Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sun Sep 13 2026
Day 1
Valid 16Z Sun Sep 13 2026 - 12Z Mon Sep 14 2026
...THERE ARE SLIGHT RISK AREAS FOR EXCESSIVE RAINFALL OVER
PORTIONS OF THE NORTHEAST & CENTRAL PLAINS...
16Z Discussion Update... Radar showed that moderate to heavy
rainfall had all but ended in northern coastal New Jersey as of
late this morning but continued from Long Island northeastward into
pasts of Massachusetts. Except to trim away the western portion of
the outlook area...few changes needed to the orientation or areal
coverage of the Slight Risk area. The threat of additional flooding
will decrease from west to east throughout the day as the system
moves out to sea.
The Slight risk over the Plain and the surrounding Marginal Risk
area needed only minor nudges given latest HREF/RRFS and radar
trends.
Expanded the areal coverage of the Marginal Risk area in vicinity
of the international border given the RAPs placement of instability
during the afternoon and the precipitable water fields from the 12Z
upper air cycle. The remainder of the area still tended to fit
within the latest chances for heaviest rainfall/rainfall rates as
shown by the 12Z HREF/RRFS guidance.
Bann
Previous Discussion...
...Northeast...
The Northeast resides beneath the right-entrance region of a
strengthening 250mb jet streak, creating strong 200-400mb layer
Q-vector divergence over the Northeast. This is allowing for
unusually strong upper-level frontogenesis atop southern New
England, in addition to the surface warm front that is forecast to
spawn a strengthening surface low over Long Island early Sunday
morning. This is a continuation of the bands of strong
thunderstorms containing highly efficient rainfall rates from
Saturday night. PWs up to 2.25" are expected and low-level theta-e
advection into the warm front will direct MUCAPE between 500-1,000
J/kg into southern New England. Area averaged soundings across
multiple CAMs members paint a concerning picture; highly saturated
profiles within a sheared environment that could support enhanced
mesoscale rainfall rates >2"/hr within an urbanized corridor. The
rainfall threat moves into RI and southeast MA late morning and
into the afternoon before exiting off the coast late afternoon.
There is greatest concern for flash flooding impacts over Long
Island and the coastal portions of CT and RI where 00Z HREF
probabilities should moderate chances (40-60%) for 3-hr QPF >3"
across the mid-late morning hours on Sunday. The 00Z HREF also
shows moderate chances (40-60%) for 6-hr QPF surpassing 6-hr FFGs.
Both the 00Z HREF and REFS show high-chance probabilities (>70%)
for rainfall totals over 3" through early afternoon along the
southern New England coast. The Slight Risk continues to remain in
good standing, with the flash flood threat likely ending throughout
southern New England by late afternoon.
...Central Plains...
A frontal boundary draped over the Central Plains and a prolonged
SWrly LLJ in the Southern High Plains will act as a source of lift
and moisture transport throughout much of the day. By evening, an
area of low pressure will work in tandem with high pressure over
the Great Lakes Sunday night to strengthen the LLJ to as much as 40kt
LLJ over the Central Plains. A plume of high theta-e air will
continue to be oriented along the frontal boundary, with
additional storms likely to develop Sunday night. 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 by early Monday morning.
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. While exact
placement of the heaviest rainfall still differs, most guidance is
now showing eastern KS and western MO becoming favored for the
locally heavier rainfall totals. 00Z HREF guidance shows low-to-
moderate chances (20-40%) for >3" of rainfall over the Kansas City
metro area. By Sunday night, the emergence of a strong closed 500mb
low over the northern Rockies will direct a rich plume of >99th
climatological percentile PWs into the Northern Plains. Given the
progressive nature of these storms and modest elevated instability,
the flash flood potential is not too concerning at the moment, but
the highly saturated profiles do support at least a Marginal Risk
through Sunday night.
...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. The footprint of saturated soils has only grown
following the active Saturday in AZ, thus making these areas more
prone to potential flash flooding. While confidence in higher
rainfall rates is lower at the moment, the region's sensitive
soils and potential for excessive rainfall rates >1.5"/hr
continues to support the Marginal Risk in place.
Mullinax
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sun Sep 13 2026
Day 2
Valid 12Z Mon Sep 14 2026 - 12Z Tue Sep 15 2026
...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDWEST & SOUTHWEST...
...Midwest...
