979
FOUS30 KWBC 281941
QPFERD

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

Day 1
Valid 16Z Thu Aug 28 2025 - 12Z Fri Aug 29 2025

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL ROCKIES AND THE SOUTHERN PLAINS INTO THE LOWER
MISSISSIPPI VALLEY...

...Southern Plains into the Lower MS Valley...

16Z Update: Organized convection across the adjacent Southern
Plains/Lower Mississippi area continues to propagate to the south
and southeast with the main shortwave involved analyzed over
northeast OK as of 15z UA analysis. Observational trends compared
to hi-res QPF footprint indicate a general north bias in the CAMs
leading to QPF maxima off-centered from where this is occurring.
The main concern will be the models playing catch up to the actual
outcome which could have some implications on placement of
relative maxima as we step forward in time. The good news is the
current model reflections in terms of magnitude are more favorable,
so the adjustments made to the inherited risk areas were fairly
minor and more often just shifted ever so slightly to the southwest
to account for recent trends and model bias. One area of interest
will be the western flank of the convective complex as it migrates
through Central OK into the Red River basin near the TX border.
This is an area that could be sneaky when it comes to potential
flash flood concerns as the mean flow aloft runs parallel to the
quasi-stationary front bisecting the region. Low-level inflow from
surface low pressure analyzed over north TX could be enough to
focus a narrow corridor of organized convection right within the
convergent zone situated between the in-flow area and along the
front. There`s some hints of this occurring within a few of the
CAMs leading to a minor extension westward near and along the Red
River to the northeast of Dallas. Not anticipated convective
impacts as far south as the metro, but the area near Dallas will be
monitored closely to see if convective trends favor any isolated
cells getting close enough to the eastern edge of the metro to
warrant attention. The MRGL risk is in place to account for the
low-end threat.

As far as the expectation for the most favorable areas moving
forward...the pattern still denotes the expected maximum to be
located across the ArkLaTex with a secondary maxima potentially
aligned west to east across northern LA into the southern half of
MS with a small minima forecast over southeast AR and northeast LA.
Assessing sat/radar composite over the Lower Mississippi signals
some gravity wave formations across southern MS with a small MCV
feature embedded within the convection situated near Jackson. These
features could be sneaky when it comes to locally enhanced
convective cores over the course of the morning and afternoon with
some training potential aligned northwest to southeast from the
Mississippi Delta down into southeast MS. The SLGT risk from
previous forecast was adjusted a bit further southeast to account
for the trend.

As for the ArkLaTex, the threat remains relatively stable with
regards to the QPF output via CAMs signaling the greatest
convective posture to end up over the aforementioned area. The core
of highest PWATs is centered over east TX over into the Lower
Mississippi Valley with KJAN and KFWD soundings both nestled
between 2-2.1" with higher PWATs focused within the progressing
complex. As the shortwave migrates south, we see the currently
analyzed warm front stall and even start to drift back south as a
quasi- stationary front as alluded above. This will be the focal
point for heavy precip threats later this evening as the LLJ kicks
in and we see increased convergence over the area. Models are
generally in agreement with secondary convective development
initiating over the area leading to additional heavy rainfall with
rates between 1-2"/hr likely in stronger cores with upwards of
3"/hr when you account for intra-hour maximum. High regional FFG`s
will be one factor working against more widespread flash flood
potential, however the threat is certainly in the SLGT risk
threshold with a lean towards the higher end of the risk spectrum.
Greatest chance for flash flooding will likely occur from small
stream responses and more urbanized settings where run off
capabilities are maximized. The overnight period will be one to
monitor for targeted upgrades if the trends warrant it. For now,
the rates and antecedent conditions point towards a continuity of
the previous SLGT risk forecast.

Kleebauer

...Rockies and Western U.S....

16Z Update: The current forecast remains on track for locally heavy
rainfall across the CO Front Range near Pueblo as models remain
consistent in their interpretation of scattered convective
development later this afternoon generating a corridor of heavy
rainfall with totals between 2-4" possible where cell mergers
materialized. Widely scattered thunderstorms over the west will
also provide the opportunity for isolated flash flood concerns with
the main focus over WY/MT late afternoon and evening.

Kleebauer

..Previous Discussion..

A small Slight risk was maintained across portions of southeast
CO. With easterly upslope flow into the Front Range high res models
indicate convective initiation this afternoon, with cells moving
eastward into the CO Plains, before diving off to the south. Some
initial slow cell motions and the possibility of a cell merger or
two will allow for areas of locally heavy rainfall. Most models
really only indicate one or two stronger cells persisting long
enough to drive a flash flood risk...so there is a question whether
the coverage of flash flooding will reach Slight risk levels. But
given the risk is already out, and the fact that models continue to
show a focus for heavier rainfall amounts over this corridor, we
opted to maintain the Slight risk.

Elsewhere a broad Marginal risk covers much of the Intermountain
West. Not expecting as much coverage of convection or as heavy of
rainfall totals as we saw on Wednesday, but localized areas of
heavy rainfall remain possible within the Marginal risk area. The
best convective focus will likely be across portions of
southern/central MT and northeast WY, but only modest instability
should keep rainfall rates down enough to keep any flash flood
risk isolated in nature.

Chenard


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