801
FOUS30 KWBC 240106
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
906 PM EDT Sat Aug 23 2025

Day 1
Valid 01Z Sun Aug 24 2025 - 12Z Sun Aug 24 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST AND FOR PORTIONS OF THE CENTRAL/SOUTHERN ROCKIES AND
THE SIERRA NEVADA...

01Z Update...
Biggest adjustment was to reduce the footprint of the outlook areas
in the Southeast, removing much of the interior Southeast, apart
from inland southern Georgia and northern Florida.

The overall consensus of the CAMs show an area of low pressure
moving north from the Georgia coast, along the South Carolina
coast overnight before reaching the southeastern North Carolina
coast tomorrow morning. Onshore flow ahead of the low is expected
to focus deep moisture and the potential for heavy rain initially
along the South Carolina coast and then later across the Outer
Banks, and potentially some areas west of Pamlico Sound. Current
radar shows a band of moderate to heavy rainfall already beginning
to organize along the South Carolina coast. The Slight Risk was
adjusted to reflect those areas where the 18Z HREF shows high
neighborhood probabilities of 3 inches or more by 12Z and where
recent runs of the HRRR show heavy amounts spreading onshore.

As previously noted, the outlook areas were greatly reduced across
the remainder of the Southeast, except across southern Georgia and
northern Florida. A lingering boundary and corresponding deep
moisture pool will continue to fuel storms producing locally heavy
rainfall rates for a few more hours before waning.

Elsewhere, a Marginal Risk was maintained from central Alabama
northward into parts of the Tennessee Valley and the central
Appalachians.

Mostly minor adjustments were made to the areas in the West.

19Z Special Update...
A special update was sent at 19Z...reflecting the monsoon-fueled
shower and storms developing over the Sierra Nevada. A Slight Risk
was added coincident with the area of greatest concern highlighted
in WPC MPD #983. HREF neighborhood probabilities indicate that
amounts of 1-2 inches are likely, with heavier amounts possible,
within the highlighted area from 18-00Z. Refer to WPC MPD #983 for
additional information regarding the near-term heavy rain and
flooding threat across this area.

Pereira

Previous Discussion...
...Southeast into the Appalachians...

Convection that occurred on Friday has increased the amount of soil
saturation and consequently the sensitivity for flooding from
additional rainfall. With similar environmental conditions
today featuring decent CAPE, lingering mid and upper level
vorticity with a weak circulation center lingering near the
Georgia/South Carolina coastline, and elevated precipitable water
values, the threat for scattered areas of flash flooding across
the region remains for portions of Georgia and extending east to
the coast. Areal averages of 1 to 2 inches will be common with
isolated maxima upwards of 4+ inches possible near the coast.

The existing Slight Risk area was extended farther northeast along
the coast to include the greater Wilmington, NC metro area, and
the Marginal up to the Albemarle Sound given latest trends in the
CAM guidance for later today. The Slight Risk was also extended
south to include the greater Tampa Bay metro area owing to radar
trends and locally higher QPF maxima in the CAM guidance. For the
central Appalachians, the QPF has generally trended upwards
compared to previous forecasts, and therefore the existing Marginal
Risk area was extended north to include central and western WV.

...Southwestern states...

Scattered to numerous showers and thunderstorms are expected across
much of the Desert Southwest and extending eastward to include
northern NM and into much of CO. Similar to earlier forecasts, the
heaviest QPF footprint is currently situated over the Sangre de
Cristos and San Juan mountains southward into northern New Mexico
given embedded shortwave energy and orographics providing areas of
focused ascent across that region. Slot canyons, burn scar
areas, and arroyos will once again be the target of interest for
flash flood prospects with model guidance continuing to show
localized QPF maxima on the order of 1-2 inches during the time of
maximum heating that persists into the evening.

Upon further examination of the latest CAM guidance suite and HREF
flash flood guidance exceedance probs, a Slight Risk has been added
for the general vicinity of the Mogollon Rim in Arizona. Slow
moving storms forming over the terrain this afternoon and moving
over the adjacent lower elevations could raise the risk of flash
flooding more so than the surrounding areas. In addition, the
existing Marginal Risk was expanded more to the west to include the
Peninsular Range of southern California and also includes more of
the Sierra Nevada where scattered slow moving storms could also
result in some localized flooding problems.

Bann/Hamrick

Day 2
Valid 12Z Sun Aug 24 2025 - 12Z Mon Aug 25 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE SIERRA,
SOUTHERN UTAH, AND THE WESTERN HIGH PLAINS...