A strong closed 500mb low, sporting 500mb heights below the 10th
climatological percentile per ECMWF, will work in tandem with a
strong 500mb ridge over the Mid-South to produce an exceptional IVT
throughout much of the North Central U.S.. The ECMWF SATs show the
IVT topping 1000 kg/m/s over the MO River Valley and into MN
Monday morning, then strengthening to >1,250 kg/ms/s over the
Upper MS Valley by Monday evening. These values are approaching
levels that could challenge the highest observed IVTs for mid-
September in the CFSR database (1979-2009), thus showcasing the
remarkable moisture transport at play. Strong theta-e advection
and anomalous PWs exceeding the 99th climatological percentile
will set the stage for an active day, consisting of widespread
showers and thunderstorms that contain highly efficient warm rain
processes.
The area of greatest concern is along the warm front,
closest to the surface low track, as veering low-level wind
profiles ahead of the approaching cold front support clusters of
organized thunderstorms. Latest WPC QPF shows widespread 1-2" of
rainfall from eastern NE to as far north as the Upper Great Lakes
with localized totals surpassing 3" possible. The inherited Slight
and Marginal Risks remain in good shape, but extended the Slight
Risk into southern NE to account for recent south and west trends
in QPF.
...Southwest...
Broad upper-level troughing over the West and upper-level ridging
in the Southeast will continue to support a healthy monsoonal flow
into AZ/NM and as far north as the Central Rockies. NAEFS shows IVT
values still topping the 90th climatological percentile for mid-
September. PWs will also surpass the 97,5 percentile for much of
eastern AZ and western NM. In addition, the Four Corners region is
ideally placed beneath the right- entrance region of a 250mb jet
streak that favors healthy upper- level ascent. The region has
remained fairly active in recent days and will continue to see
additional scattered showers and storms on Sunday, helping to
further saturate soils ahead of Monday.
QPF has trended wetter over eastern AZ and western NM, and given
soils are more likely to moisten rather than dry out in the coming
days, the Slight Risk was expanded to include more of southeast AZ
and far western NM. A broad Marginal Risk accounts for the
potential of localized flash flooding from central AZ on east
through much of NM and northward into the CO Rockies.
Mullinax
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sun Sep 13 2026
Day 3
Valid 12Z Tue Sep 15 2026 - 12Z Wed Sep 16 2026
...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS, MIDWEST, & SOUTHERN ARIZONA...
...Central Plains & Midwest...
There is increasing concern over a multi-day excessive rainfall
across portions of the Central Plains and Midwest. For Tuesday,
ensemble means (ECMWF/GEFS/EC-AIFS ENS) all show PWs above the 99th
climatological percentile taking shape from the central High
Plains and stretching towards the MO River Valley. Rounds of storms
are likely to unfold during the day Tuesday as a persistent
southerly fetch of 850mb moisture is directed towards a stationary
front that is forecast to be oriented WSW-ENE from KS to central
IL. By Tuesday night the LLJ looks to strengthen, directing high
theta-e air and an increasingly unstable air-mass towards the
frontal boundary. Moisture for this setup not only comes from the
Gulf, but as far east as the western Atlantic, as the strong ridge
over the Southeast U.S. causes an extensive fetch of easterly flow
across these bodies of water and directs the moisture into the
Nation's Heartland.
This meteorological setup will not only feature moisture from
tropical origins, but a robust 250mb jet streak over the Great
Lakes that will place its divergent right-entrance region over the
Central Plains. Excellent upper-level ascent will allow for
widespread showers and storms that, given the stagnant SWrly low-
level flow into the frontal boundary, will provide an ideal setup
for training and back-building thunderstorms. There is a Slight
Risk that extend from central KS to as far east as northern IL. The
greatest concern is for northeast KS and northern MO, where WPC
QPF now shows 2-3" of rainfall, implying localized amounts over 4"
are possible. This area is deemed to be a "higher-end" Slight Risk.
There does remain some uncertainty in the rainfall axis, which is
why no higher threat levels were issued this forecast cycle.
...Eastern AZ & Western NM...
The same robust 250mb jet streak over the Great Lakes that will
place its divergent right-entrance region over the Central Plains
will actually stretch as far south and west as the Four Corners
states. PWs remain above the 97.5 climatological percentile on both
ECMWF and NAEFS SATs, and soils continue to grow more saturated in
wake of the active day on Monday. WPC QPF has increased over
eastern AZ, much of NM, and both southern and western CO. There is
some uncertainty in how much instability will be available
following what appears to be an active Monday for much of the
region. But given the anomalous PWs, excellent synoptic scale
forcing, and at least some minor levels of instability, a Slight
Risk was introduced to highlight the potential for scattered
instances of flash flooding in portions of eastern AZ and western
NM on Tuesday.
