FOUS30 KWBC 262003
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
403 PM EDT Tue Aug 26 2025
Day 1
Valid 16Z 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 SIERRA NEVADA, THE
CENTRAL ROCKIES AND THE SOUTHERN PLAINS...
...The West...
16Z Update: SLGT risks inherited were either maintained or
expanded with the largest expansion occurring over the Great Basin
including much of eastern and south-central NV, as well as
southwestern UT. Areal buoyancy during peak diurnal heating along
with increasing convergence on the western flank of the MCV located
across the NV/UT border will lead to enhanced convective posture
across the terrain located north of the valley in southern NV. 12z
HREF probs for >1" increased substantially to 50-90% over much of
the above area with the >2" interval a solid 30-45% in the same
location. FFG's are naturally lower in these regions due to
flashier soils and complex terrain that leads to funneling,
something that will be greatly important to monitor as we move
through the day as the convective risk blossoms.
Kleebauer
..Previous Discussion..
The sprawling upper ridge over the Southwest continues to funnel
rich 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, burn scars, and nearby creeks and
streams. The Slight Risk over the northern Great Basin and
portions of the Snake River Valley remains in place. The Slight
Risk area resides within the strongest signal from the ECMWF-EFI
with 0.8-0.9 values, which give support to highly unusual QPF
totals and potential impacts for this time of year. A Slight Risk
was also introduced over the central Sierra Nevada given the
lingering threat for flash flooding over the rugged terrain that is
growing increasingly more saturated and sensitive.
Mullinax
...Arizona...
16Z Update: No major changes reflected in the area of the SLGT risk
across AZ today. Another round of convection will spawn in-of the
Mogollon Rim and the terrain of southern AZ to create another
threat of scattered flash flood occurrences later this afternoon
and evening. SLGT was expanded on the northwest edge to account for
trends in CAMs signaling higher coverage near and south of
Flagstaff later today.
Kleebauer
..Previous Discussion..
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
was not as active on Monday and should tap into modest instability
(500-1,000 J/kg of MUCAPE) and moisture aloft (PWATs up to 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 still pose a flash flood
threat, especially in dry washes and in spots that drain poorly.
Mullinax
...Central & Southern Rockies...
16Z Update: Overall pattern remains favorable for widespread
convective initiation over the central and southern Rockies this
afternoon and evening with a favorable flash flood threat given the
elevated PWATs and focused ascent present within the mid-levels.
SLGT risk was expanded to the northwest to include more of western
CO and all of southeast UT as guidances elevates the signal for
locally heavy rainfall over some fairly sensitive terrain areas in
the aforementioned locations. The SLGT was also expanded on the
southern edge to include portions of the Sacramento Mtns, mainly in
the vicinity of the burn scar near Ruidoso, NM where extreme
sensitivity persists. 12z HREF mean QPF is quite robust with great
agreement among the CAMs for locally heavy rain to spawn sometime
between 18-00z. Burn scar remnants and slot canyons will be the
most susceptible for flash flood concerns this period as rates in
the strongest convective cores will likely reach 1-1.5"/hr at their
peak intensity. This is more than sufficient for flash flood
generation in these locations.
Kleebauer
..Previous Discussion..
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 due to recent heavy
rainfall and the favorable atmospheric parameters in place, more
instances of flash flooding are expected with complex terrain and
burn scars more at-risk.
Mullinax
...Southern Plains...
16Z Update: Complex of showers and thunderstorms over OK continue
to migrate to the southeast with the main shortwave exiting the
western half of the state leading to an end in the convective
potential for parts of the state. A more stable environment awaits
further east leading to the threat of heavy rainfall sort of
petering out as it enters the Lower Mississippi Valley. Expectation
is for the disturbance to continue its progression with a general
decay in the rates and coverage over the course of the late-morning
and afternoon. A brief lull to materialize across the Southern
Plains, but more convection firing upstream will migrate over
northeast NM into the bordering TX/OK Panhandle border with another
period of locally heavy rainfall occurring late tonight, mainly
beyond 03z. Signals are mixed on the threat, but regional forcing
is sufficient for scattered heavy precip genesis, so wanted to
maintain the MRGL to account for the threat. Look for the MRGL
across OK to be removed in the next update at 01z, or sooner as the
threat across central and eastern OK comes to an end.
Kleebauer
...Western Florida Peninsula...
