320
FOUS30 KWBC 271958
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
358 PM EDT Wed Aug 27 2025

Day 1
Valid 16Z Wed Aug 27 2025 - 12Z Thu Aug 28 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER SOUTHEAST
KANSAS, FAR SOUTHWEST MISSOURI, AND FAR NORTHEAST OKLAHOMA...

16Z Update: A Moderate Risk of Excessive Rainfall was introduced
this forecast cycle for southeastern KS towards the very edges of
both southwest MO and northeast OK. Higher end SLGT risk is
forecast for the Upper Snake River across eastern ID into far
southwest MT. More on these setups below...

...Central Plains...

A sheared 500mb vort max exiting the Central Rockies will spawn a
surface low over the southern High Plains today. Working in tandem
with a 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" across much of
southeastern KS into the adjacent borders of MO/OK between the
00-12z time frame. With MUCAPE ranging between 500-1,000 J/kg and
the incoming anomalous moisture advection regime, these parameters
would support up to 2"/hr rainfall rates with some intra-hour
encroaching 3" within storms that develop more prolific meso-
cyclones. 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. 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.

There has been a notable uptick within the 12z HREF prob fields
that brought attention to a growing concern for more locally
significant flash flood and areal flood concerns during the time
frame of interest (00-12z Thu). Neighborhood probabilities for
totals rainfall >3" are now above 80% over a large area
encompassing much of southeast KS, including near the Salina and
Wichita urban corridors. There`s even a bullseye of 90+% location
to the northeast of Wichita, a testament to a strong signal for
repeated rainfall in proxy the maximized 850mb FGEN evolution. The
most prominent signal is now incorporated into the higher end
precip accums with the same neighborhood prob field indicating
between 50-70% for >5" within that aforementioned corridor
referenced above, as we as upwards of 25-30% for locally 8+" in the
area from I-35 to the northeast of Wichita down to the MO/AR/OK/KS
intersection. HREF EAS fields are also very robust for even the 2"
(50-80%) and 3" (25-40%) signals, a strong correlation between the
various CAMs input in the hi-res ensemble. Individual deterministic
output indicates some 5-7" totals within each member, but a few
more robust outputs within the ARW/Nam Nest windows indicate
upwards of 9-10" across that area north and east of Wichita. The
metro corridor is right on the edge of the heaviest when assessing
the prob fields, so it will be prudent to monitor near term trends
in guidance and general observations to determine if a higher risk
is necessary for the population center. For now, a MDT risk is now
in place within that region extending from Salina down to the
quad-state border with an inclusion of both far northeast OK and
southwestern MO.

Meanwhile the Slight Risk area in the Central High Plains and along
the Front Range of the Rockies remains in place. Many locations now
sport overly saturated soils, as shown by the NASA SPoRT-LIS 0-40cm
soil percentiles which are above the 90th percentile. PWATs remain
above the 90th climatological percentile and anywhere from
500-1,000 J/kg of MUCAPE will provide sufficient instability to
support up to 1.5"/hr rainfall rates along the Front Range, and
perhaps as high as 2"/hr in the CO High Plains. The Slight Risk
remains in place due to the combination of anomalous moisture and
modest instability triggering storms over areas that feature
sensitive soils.

Mullinax/Kleebauer

...The West...

16Z Update: The previous SLGT risk was relatively unchanged with
the signal still prominent within the latest 12z CAMs suite for
locally heavy rainfall up to 2-3" in spots. 10 year ARI exceedance
probabilities are now 80+% across the Upper Snake River basin that
includes those locations north of PIH towards the ID/MT state line.
Environmental conditions are ripe for locally enhanced rainfall
over those more sensitive locations situated across the area in
question. KBOI sounding this morning was a resounding 1.53",
putting it 5th all time and smashing the daily record by more than
0.3". These elements favor a higher than normal threat for flash
flood concerns leading to a more higher end SLGT risk wording
necessary for areas of the Snake River Basin over into the Sawtooth
Range.

Kleebauer

..Previous Discussion..

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. Sufficient instability (500-1,000 J/kg
MUCAPE) is also present 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 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 today and into this evening.

Mullinax


Day 2
Valid 12Z Thu Aug 28 2025 - 12Z Fri Aug 29 2025

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS, CENTRAL ROCKIES, & LOWER MISSISSIPPI VALLEY...

20Z Update: The main adjustments from this period were an extension
to the northwest for the SLGT risk across the Lower Mississippi
Valley and cutting out the area of northeast NM within the previous
SLGT to relegate the risk to now only CO. The situation over the
Lower Mississippi is semi-fluid with the main area of interest for
higher impacts comprised of the Ozarks across northwest AR into
far southeast KS as the evolution from D1 spills into D2 with the
shortwave progression migrating southeast through the Central
Plains. Strong mid-level ascent will continue over the course of
Thursday with the best heavy rain threat correlating with a
continued sfc-850mb FGEN pattern coupled with the large scale
ascent provided by the shortwave approach and upper jet streak
dynamics. 850mb LLJ signature will linger through the early morning
time frame, but begin mixing out after 15z leading to a less robust
frontogenetic regime which will limit rainfall intensity and
further development for a time. The break will allow for a plateau
in the threat with some degrading precip posture as we move into
the afternoon hrs across AR.