Mullinax
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sun Sep 13 2026
Day 4 and Day 5
Valid 12Z Wed Sep 16 2026 - 12Z Fri Sep 18 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST U.S, CENTRAL PLAINS, AND MISSOURI VALLEY INTO PARTS
OF THE CORN BELT AND UPPER MIDWEST...
...Day 4...
General persistence for the pattern across the CONUS will lead to
another period of heavy rainfall across the center of the nation
with scattered heavy convective potential lingering over portions
of the Southwest U.S. Consensus is a maintenance of a relatively
stable upper level setup with a lingering quasi-stationary front
bisecting the same areas as the previous period(s). The front will
struggle to lose much latitude unless propelled by stronger cold
pools emanating from the convective threat that occurs over the
previous 48 hrs. ML and ENS suites continue to maintain relative
continuity in the placement of heavy rain somewhere between the
Central Plains into the western Ohio Valley with the current mean
QPF max situated over eastern NE through much of IA. Multi- day
totals between 3-5" are forecast at range which is typically
coincident with a potential for a significant flash and hydrologic
flood threat when looking back at previous events in this region of
the CONUS. The previous SLGT was maintained from the previous
issuance, but did expand a bit further north to encompass the outer
fringes of the 90th percentile NBM QPF threshold of 1" to cover for
any wavering of the boundary as it oscillates in the general
vicinity of the Corn Belt to Missouri River Valley.
Guidance continues to remain aggressive on the threat for
widespread convection over the Southwestern U.S, namely for eastern
AZ through much of NM as the region sits right on the western
fringe of the broad upper ridge centered over the Southeast and
Mississippi Valley. Model consensus is for local areas of 1-2" in
the means situated over a broad expanse covering the above
locations, even hinting at some extension further north into
western CO. This setup is conducive for scattered flash flood
concerns given the PWAT anomalies pushing upwards of +2 standard
deviations (95-99th percentile) when assessing the latest NAEFS
forecast for the period. Given the stable signal and elevated QPF
outputs within the means, a SLGT risk was issued across
southeastern AZ into the eastern Mogollon Rim, extending through
much of NM with the highest threat centered over local urban
zones, burn scars, and flashier arroyos and small streams in the
outlined areas.
...Day 5...
By the later part of the week, we will finally begin to see some
shifts in the longwave pattern across the CONUS with some motion of
the upper ridge shifting westward and losing a bit of its vigor as
it migrates away from the Southeast U.S. Height fields off the
ensemble means tell the story with some ebb and flow of the ridge
itself and a touch of flattening on the northern periphery of the
ridge axis which could play a role in the boundary positioning as
we move into Thursday. Consensus in guidance, both ML and NWP
ensembles remain persistent in another round of convective
potential over the Midwest/Corn Belt with the highest probs for at
least 1" centered right over IA back into the Missouri River area
in NE. NBM QPF alignment in the heaviest precip notes the area
along I-80 in the above corridor as the current point of emphasis
with the 90th percentile output already depicting well over 2" of
rainfall in the region. The overall synoptic pattern supports this
prospect with the threat of heavy rainfall for potentially a 3rd or
4th day in a row over some areas during this time frame. Usually at
range, unless there was something truly prolific being shown
within the means, the forecast would maintain a lower end risk to
account for uncertainty. Given the variables referenced above, and
the fact this would be the 3rd or 4th round of heavy precip in the
vicinity of the area in question, a SLGT risk was justified given
the prospect of saturated grounds, even potentially ongoing
flood concerns when you couple the hydrologic responses likely to
occur from the episodic nature of this prolonged event. MRGL's
maintain prevalence around the perimeter of the aforementioned SLGT
to account for some uncertainty in the latitudinal element of where
the heaviest rain could occur. This continues to be a genuinely
impactful threat with an opportunity for higher risks if the
previous periods impact is appreciable leading into the D5.
Across the Southwest U.S, at least one more day of widespread
convection is forecast for areas of eastern AZ into NM with the
main axis situated over NM. This could very well be a repeat of the
previous period which would be an opportunity for perhaps more
elevated concerns as certain areas get impacted multiple days in a
row. A SLGT risk was added over similar areas to D4 with the
emphasis still over NM with the highest threat centered over local urban
zones, burn scars, and flashier arroyos and small streams.