16Z Update: No changes necessary to the previous MRGL risk forecast
over the west-central FL Peninsula. Local convergence pattern
remains prevalent in the vicinity of Tampa and surrounding locales
leading to a low-end probability for heavy rainfall between 3-5"
from any cells that migrate off the Gulf and move inland. PWATs are
hovering ~1 deviation above normal with the 12z KTBW sounding
indicating a PWAT of 2.25", right on track with the previous
forecast lending credence to maintaining general continuity in the
setup.
Kleebauer
..Previous Discussion..
A similar setup to Monday is expected to produce thunderstorms
capable of producing torrential downpours over the heavily
urbanized western Florida Peninsula today. An approaching cold
front will act to trigger thunderstorms in an atmospheric
environment that contains up to 2.25" PWATs and MLCAPE around
1,000 J/kg. Surface-500mb mean winds remain fairly uniform out of
the WSW, so there is the potential for training thunderstorms
around the Tampa-St. Pete metro area. The 00Z HREF shows moderate-
to-high chances (50-70%) for 24-hour rainfall totals >3" and low
chances (10-30%) for 24-hour rainfall totals >5". The inherited
Marginal Risk remains on track with the more urbanized areas
containing the greatest concentration of hydrophobic surfaces most
at-risk for flash flooding.
Mullinax
Day 2
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...
20Z Update: The previous forecast SLGT risk areas over the=20
interior Northwest CONUS, and Central Plains to Ozarks remain=20
favorable for their respective pattern evolutions based off the=20
latest 12z numerical suite.=20
Trends have favored scattered heavy convective signatures for the=20
SLGT risk over the interior Northwest with aid from a significant=20
moisture advection regime inferring PWAT anomalies increasing=20
closer to +3 deviations above climatological norm. Remnant=20
shortwave propagation stemming from previous convective activity on
D1 will be one of the main drivers for the potential as regional=20
forcing will be ongoing under the influence of a consolidated MCV=20
moving northward from UT into eastern ID. Guidance is consistent on
the threat lingering from D1 to D2 with the shortwave becoming=20
increasingly sheared by the afternoon time frame leading to more of
a west to east aligned convective development during peak diurnal=20 instability maxima. Low FFG's across much of ID/Eastern=20
OR/Southwest MT infer more susceptibility for flash flooding=20
concerns in these locations creating a suitable threat for flash=20
flooding during much of the D2 period. Only minor adjustments were=20 necessary, mainly expansion on the southern flank of the SLGT risk=20
into the northern fringes of UT/NV.=20
Across the Central Plains to Ozarks, consensus for the evolution=20
of the next frontal wave maintains a pretty stout signature for
heavy rainfall focused over central KS down through southeast KS
into the MO/AR Ozarks as we move into the final 6hr window in the
forecast cycle. One of the biggest changes in the forecast from the
morning update was the convective pattern trending a bit faster and
further east in terms of the heavier precip signal as the shortwave
progression and evolving LLJ should spur the convective pattern to
push downstream a bit quicker as it materializes. 12z HREF
probabilities for all relevant intervals have shifted the maximum
prob outputs further southeast into southwestern MO and even the
northwestern portions of AR. This is a significant addition
compared to the overnight iteration of the hi-res ensemble, as the
previous forecast was focused more to the northwest comparatively.
That said, there is a still a pretty strong signal overall for the
heavy rain risk across all of southeast KS with the upstream
depiction viable for the potential of >2" focused as far back as
Hays, KS. The trend was just an adjustment in the heavier QPF
output more into the MO Ozarks with a secondary QPF maxima
positioned southwest of Springfield, MO. This is in good agreement
with the latest EC AIFS Ensemble depiction which also trended ever
so slightly to the southeast with the higher probability fields for
the heavier precip notions. This was enough to warrant a small
extension into those portions of MO/AR to account for the eastern
expansion of heavier convection.=20
Kleebauer
..Previous Discussions..
...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 30-45 knots of sfc-6km
shear and sfc-1 km SRH values in excess of 200 m/2 s/2. These
vertical wind shear values support mesocyclone formation, which are
efficient rainfall producers within environments containing deep
warm cloud layers and highly saturated sfc-500mb environments, all
of which apply in this situation. Latest guidance has continued to
trend wetter in the central High Plains and even along the
foothills of the Central Rockies, leading to an expansion of the
Slight Risk into northern CO and southeast WY. But the greatest
concern lies over southern Kansas where even 75th percentile 24-hr
QPF on the NBM is topping 5". This setup bears watching as locally
significant flash flooding cannot be ruled out in parts of southern
and eastern KS, northeast OK, and southwest MO.
