724
FOUS30 KWBC 290059
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Thu Aug 28 2025

Day 1
Valid 01Z Fri Aug 29 2025 - 12Z Fri Aug 29 2025

...THERE IS A MARGINAL RISKS OF EXCESSIVE RAINFALL OVER MUCH OF
THE ROCKIES AS WELL AS FOR PORTIONS OF THE LOWER MISSISSIPPI
VALLEY...

...01Z Update...

...Colorado...

The Slight Risk area was downgraded to a Marginal with this update.
Shower and thunderstorm activity across the region has
underperformed prior forecasts, and the coverage and intensity of
storms have not nor are likely to be great enough to cause Slight
Risk levels of impacts. The Marginal Risk for the area remains in
effect for potential isolated instances of flash flooding.

Elsewhere in the West, the Marginal Risk was trimmed out of Arizona
and much of New Mexico with this update due to lack of convection.
Waning amounts of daylight from here should keep any storms capable
of producing flash flooding to isolated coverage. Further north,
the Marginal was left roughly the same, following ongoing radar
trends suggesting an isolated flash flooding threat remains into
Wyoming, portions of Idaho, and Montana.

...Lower Mississippi Valley...

The ongoing MCS across the area has become mostly a line of storms,
albeit slower moving ones across Texas, but otherwise is both
progressive and is not aligning so as to make training convection a
large concern. With the nocturnal strengthening of the low level
jet, it`s likely there will be some redevelopment of storms across
the Marginal Risk area, centered over northern Louisiana. However,
the latest guidance is in poor agreement as to the coverage of
those storms, or whether they will align in such a way as to
promote flash flooding via training. Soils are quite dry from
eastern Texas across northern Louisiana and into much of
Mississippi, which has raised flash flood guidance values
considerably, to near the highest level that flash flood guidance
goes. Thus, expect there would need to be numerous training
thunderstorms to result in Slight Risk level impacts. The Marginal
risk remaining largely covers an isolated instance of flash
flooding or two, which are most likely in any urban areas, such as
Shreveport or Alexandria.

Further west, the Marginal Risk for the Dallas-Fort Worth Metroplex
is largely covering ongoing convection in the area, with the threat
there likely ending in the next couple hours or so.

Wegman


Day 2
Valid 12Z Fri Aug 29 2025 - 12Z Sat Aug 30 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER EASTERN NEW
MEXICO INTO THE TEXAS PANHANDLE, AND OVER PORTIONS OF THE LOWER
MISSISSIPPI VALLEY AND SOUTHEAST...

...Lower MS Valley into the Southeast...

20Z Update: Models seem to be taking a more favorable step into the
realm of consistency with the axis of heaviest precip now centered
over the Lower Mississippi Valley through the Mississippi Pine Belt
and Southern Alabama. This is the general orientation of heavy
precip when assessing both CAMs and global model output with the
former indicating a better signal for precip >3" in any given
location, something missing from the global input. Considering the
environmental factors and deep layer ascent, there will surely be
areas that see >3" of rainfall during the forecast setup with
modest neighborhood probabilities for >5" (20-35%) situated across
the aforementioned areas. Higher FFG`s will curb the top end of any
threat as the recent dry pattern has relegated all 1/3/6 indices to
run very high, a state that usually leaves significant flash flood
potential muted historically. This pattern is trending to be no
different with the greatest threat likely to occur in more
urbanized zones and where there`s overlap in heavier precip
occurring D1 and D2 leading to a more cumulative evolution which
allows more favorability for flooding to occur. The SLGT risk was
adjusted a touch further south to account for trends in a northern
bias of short term QPF distribution, and to align better with the
higher neighborhood probs for >5".

Kleebauer

..Previous Discussion..

Given the above model diag, it seems prudent to go ahead with a
Slight risk across portions of the region. The environmental
ingredients will be conducive to heavy rainfall, with PWs over 2"
and a corridor of persistent lower level moisture
transport/convergence supporting some backbuilding and/or training
potential. Other than the weaker GFS, models show an area of low
pressure only slowly moving eastward along a low level frontal
axis...with upwards of 2000 j/kg of CAPE near and south of this
boundary. Thus it does seem like an excessive rainfall threat
could evolve Friday into Friday night with either a persistent
swath of convection or multiple convective rounds. The Slight risk
area was drawn from portions of central/northern LA into
central/southern MS and southwest AL...as this corridor is
generally the consensus axis of heaviest rainfall potential from
the non-GFS solutions. The high FFG keep this as just a lower end
Slight risk for now, and I would expect to see some adjusting of
the risk area as we get closer and models better converge on a
solution.

...Rockies into the High Plains...

20Z Update: A general expansion of the southern and eastern edge of
the SLGT risk across the Southern High Plains was made to account
for the expected forward propagation of an MCS that will
materialize from the northern TX Panhandle before dropping south
along the defined theta_E ridge in place across the TX High Plains.
Neighborhood probs for >2" are now upwards of 50-80% for the period
with the maxima centered just west of I-27 between Amarillo and
Lubbock. Antecedent dry conditions will help alleviate some of the
potential initially, but rates between 1-2"/hr for multiple hrs.
during time period of impact should be enough to induce isolated
to scattered flash flood potential in more urban settings, and in
more flash flood prone sections of West TX where low-water
crossings exist.

