FOUS30 KWBC 270801
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
401 AM EDT Wed Aug 27 2025

Day 1
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 & NORTHWEST...

...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" 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. 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. The latest 00Z HREF has started to show some higher
confidence in southeast KS in being the epicenter of the heaviest
rainfall. 24-hr QPF probabilities for >3" of rainfall are high
(70-90%) and probabilities for >5" are now 30-45%. While confidence
in locally significant rainfall amounts are increasing, the 
placement still remains lower in confidence. With the bulk of this 
event unfolding between 06-12Z Thurs, opted to hold off on a 
categorical risk upgrade tonight to see if new 12Z HREF guidance 
provides more supportive insight into the Wednesday night flash flood setup.

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.

...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. 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...

...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...

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
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