Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Tue Sep 29 2026
Day 1
Valid 01Z Wed Sep 30 2026 - 12Z Wed Sep 30 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM THE
SOUTHWEST INTO THE CENTRAL HIGH PLAINS...
01Z Update...
The threat of flash flooding...with some localized considerable
impacts...will continue throughout the overnight hours from the
Southwester U.S. across the southern portions of the Rockies and
out onto the adjacent central Plains. Radar and satellite trends
from the late afternoon and early evening allowed for some of the
outlook areas to be trimmed from the western extent.
However...moisture with a subtropical connection remains in place
with favorable upper flow pattern to maintain the threat. As the
moisture emerges over the plains and interacts with an amplifying
synoptic scale trough over the central portion of the
country...shifted the axis of excessive rainfall threat southward
to follow the trend first started in the 29/12Z suite of guidance,
Otherwise...the changes to the on-going outlook were generally
small.
Introduced a Marginal Risk area over the southwestern Florida
peninsula as an area of shower and thunderstorms approached the
coastline as of 00Z along an inverted trough.
Bann
16Z Update: The main premise of the forecast remains unchanged with
a significant, widespread heavy rainfall threat across much of
AZ/NM into portions of west TX as the tropical remnants of Polo
make their way northward, contributing to a prominent flash flood
prospect over the Desert Southwest into the Southern High Plains.
12z CAMs continue to pinpoint a significant heavy rain threat
across southeastern AZ, including the Tuscon area where 2"/6-hr
probabilities running >70% for much of the area, a threshold that
typically verifies more substantial flash flood prospects in that
area, historically. The environment across the Southwest remains
ripe for heavy rainfall rates between 1-2"/hr and potential for
widespread 2-4" totals, locally higher given the deep moisture
profiles stemming from advection of Polo's moisture being pulled
poleward. Latest 500mb analysis indicates the evolution we have
been anticipating the last several forecast cycles as the mid-level
energy from Hurricane Polo begins to interact and couple with the
upper level low (ULL) currently positioned over the Colorado River
basin out between AZ/CA/NV. Expectation is for the main energy from
Polo to become fully incorporated into the ULL leading to the
addition of stronger vorticity advection into the overall setup
that will materialize over the course of the D1 and beyond.
Time frames of note will include now through 00z across southeast
AZ as the initial surge of convection will occur over the area with
a quasi-linear band of heavy rainfall will initiate over eastern
Pima county and slide east-northeast through the course of the
afternoon. This will lead to widespread flash flood concerns over
the I-19 corridor from the border to north of Tuscon with the
complex of thunderstorms likely expanding east into impact areas
along and north of I-10 into western NM. This area is where the
signal is very prevalent for rainfall totals >3" with the latest
12z HREF blended mean QPF between 3-5" over the area just northeast
of Tuscon into western NM with the maximum located within the
Apache and Gila National Forests. Latest probabilities for >3" are
substantial in that same area with the extension of 60-90+%
probabilities all the way into Tuscon proper which enhances the
threat of the locally significant flash flood concerns in the
region.
As we expand further east, the rest of NM will become inundated
with widespread heavy rainfall late-afternoon through the overnight
time frame with the heaviest rainfall positioned over the southern
and eastern area of the state. Remnant moisture and vorticity
advection being absorbed into the upper pattern will co-locate
within the area from far west TX to points northeast leading to
substantial convective coverage, as well as some organization
trends that are conducive for more appreciable heavy rain
footprints. The areas of concern will lie within the Upper Trans
Pecos from Clines Corner down to the southeastern reaches of NM,
including the more flash flood prone areas along the Pecos river
(Roswell-Artesia-Carlsbad) where the combination of small stream
and arroyo flooding and urbanization will likely provide an avenue
for more locally significant flash flood prospects this evening as
a prolific line of thunderstorms moves through the area. Latest 12z
KEPZ RAOB this morning came in with PWATs of 1.55", a daily
maximum already established for the date, and well beyond the 95th
percentile for seasonal climatology. Current forecast is for PWATs
to rise further, putting the region within historical thresholds
when it comes to some of the higher PWAT readings in general. This
type of environment typically precludes more substantial flash
flood concerns, and this setup is no different. Expect the Eastern
NM High Plains, Upper Trans Pecos, Sacramento Mountains, and
adjacent West TX High Plains from the TX Panhandle into the Permian
Basin to be under the threat of enhanced flash flood chances
through the back half of D1, bleeding into D2.
