FOUS30 KWBC 250811
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 AM EDT Mon Aug 25 2025

Day 1  Valid 12Z Mon Aug 25 2025 - 12Z Tue Aug 26 2025

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR SOUTHERN
ARIZONA AND OVER PORTIONS OF THE CENTRAL ROCKIES FRONT RANGE AND
THE SOUTHERN PLAINS...

Weather Pattern Summary...

A large portion of the southwestern U.S. remains mired in a
classic monsoonal moisture flow regime with broad ridging over the
Four Corners region on south into Mexico. An expansive area of
highly anomalous moisture content will remain firmly entrenched
over the West through mid-week with some of this moisture spilling
east into the Plains as frontal boundaries and a NW-SE oriented jet
stream direct some of this monsoonal moisture eastward. The ECMWF
ENS shows PWATs above the 90th climatological percentile from the
Pacific Northwest Coast to as far east as eastern Oklahoma. This
abundance of moisture is why there is an expansive Marginal Risk
area that stretches from southern Oregon and California's Sierra
Nevada to as far east as the Lower Mississippi Valley. Look for
storms today and through Monday evening to be capable of producing
torrential downpours throughout the Marginal Risk area in the West
that could result in flash flooding. For additional information on
the Slight Risks, please see below:

...Southern Arizona...

An influx of rich monsoonal moisture (PWATs 1.6-2.0" will engulf
much of southern AZ and the Lower Colorado River Valley Monday
afternoon and evening while MLCAPE ranges between 500-1,000 J/kg.
Storm motions will generally be <10 knots and vertical wind shear
is weak, but these storms will be loaded with heavy rainfall that
generate exceptional rainfall rates in a desert climate.
Thunderstorms will be capable of producing 2"/hr rainfall rates and
in some significant cases, as much as 1.5" in 30 minutes. Storms
will initially develop as a result of daytime heating, but additional
storms will form into Monday evening due to colliding outflows
producing storms that tap into lingering elevated instability.
Areas most at risk for flash flooding are in dry washes, arroyos,
and poor drainage areas.

...Central Rockies Front Range...

Low-level moisture advection into southeast CO and northeast NM
will coincide with a nearby frontal boundary to trigger additional
strong thunderstorms Monday afternoon and evening. PWATs remain
above the 97.5 climatological percentile within the Slight Risk
area and soils are becoming increasingly more saturated following
an active weekend. With PWATS >1.2" and MLCAPE >500 J/kg, hourly
rainfall rates could range between 1.5-2.0" at their heaviest
intensity. Given these factors, flash flooding is likely in areas
of complex terrain and near burn scars.

...Southern Plains...

Unlike the Southwest and the Rockies, there are more synoptic-
scale and mesoscale factors at play that will develop numerous
areas of thunderstorms capable of producing flash flooding. The
region resides beneath the diffluent right-entrance region of a
250mb jet streak that is located over the east-central U.S.. A
strong 850mb front will reside over the Slight Risk region with
modest 850mb warm-air advection being observed over the Southern
Plains. Not only is the moisture source initially from monsoonal
flow, but there is also some modest moisture advection at low-
levels that originates out of the northern Gulf. The result is a
surge in PWATs that approaches 2.0" in central OK and central AR
that will also have anywhere from 500-1,000 J/kg of MUCAPE to work
with through Monday night. Storms will flare up initially this
morning, then following a brief lull in thunderstorm activity
through early evening, a brief restrengthening of the LLJ will
trigger new storms Monday night and into Tuesday morning. Latest
HREF guidance shows low-to-moderate chances (20-45%) for rainfall
>3" in eastern OK. The Slight Risk was adjusted to match more of
the QPF trends and the aforementioned HREF probabilistic guidance.

...Western Florida Peninsula...

A Marginal Risk was introduced over the more populated section of
western Florida given a rather noticeable 20-35% odds depicted by
the 00Z for 24-hour rainfall totals >5". The Tampa-St. Petersburg
metro area will have >2.2" PWATs and >1,000 J/kg of MLCAPE
available for storms to tap into, while at the same time, mean-
layer winds in the sfc-500mb layer are almost uniformly out of the
WSW. The concern is for training and back-building thunderstorms,
which is denoted by ECMWF area-averaged soundings around Tampa Bay
that feature Upshear Corfidi vectors that are <10 knots. Given
these factors, as well as these storms producing >3"/hr rainfall
rates over a more urbanized environment, a Marginal Risk was
introduced this forecast cycle.

Mullinax


Day 2 Valid 12Z Tue Aug 26 2025 - 12Z Wed Aug 27 2025

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN ARIZONA AND THE CENTRAL ROCKIES...