Kleebauer
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sun Sep 13 2026
Day 4 and Day 5
Valid 12Z Wed Sep 16 2026 - 12Z Fri Sep 18 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST U.S, CENTRAL PLAINS, AND MISSOURI VALLEY INTO PARTS
OF THE CORN BELT AND UPPER MIDWEST...
...Day 4...
General persistence for the pattern across the CONUS will lead to
another period of heavy rainfall across the center of the nation
with scattered heavy convective potential lingering over portions
of the Southwest U.S. Consensus is a maintenance of a relatively
stable upper level setup with a lingering quasi-stationary front
bisecting the same areas as the previous period(s). The front will
struggle to lose much latitude unless propelled by stronger cold
pools emanating from the convective threat that occurs over the
previous 48 hrs. ML and ENS suites continue to maintain relative
continuity in the placement of heavy rain somewhere between the
Central Plains into the western Ohio Valley with the current mean
QPF max situated over eastern NE through much of IA. Multi- day
totals between 3-5" are forecast at range which is typically
coincident with a potential for a significant flash and hydrologic
flood threat when looking back at previous events in this region of
the CONUS. The previous SLGT was maintained from the previous
issuance, but did expand a bit further north to encompass the outer
fringes of the 90th percentile NBM QPF threshold of 1" to cover for
any wavering of the boundary as it oscillates in the general
vicinity of the Corn Belt to Missouri River Valley.
Guidance continues to remain aggressive on the threat for
widespread convection over the Southwestern U.S, namely for eastern
AZ through much of NM as the region sits right on the western
fringe of the broad upper ridge centered over the Southeast and
Mississippi Valley. Model consensus is for local areas of 1-2" in
the means situated over a broad expanse covering the above
locations, even hinting at some extension further north into
western CO. This setup is conducive for scattered flash flood
concerns given the PWAT anomalies pushing upwards of +2 standard
deviations (95-99th percentile) when assessing the latest NAEFS
forecast for the period. Given the stable signal and elevated QPF
outputs within the means, a SLGT risk was issued across
southeastern AZ into the eastern Mogollon Rim, extending through
much of NM with the highest threat centered over local urban
zones, burn scars, and flashier arroyos and small streams in the
outlined areas.
...Day 5...
By the later part of the week, we will finally begin to see some
shifts in the longwave pattern across the CONUS with some motion of
the upper ridge shifting westward and losing a bit of its vigor as
it migrates away from the Southeast U.S. Height fields off the
ensemble means tell the story with some ebb and flow of the ridge
itself and a touch of flattening on the northern periphery of the
ridge axis which could play a role in the boundary positioning as
we move into Thursday. Consensus in guidance, both ML and NWP
ensembles remain persistent in another round of convective
potential over the Midwest/Corn Belt with the highest probs for at
least 1" centered right over IA back into the Missouri River area
in NE. NBM QPF alignment in the heaviest precip notes the area
along I-80 in the above corridor as the current point of emphasis
with the 90th percentile output already depicting well over 2" of
rainfall in the region. The overall synoptic pattern supports this
prospect with the threat of heavy rainfall for potentially a 3rd or
4th day in a row over some areas during this time frame. Usually at
range, unless there was something truly prolific being shown
within the means, the forecast would maintain a lower end risk to
account for uncertainty. Given the variables referenced above, and
the fact this would be the 3rd or 4th round of heavy precip in the
vicinity of the area in question, a SLGT risk was justified given
the prospect of saturated grounds, even potentially ongoing
flood concerns when you couple the hydrologic responses likely to
occur from the episodic nature of this prolonged event. MRGL's
maintain prevalence around the perimeter of the aforementioned SLGT
to account for some uncertainty in the latitudinal element of where
the heaviest rain could occur. This continues to be a genuinely
impactful threat with an opportunity for higher risks if the
previous periods impact is appreciable leading into the D5.
Across the Southwest U.S, at least one more day of widespread
convection is forecast for areas of eastern AZ into NM with the
main axis situated over NM. This could very well be a repeat of the
previous period which would be an opportunity for perhaps more
elevated concerns as certain areas get impacted multiple days in a
row. A SLGT risk was added over similar areas to D4 with the
emphasis still over NM with the highest threat centered over local urban
zones, burn scars, and flashier arroyos and small streams.
Kleebauer
