Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
901 PM EDT Fri Sep 4 2026
Day 1
Valid 01Z Sat Sep 05 2026 - 12Z Sat Sep 05 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHWEST AND THE OHIO VALLEY INTO THE CENTRAL APPALACHIANS...
01Z Update...
...Great Lakes to the Central Appalachians and Mid Atlantic...
Shortwave energy originating over the upper Midwest is expected to
support convective development, with storms propagating southeast
along an axis of deeper moisture and instability centered along the
outlook areas. Both the Marginal and Slight Risk areas were
adjusted further south, reflecting recent runs of the HRRR,
mesoanalysis, and HREF guidance. While individual cells are
generally expected to propagate steadily to the southeast, training
storms could contribute to heavier totals and a localized threat
for flash flooding. The 18Z HREF indicates totals of 1-2 inches are
likely with some potential for heavier amounts, especially from
northeastern Indiana to southeastern Ohio.
While much of the Mid Atlantic was removed from the outlook, did
maintain a narrow Marginal Risk from central Pennsylvania
southeastward to southern New Jersey and Delaware. Storms now
forming over northwestern to central Pennsylvania are expected to
further develop through the remainder of the evening, with some of
the guidance indicating heavy amounts accumulating around southern
New Jersey and northern Delaware.
...Southwest...
Trimmed the western extent of the previous outlook areas, but
maintained a Slight Risk from far southeastern Arizona into west-
central New Mexico, surrounded by a broader Marginal Risk where
storms that developed earlier in the day continue to fire. However,
expect the threat for additional heavy rain and flash flooding to
be short-lived, with storm coverage likely to decrease with the
loss of daytime heating.
...Southern Plains to the Southeast...
Maintained a Marginal Risk from portions of the Concho Valley
westward to the Texas Big Bend, where ongoing north-south
propagating storms are expected to continue for at least a couple
moisture hours. These storms will be fueled by deepening moisture,
supported by low level southeasterly flow into the region. Some
north-south training may accentuate the threat for localized heavy
amounts and isolated flash flooding.
Elsewhere, across the Southern Plains into the Southeast, the
Marginal Risk was dropped. While an axis of very deep moisture
remains, the combination of diminishing storm coverage with the
loss of daytime heating, fairly steady westerly storm motions, and
high FFGs, suggest a limited threat for flash flooding through the
remainder of the evening into the overnight.
Pereira
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
901 PM EDT Fri Sep 4 2026
Day 2
Valid 12Z Sat Sep 05 2026 - 12Z Sun Sep 06 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN NORTH
CAROLINA INTO THE VIRGINIA TIDEWATER...
...Ohio Valley, Appalachians, and the southern Mid-Atlantic...
20z Update: The synoptic weather pattern will feature a "back-
door" cold front sliding southward across the southern Mid-Atlantic
and upper-level northwesterly flow that will provide some bulk
shear for storm organization and also guide complexes of storms
through the region. At the surface, PWATS around 2 inches and CAPE
around 3000-4000 J/kg will support 1-2 inch hourly rainfall rates
that could support flash flooding.
The main change to this forecast package was to extend the Slight
Risk northward from North Carolina to include more of southern
Virginia. The expanded area matches well with 24 hr QPF
probabilities of exceeding 3 inches, as well as 15-20% 3hr and 6hr
flash flood guidance exceedance and closer to 30% in the more flood
susceptible Tidewater region. The Slight Risk across eastern North
Carolina remains on track with model guidance showing a broad area
of 2-3 inches of QPF, some locally higher, though it is worth
noting the 3 hour flash flood guidance exceedance is a rather high
3-4 inches along the coastal plain.
There was some model guidance to suggest a Slight Risk across
eastern Kentucky into West Virginia for Saturday with a heavier QPF
signature in this area. Ample instability and moisture, combined
with an approaching cold front, should spark thunderstorm
development capable of producing hourly rainfall rates of 1-2
inches a hour. After collaboration with JKL, a Marginal ERO was
maintained for both model uncertainty and dry antecedent
conditions in the area.
Wilder
..Previous Discussion..
A lower-end Slight was introduced with this update for
eastern/coastal North Carolina. A back-door cold front will make
steady progress southward down the Delmarva Peninsula today. Ahead
of the front in eastern North Carolina, multiple rounds of storms
are possible as instability maximizes in the area ahead of the
front. Sea breeze fronts are likely to be the predominant forcing
for any storms during the early to mid-afternoon. Then as the front
approaches in the late afternoon and evening, a second much more
widespread area of storms will impact the area. While soils there
are dry currently, expect a round of storms today to help saturate
the soils ahead of this Day 2/Saturday round. This nudges the area
into the low-end Slight category.