...The West...

The sprawling upper ridge over the Southwest continues to funnel
monsoonal moisture northward through the Intermountain West. ECMWF
guidance shows PWATs in excess of 1" advancing as far north as the
Blue Mountains and Columbia River Valley of WA/OR. Some PWATs are
also expected to top 1.25" in portions of the Great Basin and Snake
River Valley. Helping to add some upper-level support aloft will be
the presence of a meandering 700mb low positioned over the Pacific
Northwest and a healthy diffluent pattern at 250mb thanks to a
longwave trough situated along the West Coast. With such high
moisture content, not much in the way of surface based heating will
be needed to spark widespread thunderstorms Tuesday afternoon and
through Tuesday evening. Combined with MUCAPE between 500-1,000
J/kg, thunderstorms will be capable of generating up to 1.5"/hr
rainfall rates, which will pose a flash flood threat in areas of
complex terrain and in nearby creeks and streams.

...Southern Arizona...

Fresh off a busy day of thunderstorms on Monday, there is the
possibility that the atmosphere is overworked to where converage of
thunderstorms is not as expansive as Monday. However, southeast AZ
should have a greater concentration of instability (500-1,000 J/kg
of MUCAPE) and PWATs will remain as high as 1.5" in some cases.
This continues to support rainfall rates up to 2"/hr in the more
intense storms. While storms will remain more pulse-like in nature,
these potent storms will still pose a flash flood threat,
especially in dry washes and in spots that drain poorly.

...Central & Southern Rockies...

Guidance across the board shows a higher concentration of
atmospheric moisture over the region on Tuesday compared to Monday,
and that is notable considering PWATs were already above the 97.5
climatological percentile on Monday. PWATs in the tallest peaks
will generally hover near 1" at their highest values, but along and
just east of the Front Range, PWATs are likely to range between
1.3-1.5". These values are surpassing the 99th climatological
percentile while also having 250-500 J/kg of MLCAPE present. With
soils steadily growing more and more saturated and the favorable
atmospheric parameters in place, more instances of flash flooding
are expected with complex terrain and burn scars more at-risk.

Mullinax


Day 3  Valid 12Z Wed Aug 27 2025 - 12Z Thu Aug 28 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS...

...Central Plains...

A sheared 500mb vort max exiting the Central Rockies will help to
spawn a surface low over the southern High Plains on Wednesday.
Working in tandem with the dome of high pressure to the east, this
will tighten the pressure gradient and prompt a strengthening LLJ
to form over the Southern Plains late Wednesday afternoon and into
Wednesday night. This enhanced LLJ will not only act to deliver
rich Gulf moisture northward, but it will also cause 850mb
frontogenesis to ensue over the Central Plains. PWATs will surge
to 1.8-2.0" within the Slight Risk area with MUCAPE ranging between
500-1,000 J/kg. These parameters would support up to 2.5"/hr
rainfall rates within storms that could contain mesocyclones. This
is due to vertical wind profiles that sport 30-40 knots of sfc-6km
shear and sfc-1 km SRH values in excess of 200 m/2 s/2.
Mesocyclones are efficient rainfall producers particularly in
environments with deep warm cloud layers and highly saturated
sfc-500mb environments, all of which apply in this situation. This
setup bears watching as locally significant flash flooding cannot
be ruled out.

...The West...

There remains no shortage of monsoon moisture throughout much of
the West, but the focus for flash flooding will become more
pronounced in the Northwest. An organized 700mb low will have an
abundance of moisture in place from OR/WA on east into the northern
Rockies. PWATs from the Blue Mountains and Snake River valley on
east across the Bitterroots and into southern MT/western WY are
likely to top the 99.5 climatological percentile and the presence
of the 700mb low will provide additional synoptic-scale support. In
addition, modest instability for the region will be present with
the ECMWF-EFI showing a pronounced signal for unusually high CAPE
(or instability) values across the northern Great Basin, eastern
OR, and much of ID. The same tool (ECMWF-EFI) depicts an impressive
QPF footprint over much of southern ID and even as far east as
southern WY. There was some contemplation of a Slight Risk for
parts of the Northwest tonight, however the region does need those
modest instability values (500-1,000 J/kg MUCAPE) to be realized in
order to increase the flash flood threat. Given there is still
some spread in where the heaviest QPF occurs and where the greatest
instability ensues, opted to hold off on a Slight Risk at this
time. However, should confidence increase in a more unstable
atmosphere come to fruition, a Slight Risk may be necessary in
future forecast cycles.

Mullinax
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