A targeted MDT was added over the AZ/UT for the central portions of
the bordering area due to another round of convection occurring
this afternoon under the influence of the ULL. Ongoing flooding
over the area enhances sensitivities for the area with local FFG's
basically as low as they could go before more flooding ensues. In
coordination with the local SLC and FGZ WFO's, a MDT risk was added
to target this specific area with an additional 1-2" of precipitation
possible in any convection that occurs this afternoon. This threat
will wane with the loss of diurnal heating around sunset.
Broad SLGT risk continues over the Central Plains with a bit more
of an isentropic pattern materializing across KS into NE/IA which
will likely include some convective schemes, but less instability
and a less robust deep moisture presence compared to areas of the
Southwest will limit the threat to more scattered flash flood
concerns this evening. 12z HREF EAS probabilities for >1" are still
fairly robust (50-80%) over the area from nort-central KS up
through the Missouri River Valley and adjacent Corn Belt areas of
IA, but the >2" signal drops off considerably which notes a limit
to the upper threshold of the forecast which curbs the threat
closer to a SLGT for this period.
Kleebauer
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Tue Sep 29 2026
Day 2
Valid 12Z Wed Sep 30 2026 - 12Z Thu Oct 01 2026
...THERE ARE MODERATE RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL AND SOUTHERN PLAINS, AS WELL AS THE CORN BELT...
...Front Range to Southern Plains...
20Z Update: Maturing lee cyclone across the Central and Southern
High Plains will allow for a continued heavy rain threat across the
Sangre de Cristos into the San Juan Range as the flow shifts out
of the northeast along the northwest periphery of the surface low,
creating a textbook upslope flow regime to perpetuate the rainfall
over the terrain of southern CO into northern NM. The prospects of
flash flooding will be enhanced due to the antecedent moist soils
brought about by prior periods of rainfall, some of which will
likely cause flash flooding in their own right, leading to a
heightened probability for flooding given the overlapping nature of
heavy precip. 12z HREF continues to advocate for significant
totals, 2-4" with locally higher, of additional precip within the
mountains of the Southern Rockies into the Front Range as the
combination of upslope flow and enhanced axis of deformation will
lead to two primary periods of heavy precip; once in the morning
and then once again in the evening as we see the surface low
intensify and strong mid- level vorticity advection pivots around
the ULL center. HREF probabilities, both EAS and neighborhood
probabilities for >2" remain very high across the aforementioned
area leading to high confidence of the heavy rain threat with
multi-day totals likely breaching 3" over a large area over the
Front Range of southeastern CO into northern NM. The previous MDT
risk was maintained with little adjustment given the continued
signal.
Across the Southern Plains from Oklahoma down through Texas, the
setup is more classic with regards to increasing low-level
convergence along a propagating cold front that will materialize
from the surface cyclogenesis over the High Plains. The flow will
be fairly meridional ahead of the slow-moving mean trough/ULL
moving through the Southwest U.S. creating a broad area of
relatively backed flow across portions of west TX into the Red
River Valley down through the Central Rio Grande Valley (RGV) and
Edwards Plateau by Wednesday evening into early Thursday morning.
Model consensus is for the initiation of heavy convection along the
front as it migrates out of the High Plains of TX with the western
periphery of precip likely developing from the western Rolling
Plains down into the Lower Trans Pecos and adjacent Concho Valley
by late Wednesday afternoon. This will set the stage for a
broadening convective footprint over the Southern Plains as the
warm sector environment with deep tropical moisture presence will
interact with the frontal progression as it slowly moves eastward
through the area Wednesday night. Models indicate a formidable
35-45kt LLJ structure situated from the RGV up into the Central
Plains as the warm sector maturation will be well-defined with
continued moisture advection poleward until the front passes over
any given area. MLCAPE between 1500-2500 J/kg bisecting the eastern
two-thirds of the region will offer plenty of instability to
couple with 30-40kt Effective Bulk Shear, creating an environment
conducive for stronger mesocyclone cores and merging multi-cell
clusters that will provide heavy rain for a large area come
Wednesday evening.