Warm frontal positioning near the ArkLaTex will aid in a secondary
convective pattern initiating within peak diurnal destabilization
causing a heavy convective signature over the ArkLaTex. This signal
has become very well captured within the entirety of the 12z CAMs
suite with varying intensities of rainfall anticipated. Considering
the frontal alignment and anomalous moisture pooling along the
warm front, the stronger correlated signals of rainfall between
2-4" have more merit than the diffuse signature on one of the CAMs.
This allowed for a general continuity of the SLGT as FFG`s in the
region remain elevated given the drier spell leading in. Heavy rain
threat will initiate back over Central AR by the evening with
large scale forcing and weak LLJ return aiding in redevelopment
over the area. Downstream propagation into MS will materialize
overnight into early Friday leading to scattered flash flood
concerns considering some modest training potential just north of
the slow advancing cold front. The pattern seems pretty locked in
when assessing ensemble/deterministic variability, or the lack
there of leading to only the minor adjustment over the northwest
edge of the SLGT to include some overlap of the D1 MDT across
southeast KS and neighboring areas.

Across the Southern High Plains, the signal is less emphatic for
heavy rain over northeast NM when you assess the probability fields
as the convective posture further north is deemed more likely to
produce something appreciable compared to the High Plains of NM.
The signature for >1" was relatively minor compared to what was
being indicated further north towards Pueblo. In coordination with
the ABQ forecast office, the SLGT was trimmed back to a MRGL given
the trends. The SLGT over CO was generally maintained with only
some modifications closer to the leeward side of the Sangre de
Cristos.

Kleebauer

..Previous Discussions..

...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 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 MLCAPE 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. Aside
from minor tweaks based off latest WPC QPF trends, no significant
adjustments were made this forecast cycle.

...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. Given the lingering
spread in guidance regarding the placement of the strongest
thunderstorms, a Slight Risk was not issued as of this forecast
cycle. Should confidence increase in both heavier totals and
placement, and should antecedent soil moisture conditions continue
to grow more sensitive following Wednesday`s rainfall, a
categorical risk upgrade may be necessary.

Mullinax


Day 3
Valid 12Z Fri Aug 29 2025 - 12Z Sat Aug 30 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A LARGE
PORTION OF THE GREAT PLAINS, THE FRONT RANGE OF THE ROCKIES, AND
ACROSS MUCH OF THE DEEP SOUTH...

20Z Update: The D3 time frame can be described as "chaotic" within
the grand scheme of the CONUS pattern and the 12z numerical suite
offered no help in discerning potential maxima placement across the
South and Central/Southern High Plains. Individual deterministic
detailed more of the variability of the pattern with maxima for the
Southeastern U.S. situated anywhere from SC to TX with the ensemble
means relatively muted thanks to the spread. High Plains from the
CO Front Range down to the TX Panhandle was a little better
defined, but still lacked consistency in the expected magnitude of
rainfall in the vicinity. Overall, the threats exist in a vacuum,
but nailing down the exact placement of the maxima is closer to
"throwing darts at a dart board" as much of the setup in the
Southern U.S. is contingent on the convective evolution the period
prior. For now, maintained the continuity in the broad MRGL
situated over the Southeast extending back up into the Northern
Rockies. As we move forward in time, an upgrade or two is certainly
plausible, most notably over the High Plains and/or Southeast
CONUS.

Kleebauer

..Previous Discussion..

A pool of anomalous atmospheric moisture will maintain a
stronghold over the Northern Rockies and Plains on south through
the Southern Plains. A series of 500mb disturbances exiting the
Great Basin will provide some upper-level support for thunderstorms
from the northern Rockies and Plains on south to the Central
Rockies Front Range and Plains. Farther south, a nearby frontal
boundary in the southern High Plains will be the primary trigger
for thunderstorms. This area will have a greater concentration of
PWATs to work with (1.3-1.5" in the TX/OK Panhandles into eastern
NM) and greater MLCAPE values (topping 1,000 J/kg). These areas
will have seen soils dry up following a couple days of drier
weather, and given some global guidance members differ on the
location of the storms, there was no categorical risk upgrade this
forecast cycle.

Farther east, a wave of low pressure will track east along a
frontal boundary that stretches from the Front Range of the
Rockies to Florida. Numerous showers and thunderstorms are likely
to envelope the South, with the Lower Mississippi Valley favored to
see the heaviest rainfall at this time. Guidance shows the Lower
Mississippi Valley sporting the highest PWATs (2.0-2.25") and
instability (MLCAPE 1,000-2,000 J/kg) while also being more
favorably located closer to the emerging area of low pressure in
eastern TX. The GEFS/GEPS camps favor the Lower Mississippi Valley,
but many ECMWF members were still showing storms that could
produce locally heavy rainfall as far east as eastern GA. Given the
lingering spread in model guidance, and the Lower Mississippi
Valley`s currently higher 1-3hr FFG exceedance values, there was no
categorical risk upgrade hoisted as of this forecast cycle. But
the >2.0" PWATs and modest instability does support a localized
flash flood threat across much of the South during a time that also
just so happens to be the kickoff to Labor Day weekend. Flash
flooding could result in some locally hazardous travel conditions,
especially along roads that drain poorly.

Mullinax


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt