FOUS30 KWBC 260038
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Mon Aug 25 2025
Day 1
Valid 01Z Tue Aug 26 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 and Interior Central-Southern California...
16Z update... Guidance depicts a slight uptick in amounts possible
for southeast California. Some of these cells may produce up to
1.5 inches/hour. The Slight Risk was nudged a bit further northwest
over southeast California and across the northern Sierra to=20
account for this trend.
Campbell
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...
0100Z update... Based on the latest observational and guidance
trends (including recent HREF and RRFS QPF exceedance
probabilities), have cut back on the northern portions of the
Slight and Marginal Risk areas across the Southern Plains. With the
surface front now south of the Red River, any deep-layer elevated
instability supportive of excessive rainfall will exist in a=20
relatively narrow corridor along and north of the Red River.
Hurley
Previous discussion..
Unlike the Southwest and the Rockies, there are more synoptic-=20
scale and mesoscale factors at play that will develop numerous=20
areas of thunderstorms capable of producing flash flooding. The=20
region resides beneath the diffluent right-entrance region of a=20
250mb jet streak that is located over the east-central U.S.. A=20
strong 850mb front will reside over the Slight Risk region with=20
modest 850mb warm-air advection being observed over the Southern=20
Plains. Not only is the moisture source initially from monsoonal=20
flow, but there is also some modest moisture advection at low-=20
levels that originates out of the northern Gulf. The result is a=20
surge in PWATs that approaches 2.0" in central OK and central AR=20
that will also have anywhere from 500-1,000 J/kg of MUCAPE to work=20
with through Monday night. Storms will flare up initially this=20
morning, then following a brief lull in thunderstorm activity=20
through early evening, a brief restrengthening of the LLJ will=20
trigger new storms Monday night and into Tuesday morning. Latest=20
HREF guidance shows low-to-moderate chances (20-45%) for rainfall=20
3" in eastern OK. The Slight Risk was adjusted to match more of=20
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, THE NORTHERN GREAT BASIN, THE CENTRAL ROCKIES AND
THE SOUTHERN PLAINS...
...The West...
21Z update... In coordination with the local forecast offices
across the northern Great basin/Intermountain West, a Slight Risk
was hoisted for this period. Strong, diffluent flow aloft combined
with broad forcing for ascent and PW values near the 99th
percentile will support enhanced rainfall across a broad area.
Local 1hr/6hr FFG are about 1.00/1.5 inches respectively and
guidance is signaling 1 to 2 inches with isolated maxes of 3+
inches. Steep terrain, burn scars and other sensitive locations
will be vulnerable to these rainfall intensities.
Campbell
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...
21Z update... Latest WPC QPF and model guidance had an increase in
amounts near the New Mexico/Texas Panhandle border, therefore a
minor expansion of the eastern boundary was made. In addition, a
new Slight Risk area was hoisted for portions of Texas and
Oklahoma. Multi-day rain event will keep this part of the country
near saturation with an increase threat for flash flooding
concerns. This new area overlaps much of the area defined during
the Day 1 period.
Campbell
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
...Western Florida Peninsula...
The environmental setup will be very similar to those described
for Day 1 period and convection is expected to persist within the
Tampa Bay vicinity. Recent heavy rain with added potential for
another couple of inches will keep the threat for excessive
rainfall and isolated flash flooding elevated. A Marginal Risk area
was raised.
Campbell
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...
21Z update... The footprint of the QPF in the Plains trended more
oblong, with the eastern side reaching the central parts of the
Ozark mountains. Meanwhile guidance increased amounts, and to some
degree areal coverage, across eastern Colorado. Opted to alter the
Slight Risk by elongating it to span from eastern Colorado to
Northwest Arkansas. As such, the Marginal was adjusted further
into north-central Arkansas. The best concentration of QPF
continues to be over central and southeast Kansas. This part of the
region will continue to be monitored for any future upgrades in
Risk level.
Campbell
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...
21Z update... The environment will continue to be more than
abundant with moisture, forcing and instability. Refer to details
below from the previous discussion. A Slight Risk was hoisted to
cover from far eastern Oregon to western Wyoming. The exact
location of the storms are uncertain, but will have the potential
to drop another to 1 to 3+ inches.
Campbell
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
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!82zqSKH7bAFaHzoOFVk_YXxMxKK4DImhCGm9J_V64RAK= LXn_Ag_3mkDiCGS3tXUbqO8AJsOmcLpODmyqSfbNWTqwKeA$=20
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!82zqSKH7bAFaHzoOFVk_YXxMxKK4DImhCGm9J_V64RAK= LXn_Ag_3mkDiCGS3tXUbqO8AJsOmcLpODmyqSfbNqS9Owtg$=20
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!82zqSKH7bAFaHzoOFVk_YXxMxKK4DImhCGm9J_V64RAK= LXn_Ag_3mkDiCGS3tXUbqO8AJsOmcLpODmyqSfbN--vKXiE$=20
=3D =3D =3D
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