HREF neighborhood probs for >3" are upwards of 60-90% over the
area from the Lower Trans Pecos to the central Red River Valley
between TX/OK with an average of 40-60% over the entire stretch
from Eagle Pass, TX up towards Norman, OK. This is a substantial
area of heightened probs for 3" with modest probs (25-50%) for >5"
locally with the best signal over the Lower Trans Pecos and
northern Hill Country between San Angelo and Fort Worth. This is
more than sufficient for a continuation of the MDT risk with some
adjustment to account for a bit slow forward propagation forecast
on the northern edge of the line that should keep the worst of the
rainfall just west of the DFW metroplex. This will be an area to
keep watching over the next 24 hrs as the threat for more
appreciable urban flooding in a scenario where the line makes
headway into the metro.
...Central Plains into the Corn Belt...
The maturing cyclone over the High Plains will lead to a
progressive warm front on the eastern flank of the low with a
general warm sector evolution occurring from the Southern Plains
into the southern reaches of the Midwest. Rainfall from isentropic
ascent will already be occurring on the northern periphery of the
low, but limited instability will maintain more "subdued" rates
that are coincident with more appreciable flash flood prospects.
By Wednesday evening, that will no longer be the case as the warm
front progression will open the door to a more favorable
environment with modest instability development along and south of
the front which is classic for these maturing lee cyclones. PWAT
anomalies of +2 to +3 deviations will have advected into the region
creating a formidable deep layer moisture profile to couple with
the tall, skinny CAPE signatures expected over the Central Plains
into northern MO and southern to central IA.
As of this time, there is a strengthening consensus for heavy
rainfall potential across northern KS into southeastern NE a strong
shortwave ejection is anticipated on Wednesday evening with a
budding LLJ occurring ahead of the trough located over the Rockies.
Convection will breakout by Wednesday afternoon and expand
northeastward into the Missouri River Valley and neighboring Corn
Belt by Wednesday night with training convection within proximity
of the warm front. Cells will develop across the warm sector and
move northeast, in tandem which will eventually feed into the
likely convective cluster migrating along the front from north-
central KS into southern NE, eventually making headway into western
and central IA after nightfall. The sufficiently buoyant
environment and deep moisture presence will lead to hourly rainfall
rates between 1-2"/hr in the strongest cell cores with some
repeating action along the warm front. This has led to some robust
signals in the deterministic CAMs and associated HREF blended mean
QPF output which shows a general 2-4", locally as high as 6" over
the course of Wednesday afternoon into early Thursday morning. The
most notable area was that corridor from the KS/NE border up
through Omaha over into Des Moines with a higher QPF footprint
situated along and south of I-80. There will likely be a sharp
cutoff in the greatest threat for flash flooding as north of the
warm front will allow for a more stable environment and less of the
threat of the higher rates coincident with flash flooding. 12z HREF
FFG exceedance probabilities over 3hrs notes a maximum prob of
30-50% from Hastings over into Omaha and surrounding areas from
00-09z, aligning well with a prolonged 1"/hr or great rate
forecast within that zone over the period of time in question.
In coordination with several WFO's over the Central Plains and
Midwest, a MDT risk was added for the new D2 update with highest
confidence in that area between I-70 to I-80 over KS/NE/IA.
Kleebauer
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Tue Sep 29 2026
Day 3
Valid 12Z Thu Oct 01 2026 - 12Z Fri Oct 02 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN PLAINS...
