Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1200 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
1200 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 D2 period will continue to exhibit the threat for heavy
rainfall across portions of the Central U.S. with the area located
from northeastern KS up through southeastern NE and northwestern MO
as the primary area of impact. Cold front will slowly sag southward
with the aid of a high pressure building in from the Canadian
Prairies, edging southward with the push of cooler, more stable
continental air leading to the threat of heavy rain over much of IA
to cut off by Tuesday afternoon. The setup is a bit similar to the
previous period with increasing convergence in vicinity of the
front with the main time frame of interest centered over Tuesday
evening with the additional LLJ element. Forecast soundings from
various NWP indicate a prevailing wind field aloft that would align
parallel to the boundary leading to another round of training
convection from KS to points east-northeast. PWAT anomalies will
remain around +2 to +3 deviations with true values hovering around
that 2" marker. There's a general consensus on a swath of 2-3" of
rainfall over that area of northeastern KS and points eastward with
much of the global deterministic solidly in the 2-4" range which
agrees with the latest ML outputs. This agreement among models is a
solid signal for at least scattered flash flood prospects as the
environmental conditions favor areas of 1-2"/hr rates as convection
motions over the area. A high-end SLGT is forecast for this region
of the CONUS with a chance at a higher risk pending future
forecasts. We are just outside the full temporal range of the CAMs
for entire D2 period, so its plausible to see a higher risk added
if the CAMs indicate a higher risk for 3+ inches of rainfall. Until
then, maintained continuity from the previous forecast.
...Southwest U.S...
The Southwest CONUS will remain active with a multitude of small
shortwave perturbations rounding the western periphery of the broad
ridge positioned over the Southern U.S, entering into the region
with PWAT anomalies firmly +2 or greater across eastern AZ into NM
and southern CO. Models continue to pinpoint areas of northern and
western NM as the beneficiaries of the setup with scattered 1+
inch totals depicted across much of the global deterministic. This
will be the 4th day in a row with this pattern persistence leading
to some areas becoming more and more vulnerable for flash flood
prospects. Continue to see the 18-06z period as the time range of
interest which is coincident with the diurnal instability maximum.
A broad SLGT remains positioned over the aforementioned areas with
an emphasis on eastern AZ and the western half of NM.
Kleebauer
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Mon Sep 14 2026
Day 3
Valid 12Z Wed Sep 16 2026 - 12Z Thu Sep 17 2026
...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS, MIDWEST, & SOUTHWESTERN U.S...
...Central Plains & Midwest...
Our multi-day setup across the Central U.S. continues through mid-
week with the pattern shifting back north into the Corn Belt and
adjacent areas of the Midwest with the synoptic evolution entailing
another approach of our quasi-stationary front, although this time
from the south as the high pressure to the north slides eastward
and brings back return flow to the Plains. Aloft, a strengthening
250mb jet streak will situate itself over the Upper Midwest and
Great Lakes providing excellent RER jet dynamics under the
influence of a 120kt speed max centered over the northern tier of
the CONUS. The next wave of convection will likely generate with
the addition of yet another prominent LLJ development, this one
likely to be with more vigor as the jet has influence from the
return flow regime on the western side of the surface ridge to the
northeast. Deep moisture plume will advect poleward leading the
convergent pattern to materialize, once again, for the area between
northeast KS, eastern NE, through much of IA for Wednesday evening.
This area is increasingly likely to see a broad axis of 2-3" as an
areal average with embedded maxima likely to breach 4" when looking
at the different global deterministic outputs and 90th percentile
ensemble means. This setup is favorable for overlap with some areas
that will be impacted on D1 in the region of IA which could very
well lead to more widespread flash flood concerns for the period.
As of now, elected to maintain the broad SLGT risk over the Midwest
back into portions of the Central Plains, however a high-end SLGT
is now forecast for much of IA, especially areas along and south of
I-80 between Council Bluff to Davenport. There is a potential for
an upgrade in future updates, so stay tuned for the latest forecast
over the next 48hrs.
...Southwest U.S...
Another period of scattered to widespread convective activity is
forecast for areas of the Southwestern U.S. with the general axis
aligned over eastern AZ into NM and southern CO. Wednesday is
leaning towards the peak of impacts, however, as a more robust
shortwave ejection out of Mexico will likely provide further mid
and upper level ascent that can exacerbate an already formidable
environment. PWAT anomalies will remain in the +2 to +3 standard
deviation range which is already an asset to more appreciable
rainfall potential when you couple with a primed thermodynamic
environment. Ensemble means are aggressive for Wednesday afternoon
into early Thursday morning with some models printing out 2-3" in
spots across western and northern NM, areas where flash flood
concerns will be elevated from the repeated nature of this pattern,
as well as the location of the more sensitive burn scar locales
within the state. This threat has led to an introduction of higher
end SLGT risk wording with a chance for even an upgrade further if
the models show more appreciable rainfall forecasts in the coming
days.
Unlike previous periods, there is more of a threat further west
with this evolution leading to an extension of a MRGL back into
southwestern and southern UT to cover for the threat of flash
flooding within the slot canyon regions of the west. SLGT risk
expands west through the Mogollon Rim, as well, thanks to the
surge in PWATs and added forcing.
Kleebauer
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1200 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...
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
1200 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...
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
