
to northern AZ and the Central Rockies.
Mullinax
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 PM EDT Mon Sep 14 2026
Day 4 and Day 5
Valid 12Z Thu Sep 17 2026 - 12Z Sat Sep 19 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST U.S, CENTRAL PLAINS, MISSOURI VALLEY, AND INTO THE UPPER
MIDWEST...
20Z Update...
...Southwest...
Overall, no large-scale changes were made to the previous outlook.
For Day 4, confidence is increasing that parts of the Southwest --
notably southwestern to north-central New Mexico -- may be
categorized as a "high-end Slight." Following what is expected to
be a very active Wednesday, areal averages are expected to drop
off some on Thursday, but the coverage of impactful amounts still
looks to be significant. PW anomalies on Thursday are forecast to
remain 2-3+ standard deviations above normal across much of New
Mexico and eastern Arizona on Thursday. Meanwhile, mid-to-upper
level energy embedded within southerly flow between an approaching
low/trough and a downstream ridge will help produce areas of
enhanced ascent. One limiting factor may cloud cover from the
previous day's activity and reduced instability. However, even with
reduced rainfall rates, soils are likely to be remain vulnerable
to excessive runoff given the previous days' rainfall.
...Upper Midwest...
Made mostly minor adjustments here as well. While the 12Z guidance
continued to show a good signal for heavy amounts extending east
across parts of the northern Plains toward the upper Mississippi
Valley on Day 4, and then from the upper Mississippi Valley toward
the Great Lakes on Day 5, they also showed notable latitudinal
spread regarding its axis. The Slight Risk areas remains near the
middle of the envelope of solutions.
Pereira
Previous Discussion...
The Days 4-5 period (Thu. Sept 17 - Fri. Sept 18) can be
summarized by an expansive and highly anomalous upper- level ridge
of high pressure taking hold over much of the Southern U.S., while
an upper-level trough in the West makes its way from the West Coast
towards the Northern Rockies. There is also a deep upper- trough
tracking through eastern Canada, which in contrast with the upper-
ridge to the south will create tight thermal and moisture gradient
from the Great Lakes to the Northeast. The 850mb ridge will remain
positioned over the Mid- South and is ideally located to produce an
extended fetch of southerly flow over the Southern Plains. This
results in unusually strong IVTs over the Great Plains both Days 4
and 5. The ECMWF SATs show >90th climatological percentile IVTs
aimed at the Great Lakes and Northeast on Day 4, which combined
with increasing instability and a nearby frontal boundary present,
could lead to scattered thunderstorms capable of producing
localized flash flooding.
By Day 5, the Midwest becomes the focus for flash flood potential
as a pair of jet streaks; one over the Great Lakes and another
emerging from the Rockies, place their divergent jet streak
quadrants overhead. Combined with PWs >1.75", which is above the
99th climatological percentile, this serves as an ideal setup for
additional rounds of excessive rainfall. The Midwest will have also
dealt with heavy rainfall the first half of the week, making
overly saturated soils a key player in potential flash flooding
late week. A pair of Slight Risks are in place over parts of the
North Central U.S. for both Thursday and Friday to cover this
prolonged window of excessive rainfall.
Lastly, the Southwest remains mired in a steadfast monsoonal flow
regime that is pumping copious amounts of moisture into the region.
Day after day, scattered thunderstorms will produce locally
excessive rainfall rates in excess of 1"/hr, keeping the threat for
flash flooding in place through the end of the week. A Slight Risk
remains in place for Day 4 (Thursday) while a large Marginal
encompasses an area that extends from southern NM and western TX,
to northern AZ and the Central Rockies.
Mullinax