20Z Update: Little changes were necessary for the previous MDT risk
issuance as the overall synoptic setup remains relatively
unchanged. Expectation is for considerable rainfall to continue
across south-central TX up through the DFW metroplex into Eastern
OK on Thursday as the cold front slowly advances through the
Southern Plains. The primary instability axis is positioned right
across the Southern Plains into the the southern area of the
Midwest within a broad warm sector that developed over the course
of the prior 24 hrs into the new D3. Rainfall potential over the
Hill Country is the most notable area for appreciable impacts as
the Thursday afternoon and evening time frame could spell another
period where 2-4", locally higher totals are plausible leading to
locally significant flash flood prospects given the sensitivity of
the area in general. PWATs over 2" with sufficient low-level
buoyancy will create a suitable environment for heavy rainfall with
rates likely between 1-2"/hr with as high as 3-4"/hr for intra-hour
rate potential, a signature notorious for elevated flash flood
concerns within the Hill Country and the I-35 corridor. This has
led to a maintenance of the MDT risk over the area, expanding
northward into portions of eastern OK.
The area further north into IA/IL/MO will be another area to
monitor closely as the prolonged diffluent flow over the Plains
into the Midwest will leave the door open for more heavy rainfall
over a broad area of uncharacteristically high PWATs thanks to an
airmass coincident with the remnants of Tropical Cyclone Polo.
PWATs between 2-2.25" over the Midwest and Mid-Mississippi Valley
will be upwards of 3-4 standard deviations above normal, an anomaly
that will likely flirt with some of the daily and even monthly
maxima for PWATs over this general area as noted via the NAEFS
anomaly percentiles. A broad SLGT risk with high-end SLGT forecast
over the area just west of Chicago back down through the east-
central Plains and Mid-Mississippi area of MO will be a focal point
for convection and scattered to potentially widespread flash
flooding concerns by Thursday afternoon and evening. This area will
be assessed in future updates for possible upgrades.
Kleebauer
..Previous Discussion..
During this period a postively-titled upper level trough is
expected to weaken, then stall over the Central and Southern
Plains. Along the surface frontal boundaries heavy rainfall will
persist with potentially numerous instances of flash flooding for
the Southern Plains. A Moderate Risk was maintained southeast
Oklahoma to the Texas Hill Country and South Texas where large-
scale ascent afforded in part by favorable upper jet forcing will
continue to interact with an axis of anomalous moisture (2-2.25+
inch PWs). This multiple day event will have possible overlap, thus
keeping an elevated risk for flooding.
A broad Slight Risk extending north from Texas to Wisconsin and
northern Indiana and then west to eastern Colorado. While areal
average rainfall amounts are expected to be less in this region,
moisture drawn north and focusing ahead of a cold front settling
south across the region is likely to fuel the potential for
widespread moderate to localized heavy rainfall, resulting in at
least isolated runoff concerns.
Campbell/Pereira
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Tue Sep 29 2026
Day 4 and Day 5
Valid 12Z Fri Oct 02 2026 - 12Z Sun Oct 04 2026
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHERN TEXAS ON DAY 4 AND FROM THE CENTRAL GULF COAST INTO THE
TENNESSEE VALLEY ON DAY 5...
2030Z Update...
Deterministic and ensemble guidance maintained decent run to run
continuity that resulted in few changes needed to the previously
issued Excessive Rainfall Outlook on either Day 4 or Day 5. The
large scale picture still features deep moisture streaming
northward from the Gulf of American into place ahead of a large
scale mid-latitude trough digging south- and east-ward, Spaghetti
plots from the EC ensembles suggested expanding the outlook areas
westward a bit...moreso on Day 4. But the change does not reflect a
fundamental shift in forecast reasoning.
Bann
Previous Outlook Discussion...
Day 4...
Mid-level energy associated with the trough moving into
the central and southern Plains on Thursday will remain over the
region into at least the early part of the weekend. Deep moisture
will remain as well over parts of the southern Plains --
maintained by backing low level winds tapping Gulf moisture and
drawing it westward into a lingering low level boundary. As this
moisture interacts with the slow-moving mid-level energy that is
forecast to remain over the area, there will continue to be some
potential for heavy rainfall producing storms, impacting some areas
that are likely to be left vulnerable to excessive runoff by the
previous day(s) heavy rains. While forecast confidence has
diminished some, there remained enough of a signal in the
overnight guidance that showed additional heavy accumulations
centered across parts of South to South-Cental Texas to maintain a
Slight Risk over the region. Farther north and east, a cold front
will continue to settle southeast through the Ohio Valley, with
some potential for heavy rain along and ahead of the boundary.