Further west into the Ohio Valley, storms along the western
extension of the front will also push southward with time. The
front will be much slower-moving to the west of the Appalachians.
Any associated storms impacting the Ohio Valley may produce
localized flash flooding in urban areas and other low-lying and
flood prone locales.
...Gulf Coast...
20z Update: The forecast remains on track for localized flooding
along much of the upper Gulf Coast and the Florida Peninsula
courtesy of a 20-40% chance of seeing 2+ inch hourly rainfall rates
in thunderstorms, according to the newest HREF guidance. The mid-
level low should continue to progress towards the western Gulf of
Mexico and enhance storm coverage across Louisiana and eastern
Texas. Sea breeze collisions and daytime heating of a tropical air
mass should be another source of localized heavier rainfall
totals, especially in South Florida.
Wilder
..Previous Discussion..
Another round of typical afternoon convection may cause localized
flash flooding in urban and flood prone areas. Once again, most
forcings for the storms will be localized, such as sea breeze or
old outflow boundaries. An abundance of moisture, with PWATs over 2
inches over a large areas will support any storms being capable of
very heavy rainfall. Generally flat terrain with sandy soils should
largely hinder any flooding to any urbanized areas.
...Southwest...
20z Update: The ingredients for flash flooding are not as
favorable across Arizona and New Mexico compared to yesterday with
slightly lesser moisture and instability available for afternoon
storms in the Southwest Monsoon. However, PWATS around an inch and
surface CAPE around 1000 J/Kg should still support chances for
hourly rainfall rates to near or exceed 1 inch per hour. These
rates still support a flash flood risk over burn scars and dry
washes. A Flood Watch has been issued by the Albuquerque WFO for
the Ruidoso area burn scars for the potential of slow moving storms
with high rainfall rates. In general, southern Arizona and New
Mexico will be favored for flash flooding.
Wilder
..Previous Discussion..
A Marginal Risk covers from southeastern Arizona across much of
New Mexico and into far southern Colorado. Storms Saturday
afternoon should largely be confined to the slopes of mountain
ranges. This should keep any resultant flash flooding localized.
Wegman
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
901 PM EDT Fri Sep 4 2026
Day 3
Valid 12Z Sun Sep 06 2026 - 12Z Mon Sep 07 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE COLORADO RIVER VALLEY FROM THE ARIZONA/NEVADA BORDER NORTHEAST
INTO CENTRAL UTAH AND ACROSS MOST OF SOUTH CAROLINA...
...West...
20z Update: Only cosmetic changes to the forecast were made based
on the latest model QPF guidance. Northern Arizona, southeast
Nevada,and southern Utah have the best chances for flash
flooding,especially in sensitive terrain such as slot canyons.
Greater moisture will be advected into the region from the
tropical Pacific and there will be greater synoptic lift as a
shortwave ejects into the region from the West Coast. Instability
does look to remain below 1000 J/Kg, which could help reduce
rainfall rates some.
Across the remainder of the West, a broad Marginal extends across
the interior Pacific Northwest, the northern Rockies, and the
northern Plains. An early fall upper-trough and plentiful moisture
will support half an inch to an inch of rainfall over burn scars
across the Inter-Mountain West that may lead to flooding. However,
instability will be minimal and will likely not support hourly
rainfall rates over an inch. The northern Rockies and Plains will
have more instability and upper-level divergence ahead of the
trough to support thunderstorms capable of producing 1-2 inch
hourly rainfall rates and some flash flood risk.
Wilder
..Previous Discussion..
Sunday looks to be a particularly active day over much of the West.
Some moisture from the large plume associated with Hurricane Marie
well out into the Pacific will be drawn into the Southwest.
Meanwhile the large longwave trough over the West will capture and
fling a few disturbances around the base and across the West. The
result will be much more widespread convection, especially over the
Slight Risk area from Las Vegas northeast over much of Utah. The
Slight Risk was expanded southwestward to include more flood prone
areas near Las Vegas, the Grand Canyon, and some of the western
National Parks in Utah, including Zion and Bryce Canyon. Slot
Canyons may be a particularly dangerous place to be on Sunday
afternoon with the more widespread convection expected. Additional
expansions and a potential inclusion of a higher end Slight are
probable with future updates given the very favorable pattern.
