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
404 AM EDT Wed Sep 30 2026
Day 1
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...
There will be 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 an upslope flow regime to perpetuate the rainfall over
the terrain of southern Colorado into northern New Mexico. 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. Accumulations of 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 precipitation; once in the morning and then once again in
the evening as the surface low intensify and strong mid- level
vorticity advection pivots around the ULL center.
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 Texas into the Red
River Valley down through the Central Rio Grande Valley and
Edwards Plateau by evening and into early Thursday morning. Heavy
convection expected along the front as it migrates out of the High
Plains of Texas 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, Texas up towards Norman, Oklahoma. 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.
A Moderate Risk was maintained along with an expansion of the
eastern flank for central Texas.
...Central Plains into the Corn Belt...
A mature cyclone will be moving through the central U.S. with a
progressive warm front extending from the central low pressure
across the Plains. 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. PW 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 Missouri and southern to central Iowa.
Heavy rainfall will likely align along a SW-NE axis from northern
Kansas to eastern Iowa as a strong shortwave ejects out across the
Plains coinciding with an increasing low level jet ahead of the
approaching trough over the Rockies. Convection is expected to
develop during the afternoon hours while spreading northeast into
the Missouri River Valley and neighboring Corn Belt by
evening/overnight. Training of cells near the warm front is
anticipated and move northeast, in tandem which will eventually
feed into the likely convective cluster migrating along the front
from north- central Kansas into southern Nebraska, eventually
making headway into western and central Iowa after nightfall. Hourly
rainfall rates are depicted to reach between 1-2"/hr in the
strongest cell cores with some repeating action along the warm
front. 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
Kansas/Nebraska 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.
Campbell/Kleebauer
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Wed Sep 30 2026
Day 2
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...
Considerable rainfall to continue across south-central Texas into
Eastern Oklahoma 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 hours. Rainfall potential over the Hill Country
is the most notable area for appreciable impacts as 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. Hourly 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. The Moderate Risk area was maintained from South
Texas to southeast Oklahoma.
Prolonged diffluent flow over the Plains into the Midwest the
stage for heavy rainfall over a broad area of uncharacteristically
high PW values thanks to an airmass coincident with the remnants of
Tropical Cyclone Polo. PW values 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 PWs over this general
area. A broad Slight Risk covers Texas to Wisconsin and northeast
Indiana for scattered to potentially widespread flash flooding.
Campbell/Kleebauer
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Wed Sep 30 2026
Day 3
Valid 12Z Fri Oct 02 2026 - 12Z Sat Oct 03 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHERN TEXAS...
Gulf moisture will continue to drawn into a lingering low level
boundary to continue to produce heavy rainfall, impacting some
areas that are likely to be left vulnerable to excessive runoff by
the previous day(s) heavy rains. Guidance continues to depict a
signal for additional heavy accumulations centered across parts of
South to South-Cental Texas. Therefore, a Slight Risk for
excessive rainfall was maintained for this period.
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. A broad Marginal
spans from Texas across the Gulf to western Florida and then
northeast to Ohio and Kentucky.
Campbell
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Wed Sep 30 2026
Day 4 and Day 5
Valid 12Z Sat Oct 03 2026 - 12Z Mon Oct 05 2026
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FROM THE CENTRAL
GULF COAST NORTHWARD INTO THE TN VALLEY DAY 4 AND ALONG THE CENTRAL
GULF COAST DAY 5...
A broad upper trough will continue to shift east from the Plains
and Mississippi Valley, gradually amplifying as it moves into the
eastern U.S. over the weekend. Deep, anomalous moisture (2-2.5+
std dev above normal) will be drawn along and ahead of a wavy front
that is forecast to extend southwestward from the southern
Appalachians back into the western Gulf early in the period. Mid-
to-upper level energy moving along the base of the broader scale
trough will have the potential to produce organized, highly-
efficient storms, resulting in areas of heavy amounts. Following
the axis of deepest moisture and the anticipated position of the
front, a Slight Risk extending from the central Gulf Coast
northeastward into southern portions of the Tennessee Valley was
maintained on Day 4. With the front continuing to push southeast,
the focus for heavy rainfall is likely to become confined more
along the Gulf Coast, where a Slight Risk was focused on Day 5.
Forecast confidence was limited by model spread with regards to
the timing the upper trough and the embedded shortwave elements.
While the outlook areas were drawn according to where the ensemble
guidance currently showed the greater threats for heavy amounts,
significant adjustments may be required in future updates.
Pereira
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Wed Sep 30 2026
Day 4 and Day 5
Valid 12Z Sat Oct 03 2026 - 12Z Mon Oct 05 2026
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FROM THE CENTRAL
GULF COAST NORTHWARD INTO THE TN VALLEY DAY 4 AND ALONG THE CENTRAL
GULF COAST DAY 5...
A broad upper trough will continue to shift east from the Plains
and Mississippi Valley, gradually amplifying as it moves into the
eastern U.S. over the weekend. Deep, anomalous moisture (2-2.5+
std dev above normal) will be drawn along and ahead of a wavy front
that is forecast to extend southwestward from the southern
Appalachians back into the western Gulf early in the period. Mid-
to-upper level energy moving along the base of the broader scale
trough will have the potential to produce organized, highly-
efficient storms, resulting in areas of heavy amounts. Following
the axis of deepest moisture and the anticipated position of the
front, a Slight Risk extending from the central Gulf Coast
northeastward into southern portions of the Tennessee Valley was
maintained on Day 4. With the front continuing to push southeast,
the focus for heavy rainfall is likely to become confined more
along the Gulf Coast, where a Slight Risk was focused on Day 5.
Forecast confidence was limited by model spread with regards to
the timing the upper trough and the embedded shortwave elements.
While the outlook areas were drawn according to where the ensemble
guidance currently showed the greater threats for heavy amounts,
significant adjustments may be required in future updates.
Pereira