Supported by deep southerly flow along the western extent of the
subtropical ridge, guidance continues to show a broadening pool of
anomalous moisture spreading from the Gulf Coast into this
boundary. Mid-level energy embedded within the flow may help
produce areas of heavy rain, with some guidance indicating heavy
amounts along the Gulf Coast from Mississippi to the Florida
Panhandle this period.
Day 5...
Moisture will continue to deepen from the Gulf Coast northward into
the Tennessee Valley -- with PW anomalies increasing to 2-3 std
dev above normal across much of the region during this period. A
Slight Risk was drawn along the axis of deepest moisture (PWs at
or above 2.25 inches) which is expected to extend from the central
Gulf Coast northward into an area of enhanced ascent centered in
the vicinity of the Tennessee Valley. The previously noted front
dropping south into the area along with mid-level energy emanating
from the Plains are expected to help focus heavy rainfall
development over this region. Forecast confidence was limited in
part by model spread regarding the placement of the front, which is
likely to play a role in where the heaviest amounts occur.
Therefore, notable adjustments to at least the northern extent of
the Slight Risk may be required in future updates.
Pereira
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Tue Sep 29 2026
Day 4 and Day 5
Valid 12Z Fri Oct 02 2026 - 12Z Sun Oct 04 2026
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHERN TEXAS ON DAY 4 AND FROM THE CENTRAL GULF COAST INTO THE
TENNESSEE VALLEY ON DAY 5...
2030Z Update...
Deterministic and ensemble guidance maintained decent run to run
continuity that resulted in few changes needed to the previously
issued Excessive Rainfall Outlook on either Day 4 or Day 5. The
large scale picture still features deep moisture streaming
northward from the Gulf of American into place ahead of a large
scale mid-latitude trough digging south- and east-ward, Spaghetti
plots from the EC ensembles suggested expanding the outlook areas
westward a bit...moreso on Day 4. But the change does not reflect a
fundamental shift in forecast reasoning.
Bann
Previous Outlook Discussion...
Day 4...
Mid-level energy associated with the trough moving into
the central and southern Plains on Thursday will remain over the
region into at least the early part of the weekend. Deep moisture
will remain as well over parts of the southern Plains --
maintained by backing low level winds tapping Gulf moisture and
drawing it westward into a lingering low level boundary. As this
moisture interacts with the slow-moving mid-level energy that is
forecast to remain over the area, there will continue to be some
potential for heavy rainfall producing storms, impacting some areas
that are likely to be left vulnerable to excessive runoff by the
previous day(s) heavy rains. While forecast confidence has
diminished some, there remained enough of a signal in the
overnight guidance that showed additional heavy accumulations
centered across parts of South to South-Cental Texas to maintain a
Slight Risk over the region. Farther north and east, a cold front
will continue to settle southeast through the Ohio Valley, with
some potential for heavy rain along and ahead of the boundary.
Supported by deep southerly flow along the western extent of the
subtropical ridge, guidance continues to show a broadening pool of
anomalous moisture spreading from the Gulf Coast into this
boundary. Mid-level energy embedded within the flow may help
produce areas of heavy rain, with some guidance indicating heavy
amounts along the Gulf Coast from Mississippi to the Florida
Panhandle this period.
Day 5...
Moisture will continue to deepen from the Gulf Coast northward into
the Tennessee Valley -- with PW anomalies increasing to 2-3 std
dev above normal across much of the region during this period. A
Slight Risk was drawn along the axis of deepest moisture (PWs at
or above 2.25 inches) which is expected to extend from the central
Gulf Coast northward into an area of enhanced ascent centered in
the vicinity of the Tennessee Valley. The previously noted front
dropping south into the area along with mid-level energy emanating
from the Plains are expected to help focus heavy rainfall
development over this region. Forecast confidence was limited in
part by model spread regarding the placement of the front, which is
likely to play a role in where the heaviest amounts occur.
Therefore, notable adjustments to at least the northern extent of
the Slight Risk may be required in future updates.
Pereira