Heavy rain threat also exists over the Northern Plains with the
maxima likely centered into west-central SD. Rates will be the
deterrent for a SLGT risk initially as the threat is more long
fused in the nature of convective impact meaning a generally slower
accumulation threat and less of a flash signal by definition. As a
result, felt it was okay to maintain continuity of the MRGL risk
with still a chance at a SLGT risk if the rates forecast tick up
within hi-res guidance.

Kleebauer

..Previous Discussion..

A Slight risk was also added for eastern NM into the TX Panhandle.
We should see an uptick in both CAPE and PW compared to Thursday,
and a shortwave feature and upper jet on the northern periphery of
the upper level ridging will help initiate convection Friday
afternoon. It also looks like some mid/upper level moisture
streaming well north and east of Juliette in the Eastern Pacific
will move into these areas Friday, likely helping increase rainfall
efficiency. High res models indicate convection initiating over
eastern NM near higher terrain Friday afternoon, before propagating
off to the east. Cell motions look rather quick off to the east,
which may end up limiting the extent/magnitude of the flash flood
risk. However it looks like enough of a surge in instability and
PWs to suggest high rainfall rates, and we could be looking at
enough convective coverage to support some upscale convective
cluster development. So there seems to be enough there to justify
going with a Slight risk over these areas.

Elsewhere, areas of convection across portions of NE into SD and
MT will pose a localized flash flood risk. Convective coverage and
intensity here also looks higher than Thursday, and can not rule
out eventually needing another targeted slight risk area, but for
now even with the greater convective coverage, any FFG exceedance
looks isolated in nature.

Chenard


Day 3
Valid 12Z Sat Aug 30 2025 - 12Z Sun Aug 31 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN NEW
MEXICO INTO WEST TEXAS...

20Z Update: SLGT risk across the Southern High Plains was
maintained with minor adjustments on the southern flank of the risk
based on QPF trends from the latest national blend and bias
corrected QPF. Sufficient boundary layer buoyancy coupled with
elevated PWATs and proxy to a robust theta_E ridge in place across
West TX will be more than enough to attain a widespread convective
signal with eventual upscale growth over the TX Caprock. Current
setup would lead to a cumulative precip threat for an areal average
of 2-4" with locally higher in the D2-3 periods, a signal more
coincident with an escalation of flash flood prospects for areas
that see a multiple round impact from organized convective
patterns. Given the favorable environment forecast and the
increasingly aggressive signal for heavy rain overlap across the
TX Panhandle, the SLGT risk will be deemed more on the upper end of
the threshold and will be assessed in the coming days for a
potential targeted upgrade.

Guidance across the Southeast U.S. remains a bit uncertain in
terms of where the heaviest precip potential will occur for
Saturday with the threat seemingly pushing closer to the Gulf coast
through southern GA as we step through the model trends over the
past 48 hrs. ML output is favoring the Central Gulf coast and
southern GA with a relative min in QPF across LA. If this were to
occur, the prospects for flash flooding would become very localized
with the best threat in urban areas given such high FFG indices in
place across the MS/AL Gulf coast into the FL Panhandle. There`s
not enough of a prominent signature to deviate from what is
currently forecast, so the main adjustment was to trim the northern
extent of the MRGL where a distinct northern bias is present in
global deterministic with a better signature closer to the
instability maxima situated along the immediate Gulf coast. A
future upgrade in proximity to the coastal plain is plausible if
the signal becomes more robust in its presentation, especially in
range of the CAMs.

Kleebauer

..Previous Discussions..

...Rockies into the Plains...

Mid/upper level riding will gradually breakdown by Saturday over
the Plains resulting in increased troughing and embedded shortwave
energy. This should drive rather widespread convection from TX/NM
all the way into the Dakotas. A Slight risk was maintained from
eastern NM into the TX Panhandle and west central TX where the
ingredients appear better for an excessive rainfall threat. This
area will see an uptick in instability compared to Friday, with
PWs also trending slightly higher and well within the range of
supporting heavy rainfall rates. The right entrance region of an
upper jet and an increase in low level moisture transport Saturday
evening support an organized convective threat. Convective details
this far out are uncertain, but the overall pattern supports
terrain driven convection over NM growing upscale as it moves
eastward into western TX resulting in an isolated to scattered
flash flood threat.

...Southeast...

More uncertainty exists over the Southeast by Saturday. As was
described in the day 2 discussion, still think the 00z GFS is an
outlier and lower probability solution. Most other models indicate
a lingering frontal boundary with a surface low and enhanced
moisture transport. Thus it seems more likely than not that at
least localized areas of heavy rainfall will remain possible from
the Gulf Coast and possibly into portions of GA and SC. The
northern extent of this risk is more uncertain, and quite possible
the 00z ECMWF (and other recent ECMWF runs) is too far north and
east. But somewhere from the central Gulf Coast into north FL and
southern GA still looks in play for heavy rainfall Saturday into
Saturday night. The overall environmental ingredients would support
Slight risk level rainfall...however the combination of high FFG
and above average uncertainty suggests sticking with a broad
Marginal risk is the best course of action for now.

Chenard


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