20Z Update: The existing outlook has been updated to include a
Slight Risk area centered over the central Sierra Nevada, with
additional moderate to heavy showers/storms over areas that are
getting noteworthy rainfall today. There is also a strong signal in
the CAM guidance for enhanced QPF maxima here. For the central High
Plains, there will likely be a strong MCS that develops over
eastern Colorado and then emerges over Kansas, with good overall
model agreement on this scenario. Some of the CAM guidance has very
heavy rainfall reaching the Texas Panhandle, but the ensemble
guidance suggests the QPF max should be over western Kansas. For
eastern North Carolina, the core of heaviest rainfall should be
just off Hatteras Island, with most of the heaviest rainfall in the
time period prior to 12Z Sunday. Therefore, the Marginal Risk area
remains valid for now, but should things trend westward, a Slight
Risk area may eventually become necessary. /Hamrick
---------------------

...Intermountain West to the Central Plains...

Maintained the Slight Risk area in parts of Utah and northern
Arizona where models still depict convection occurring over/near
vulnerable slot canyons, burn scars and normally dry washes during
the afternoon and evening hours, much like the placement and
timing of storms that occurred on Friday. The driving forces will
continue to be weak shortwave energy embedded within the
anticyclonic flow across the Intermountain region.

Father east, the risk of excessive rainfall starts to spread out
away from the Front Range of the central Rockies, and out over the
adjacent Plains. Deep layer shear is going to be enhanced as the
flow aloft is forecast to be northwesterly at this point while low
level flow from the southeast advects anomalous PWs upslope.
Spaghetti plots along with the RRFS continue to show potential for
2 to 3 inch rainfall amounts extending as far east as western
Kansas by the end of the period. Given the high PWs, the most
active convection could drop 1 to 2 inch rainfall amounts in an
hour or two.

...Appalachians to Mid-Atlantic and Pennsylvania/New York...

A deepening area of low pressure north of the border will force a
cold front to sweep eastward from the Great Lakes into the eastern
third of the country on Sunday. The airmass ahead of the front
across the central and southern Appalachian chain is forecast to
have 500 to 1000 J/kg of MLCAPE and precipitable water values
generally in the 1.5 to 1.75 inch range at the start of the day
and then decreasing by evening. The area over portions of
Pennsylvania and New York continues to have a longer window of
opportunity for locally heavy rainfall. As a result, maintained the
northern end of the Marginal risk area near a region of lower
flash flood guidance due to the potential for heavier rainfall
rates and amounts overlapping from earlier rains.

...Coastal North Carolina...

A weak surface feature in the vicinity of the coastline will focus
and support locally heavy rainfall near the coast of North
Carolina on Sunday and Sunday night. Present indications are that
the activity should remain off-shore. However...even a subtle
slowing of its forward speed or shift in track could bring moderate
to heavy rainfall to the immediate coastline of North Carolina
late Sunday.

...Southern Georgia and northern Florida...

On-shore flow of moist air interacting with the southern end of the
front associated with the low pressure/amplifying upper trough to
the north should combine for convection capable of producing heavy
rainfall Monday and Monday evening across portions of southern
Georgia into the northern Florida peninsula.

Bann/Hamrick

Day 3
Valid 12Z Mon Aug 25 2025 - 12Z Tue Aug 26 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL ROCKIES FRONT RANGE TO THE SOUTHERN PLAINS ON MONDAY...

20Z Update: The main change was to separate the existing Slight
Risk area into two separate areas as the QPF signal in the 12Z
model guidance suite has become better resolved. The first Slight
Risk is centered over the Sangre de Cristo Range and the Plains
adjacent to it, and the second area over north-central Oklahoma
into the first row of counties across southern Kansas, with a
relative minima in QPF over the Oklahoma panhandle. Elsewhere, the
Marginal Risk area was extended northwestward to include more of
northwestern California and into southern Oregon where additional
convection across areas that will have it on Sunday may lead to
some flooding concerns there. /Hamrick
-----------------

Previous Discussion: Broad cyclonic flow aloft will be in place
across the Plains on Monday while low level flow should continue to
be drawn northward at low levels across the Southern Plains, with
a boundary extending from northern Texas into northeast New Mexico
and southern Colorado helping to focus late day storms. The airmass
south of the front is expected to have PWs approaching 2 inches
across the region and values potentially in the 2 to 2.25 inch
range near the Red River. Instability does not appear impressive at
this point, but the atmosphere should support locally heavy
rainfall should convection develop anywhere between
Oklahoma/northern Texas westward into the plains of eastern
Colorado and northeast New Mexico.

The Slight Risk area was embedded within a broader Marginal Risk
area extending across much of the central and southern portion of
the Great Basin and eastward to the central and southern Rockies
Front Range. Once again...weak shortwave energy embedded within a
broad upper leads to yet another round of late day showers and
thunderstorms. Slot canyons...normally dry washes and burn scars
remain most at risk for flooding.

Bann/Hamrick


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