FOUS30 KWBC 160752
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Wed Sep 16 2026

Day 1  Valid 12Z Wed Sep 16 2026 - 12Z Thu Sep 17 2026

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL OVER PORTIONS
OF SOUTHERN AND CENTRAL ARIZONA, AND THE SACRAMENTO MOUNTAINS OF NEW MEXICO...

...Southwest...

Current WV satellite continues to pinpoint a robust shortwave
across the Gulf of California as it slowly migrates to the north
over the course of the evening. Guidance has been hinting at this
particular feature for several runs the past few days which will
play a significant role in the forecast over the Southwestern
CONUS, and beyond. The latest model 500mb vorticity forecasts are
noting the area of organized vorticity lifting north out of the
Baja over the next 24 hrs with sights on AZ as the likely path
of the disturbance will take it up through the lower desert and
slide north-northeast through the period. The progression of the
disturbance is key in where more organized convective elements will
materialize as the shortwave provides enhanced mid-level forcing
and is coincident with more appreciable moisture advection into the
region. PWAT anomalies are forecast to reach firmly into the +2 to
+3 standard deviation range, perhaps even flirting with upwards of
+4 standard deviations as the latest ECENS output reflects areas
of AZ and NM approaching monthly max PWATs when assessing the
climatological percentiles.

Hi-res ensemble (HREF) means remain very aggressive for later this
afternoon into early Thursday morning with some models printing out
2-3", locally higher, in spots across central and southwestern AZ
with a northern inflection into southern UT and southwestern CO
given the noted moisture advection and strong 850-500mb winds
accelerating northward and banking up into the terrain. This has
been a feature of this setup that has only improved in its
depiction as we move closer to the event leading to relative maxima
positioned within those slot canyon areas of southern UT from Zion
National all the way over towards Moab and the Arches area of UT.
This will extend as far north as the Uintas by the second half of
the forecast leading to widespread convection potential through
much of the state of UT. 700mb wind pattern is generally orthogonal
to the terrain across the state line into CO where the San Juan's
have seen a significant uptick in the forecast QPF over the last
12-24 hrs. Models are pinpointing a secondary max within that area
of the West with the upslope flow becoming a dominant regime for an
extended period of time within the mountains of northern NM into
CO. HREF probs for >2" are extremely robust for the aforementioned
areas in AZ/UT/NM/CO with probabilities between 60-90% for the 2"
threshold over the south-central AZ desert, including the Phoenix
metro and surrounds, the entire stretch of southern UT, the Uinta's
in northeastern UT, the San Juan Mountains of CO, and finally the
Sacramento Mountains in NM. Every single one of these noted
locations maintains a probability of >60% when assessing the threat
for 2" locally, a signal that typical notes a more appreciable
rainfall threat that typically leads to higher impacts when looking
into historical relevance in verification of flash flooding across
the Southwest.

One area referenced above that has the attention of another
potential high end impact is the sensitive areas within the
Sacramento Mountains as remnant burn scars continue to plague the
area in and around Ruidoso several years later. The combination of
upslope flow, PWAT anomalies of +3 or greater, and a very buoyant
environment locally will aid in the initiation of a round of heavy
thunderstorm development over the terrain and adjacent valley's
after 17z today. This setup is likely to produce rates >1"/hr for
multiple hours with the general storm motion likely to settle or
repeat over the terrain before finally winding down towards sunset.
Models have been printing out 2-3" of rainfall consistently in the
past succession of CAMs runs with even individual global
deterministic noting similar rainfall totals with their respective
outputs. This pattern is noteworthy as the environmental factors
are the tipping point that create a more favorable scenario for
higher rates and flash flooding given the airmass advecting into
the region is of a more tropical origin. These environments yield
more efficient warm rain processes and can create flash flooding
prospects very easily when you mix them with complex terrain and
higher run off capabilities. At this juncture, there is enough
consensus to add the Sacramento Mountains area in a small Moderate
Risk to account for the elevated flash flood threat with the
potential for significant impacts in-of the burn scar region and
neighboring towns.

The previous Moderate Risk upgrade from prior forecast was expanded
a bit further to the west and southwest over the Deserts of AZ with
the Phoenix metro firmly in the middle of the risk area with this
update. The MDT also extends into the central Mogollon Rim where
probabilities for 2 and even 3 inches remain very high and will
likely exhibit more flash flooding concerns as has been the case
the past several days. A broad high-end SLGT is forecast across
much of AZ up into southern UT, extending east into southwestern CO
where the San Juan Mountains will likely see copious amounts of
rainfall during the D1 period. The high-end SLGT encompasses much
of NM, as well with any convective development potentially offering
rates >1"/hr, a threshold that typically spells issues for much of
terrain and urbanized areas of the state. A further upgrade in any
of the above areas is plausible, so stay tuned for future updates
as this event evolves.

...Central Plains to Ohio Valley...

