FOUS30 KWBC 260802
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Tue Aug 26 2025

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...

...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...

...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
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