...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 eastern OR, southern ID, northern NV,
and into much of WY. There is increasing confidence in enough
instability (500-1,000 J/kg MUCAPE) for storms to tap into from
the northern Great Basin on north into the Northern Rockies to
justify rainfall rates that approach 1.5"/hr. The inherited Slight
Risk remains on track given the forecast remains largely unchanged,
and thus provides additional confidence in the flash flood
potential in the Bitterroots, Tetons, and Snake River Valley on
Wednesday.
Mullinax
Day 3
Valid 12Z Thu Aug 28 2025 - 12Z Fri Aug 29 2025
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS & LOWER MISSISSIPPI VALLEY...
20Z Update: Main change this period was the removal of the SLGT
risk across AL given the current trends maintaining the heaviest
QPF signature further west into MS with a more pronounced south-
southeastern push of the convective regime anticipated over the D3
time frame. The signal for significant rainfall still exists across
much of Arkansas and the threat could very well be upgraded pending
the influence of the CAMs and their more defined convective outputs
for the threat. At this juncture, there was not enough of a robust
QPF signature to warrant an upgrade over portions of the Lower
Mississippi Valley, especially when you account for the
substantially higher FFG's situated over the region. This period
will be one to monitor closely given the areal average QPF at these
leads which can infer at least the potential for a targeted
upgrade.=20
For the Central Rockies and adjacent Front Range of CO, the signal
for heavy rainfall remains consistent in the assessment of run to
run variability, so there was little reason to deviate from the
previous forecast. Decided to maintain continuity given the
consistency from previous forecast(s).=20
Kleebauer
..Previous Discussions..=20
...Central Plains & Lower Mississippi Valley...
A 500mb vorticity maximum tracking towards the Ozarks on Thursday
will work with the diffluent right-entrance region of a 250mb jet
streak to provide excellent upper-level ascent over the Lower
Mississippi Valley. Further supporting the synoptic-scale forcing
over the Southern Plains and Lower Mississippi Valley is the
GFS/ECMWF showing a healthy amount of 700mb Q-vector convergence
over the region. This is all occurring in the same area where low-
level WAA is occurring amidst WSW flow into a warm front that will
meander around the ArkLaTex on Thursday morning. This WAA will also
usher in rich Gulf moisture into the region that increases PWATs to
2.0-2.25" throughout the Southern Plains and Lower Mississippi
Valley. There is also no shortage of instability, highlighted by a
plume of 1,000-2,000 J/kg of MUCAPE for storms to work with. Given
these parameters, maximum rainfall rates could range between
2.5-3.0"/hr within the most intense thunderstorms. These storms
will also have sufficient vertical wind shear (35-45 kts) and low-
level SRH above 200 m2/s2, fostering mesocyclones within some
thunderstorms just north and east of the warm front. All these
factors support the potential for flash flooding within the Slight
Risk area, but even as far east as MS/AL and as far west as central
OK could also be at-risk for flash flooding on Thursday.
...Intermountain West...
A large pool of monsoonal moisture remains anchored over the
Rockies and Great Basin, highlighted by PWATs that are routinely
above the 90th climatological percentile in these regions. The
Northern Rockies will still have the lingering 700mb low over the
region that helps provide modest upper-level ascent to support
developing thunderstorms. Farther south, the Central Rockies are
located at the nose of a 250mb jet streak and low-level convergence
along the Front Range will foster widespread thunderstorms. Soils
will continue to saturate further in the days leading up to
Thursday over the Northern and Central Rockies that will further
exacerbate the flash flood threat. A Slight Risk is in place over
the Central Rockies that spills over into the the central High
Plains given the higher PWATs and instability in the region,
coupled with the gradually moistening soils. A potential
categorical risk upgrade may be necessary in the northern Rockies,
but it will come down to how much instability is available and what
the state of antecedent soil moisture conditions are following
Tuesday and Wednesday's rainfall.
Mullinax
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-syH69R1vEl_MWFMl9EZmCgdFKSdvuo96pG0O4VtMH4d= bewTc8bah5Pk_v9fXXFiu3-MJlGXsCUzWh_XcpaRlSz2ac8$=20
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-syH69R1vEl_MWFMl9EZmCgdFKSdvuo96pG0O4VtMH4d= bewTc8bah5Pk_v9fXXFiu3-MJlGXsCUzWh_XcpaRNTDAQQM$=20
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-syH69R1vEl_MWFMl9EZmCgdFKSdvuo96pG0O4VtMH4d= bewTc8bah5Pk_v9fXXFiu3-MJlGXsCUzWh_XcpaRa6AeQGc$=20
=3D =3D =3D
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