The upper ridge centered over the Southeastern U.S. will begin to
shift more to the west and center over the Lower Mississippi Valley
tomorrow while also strengthening with global deterministic
forecasting 500mb heights encroaching on 600dm for the center of
the ridge. This will allow for increasing heights to the north as
the ridge flexes and provides a greater influence in the
latitudinal positioning of our frontal boundary situated over the
Central U.S. extending into the Ohio Valley, currently. We will
continue to see heavy rain over northern MO as the previous
period's activity lingers and migrates eastward before dissipating
with the loss of the LLJ influence as we move forward in the
period. By the late-afternoon and evening hours, expectation is for
the front to stall and eventually lift north slowly as move into
the evening hours. Scattered convection will occur over the Ohio
Valley during the peak of diurnal heating with an increase in
coverage anticipated with even more activity likely to migrate
eastward across the region thanks to a litany of smaller mid-level
perturbations rippling through the mean flow aloft. A relatively
buoyant environment and elevated PWATs situated over the area will
allow for some isolated to perhaps widely scattered flash flood
prospects across IL/IN/OH with the current forecast depicting
localized areas of 1-3" where impacted by convection. Latest HREF
probs point to an stronger potential for >2" across the IN/OH
border, and across northern IL where multiple rounds of convection
are likely during the forecast period. These areas are noteworthy
due to their positioning along the front, creating a greater
propensity for cells to anchor and sit over the region, or train
over the same area. A secondary area of interest in the Ohio Valley
is now centered across north-central to northeastern Ohio thanks
to a stronger convective element expected the following evening
that could lead to cells hitting these areas where CAMs are more
aggressive with the signal given increasingly favorable instability
fields and higher PWATs advecting into the area to provide a more
suitable environment for higher rates and flash flooding potential.

By the evening, another shortwave from the Central Plains will
rotate around the northern periphery of the ridge and help initiate
a nocturnal round of convection in-of IA into northern IL leading
to another scattered flash flood concern around the vicinity of the
I-80 corridor. The combination of increasingly favorable ascent due
to the stronger PVA signal with the incoming shortwave, plus the
influence of a strengthening LLJ positioned over the Corn Belt will
provide a fairly robust convergence signature along the expected
frontal positioning by tonight through early Thursday morning. 00z
CAMs have come into better agreement on the threat maturing over
the course of the 00-06z time frame with the threat likely to reach
peak impact potential at the very end of the D1 period as the LLJ
reaches max intensity (40-50kts) and multi-cell organization
defines a more structured convective threat. Mean flow will take
cells from west to east with a potential to impact the Chicago
metro, but there is a growing threat for more appreciable rainfall
around the area of the Quad Cities and the southeastern portion of
IA. This area has seen a fair amount of rain recently which could
elevate the flash flood threat the second half of the forecast. A
bit of discrepancy in exactly where the main axis of convection
motions overhead with some models a bit further north of I-80 with
others south. The mean QPF puts the I-80 corridor as the primary
area of concern, so the SLGT risk from previous forecast remains
over the region. The SLGT was extended to cover the northern
portions of OH to account for the uptick in the QPF as the pattern
becomes more favorable for the northern OHV.

Kleebauer
 

Day 2  Valid 12Z Thu Sep 17 2026 - 12Z Fri Sep 18 2026

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTH-CENTRAL PLAINS, UPPER MIDWEST, & SOUTHWESTERN U.S...

...Northern Plains & Midwest...

Our broad upper ridge will continue to flex over a large portion of
the CONUS as we head into Thursday with the northern fringes of
the height field extending into the Upper Midwest and expanding as
far west as the Central High Plains. As we move through Thursday, a
robust shortwave, with origins from the tropics, will eject out of
the Southwestern U.S, pivoting up through the Central Rockies and
rounding the northwestern flank of the ridge and entering into the
Northern Plains as we head into Thursday evening. This shortwave
will also provide a significant PWAT increase as noted via both the
the ensemble anomaly plots and IVT climatological percentiles
approaching the 95-99th percentile up across South Dakota into
southern portions of MN by the end of the forecast period. Given
the strength of the forecast shortwave ejection, regional forcing
will increase significantly as the disturbance approaches leading
to more organized convective clusters to materialize by Thursday
afternoon and evening, lingering through the overnight hours to
Friday morning as the disturbance migrates across SD.

Model outputs continue to be bullish on a swath of 2-4", locally
higher occurring within the confines of the shortwave impulse as
the environmental conditioning and ascent pattern remain favorable
within proximity to the disturbance. Assessment of local forecast
bufr soundings indicate a traditional tall, skinny CAPE signature
with a saturated column typical of tropical-like air being pulled
poleward. These types of environments become more privy to enhanced
rainfall rates and more efficient warm rain processes that can
cause issues for areas further north. This setup is more favorable
for at least scattered flash flood prospects over the Northern
Plains into the Upper Midwest with the primary threat likely
occurring over the terrain areas of SD, extending east-northeast
into the area of I-90 and north, especially within close proximity
of the center of the disturbance. A high-end SLGT is now forecast
across southwest to south-central SD with a chance for an expansion
into eastern zones of the state if the general storm motion allows
for the heavier rainfall to make it east of the longitude of the
Missouri River which some models are depicting. Something we will
keep an eye  in subsequent forecasts.

...Southwest U.S...

Prevailing southerly flow will maintain a steady plume of elevated
moisture across the Southwestern CONUS, but the shortwave
disturbance associated with the proliferation of the previous
period's convection will vacate to the northeast allowing for a
gradual de-escalation of the convective magnitude over the region
in time for Thursday. That said, lingering PWAT anomalies ranging
from the 90-99th percentile of climo will allow for another busy
afternoon of widespread convective potential over much of NM and
eastern AZ leading to an elevated flash flood concern, once again.
Precip totals between 1-3" are forecast for the Sangre de Cristos
and San Juan's down through much of the terrain focused in the
interior areas of NM. This allowed for a high-end SLGT forecast to
remain across these aforementioned areas of NM into southern CO.
Pending the impacts from the prior period, a higher risk is also
non-zero and could be added in future forecasts as sensitive
grounds could enhance the threat for flash flooding further within the region.

Kleebauer
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