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FOUS30 KWBC 280825
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Thu Aug 28 2025

Day 1
Valid 12Z 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...
Convection will be ongoing at 12z this morning across portions of
southeast KS, northeast OK and into central and northern AR. Some
northwest to southeast training of convection is expected, and
scattered instances of flash flooding will probably be ongoing and
continue into the morning hours. Instability is a limiting factor,
and interesting to note that the 05z HRRR has significantly less
CAPE at 12z than the 00z HRRR and other 00z HREF members. This
downward trend in instability during the morning hours should
limit the coverage and magnitude of the flash flood risk from
eastern OK into AR...however convection should still train enough
to produce isolated to scattered flash flooding, especially over
any more susceptible areas, with amounts of 2-4" still probable.

By later this morning into the afternoon hours indications are
that convection should begin to forward propagate off to the south,
picking up speed as it pushes into and through southern AR.
Instability should be on the increase by this time resulting in
higher rainfall rate potential, but the expected quicker cell
motions should still cap the flash flood risk in the isolated to
scattered range.

Another uptick in convection is likely Thursday night into Friday
morning as low level convergence increases. More uncertainty with
the details by this time, but an area of expanding backbuilding
convection could develop over the ArkLaTex during this timeframe,
potentially driving and increased flash flood risk.

Model QPF forecasts are getting to the level of MDT risk
consideration. However do note that HREF probabilities of exceeding
3hr FFG do not get much above 25%, and probabilities of this
magnitude are not widespread. Typically we see higher HREF FFG
exceedance probabilities in MDT risk level events. The high FFG
over much of the area is likely the primary driver of these lower
probabilities, however the aforementioned lower instability in
place this morning is also likely to keep rainfall rates a bit
lower and thus less likely to exceed FFG on a more
numerous/widespread basis. Nonetheless will need to continue to
closely monitor model/observational trends today. At the moment,
the activity tonight into Friday morning over the ArkLaTex seems to
have the better chance of reaching MDT risk levels, especially if
rainfall this afternoon is able to lower FFG ahead of what should
be an uptick in backbuilding/training tonight into Friday morning.
Not enough confidence in the details to go MDT, but still think
higher end Slight risk probabilities are justified across portions
of eastern OK into western AR, far northeast TX and far northwest
LA.

...Rockies and Western U.S....
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...
The mesoscale and synoptic features responsible for the day 1
convection over the Southern Plains and Lower MS Valley will shift
southeast on Friday. More model spread is noted by this
time...with the 00z GFS remaining an outlier. The GFS continues to
be more suppressed and weaker with this feature than the other
global model guidance. The ECMWF, CMC, UKMET, ICON and NAM are all
farther northeast with the heavy rainfall axis stretching into
portions of MS and AL than the GFS is. The AIFS and both the ECMWF
and GFS initialized Graphcast are also farther northeast than the
GFS. Thus, it seems more likely than not that the GFS is too
suppressed and it represents a less likely outcome at the moment.
The ECMWF has been the most aggressive with a farther northeast
axis, but do note that is has been gradually trending southwest the
last few runs...and so the ECMWF may be a bit too far northeast.

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...
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...

...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 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