A slow-moving front along the Canadian border will make for a wet
go over much of northern Montana and into North Dakota, then
spreading south over much of Minnesota. A Marginal Risk remains in
place, but here too it's probable a Slight may be needed with
future updates as the convective evolution in this area becomes
more clear.
...Southeast...
20z Update: A Slight Risk was introduced in South Carolina to
include the Columbia and Charleston CWAs. Afternoon instability
will likely lead to surface CAPE around 3000-4000 J/Kg and PWATS
near 2 inches will interact with a cold front moving into the
area. Abundant, Locally heavy thunderstorms are likely to form and
may be capable of producing 1-2 inch hourly rainfall rates. This
could lead to flash flooding over the Carolina Piedmont, as well as
urban areas such as Charleston and Columbia. A Marginal remains
across the rest of the Gulf Coast and was unchanged.
Wilder
..Previous Discussion..
The Marginal Risk was expanded with this update to include some
areas where TS Edouardo just hit, another afternoon with widespread
convection over the Florida Peninsula, and some chance that
convection could spread over North Carolina's southern coast. The
area of heaviest rain could be over much of South Carolina's coast.
A Marginal was left for Charleston and adjacent areas in
coordination with the forecast office there. Better consistency
with convective behavior at the end of a front there may allow for
future upgrades, but for now given the difficulty for rain to cause
flash flooding in portions of the Low Country, the Marginal is
sufficient.
Wegman
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
901 PM EDT Fri Sep 4 2026
Day 4 and Day 5
Valid 12Z Mon Sep 07 2026 - 12Z Wed Sep 09 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHWEST INTO THE SOUTHERN GREAT BASIN, ACROSS THE UPPER LAKES
INTO THE UPPER MISSISSIPPI VALLEY AND OVER THE SOUTHEAST/NORTH
FLORIDA...
Daytime update...
With many of the models now showing tropical moisture from Marie
remaining largely offshore, PW anomalies and QPF have dropped off
notably in much the latest guidance across California into the
lower Colorado Basin on Day 5. Therefore, the Marginal Risk was
reduced to include only portions of eastern Arizona, New Mexico,
and southern Colorado with the daytime update.
Elsewhere, made only minor adjustments to the previous forecast.
Pereira
Previous discussion...
...Southwest, Southern-Central California, Southern Great Basin...
A mid to upper level trof in the mean will persist across the
western U.S days 4 and 5. A lead closed low will be ejecting
northeastward from the northern Great Basin into the Northern
Plains, while additional height falls reform the mid to upper level
trof along the West coast. The net effect will be to maintain an
axis of much above average PW values from Southern CA into the
Southwest day 4 and expanding northward into Central CA and the
Southern Great Basin by day 5 in response to the re- amplifying
West coast upper trof. A marginal risk is maintained day 4 over
eastern AZ into western to central NM/far southern CO in the model
consensus qpf axis. Scattered convection across this region will
support locally heavy rains and isolated runoff issues. With the
expansion of the anomalous PW farther to the north and west day 5
into Central CA and the Southern Great Basin, the marginal risk was
also expanded from day 4, stretching from southwest CO, western NM
into all of AZ, far southern NV and southeast CA. Increasingly
favorable right entrance region jet dynamics on the east side of
the re-amplifying upper trof will support locally heavy rains in
the widespread scattered convection, supporting isolated runoff
issues.
...Northern Plains into the Upper Mississippi Valley/Upper Lakes...
The above mentioned northeast ejection of the western trof from
the Northern Great Basin/Northern Rockies into the Northern Plains
at the beginning of day 4 will support potential for organized
convection along and to the northeast of the northwest to southeast
oriented frontal zone across the Northern Plains. Strengthening
low level southerly flow into this front in an axis of anomalous PW
values, 2-3+ standard deviations above the mean, expected across
this region, supporting potential for heavy rains and isolated
runoff issues day 4. By day 5, the associated surface frontal
boundary is expected to become more southwest to northeast
oriented along the southern edge of the westerlies. At this time
the low level inflow into this front is also expected to become
more parallel to the boundary, supporting potential for a period
of training along the front Tuesday night into early Wednesday.
...Southeast/North Florida...
Widespread scattered convection likely day 4 in the vicinity of
the slowly southward moving frontal boundary across the
Southeast/North Florida. Low confidence in qpf details in the
vicinity of this front, but with PW values 1.75-2.00", locally
heavy rains and isolated runoff issues possible.
Oravec
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
901 PM EDT Fri Sep 4 2026
Day 4 and Day 5
Valid 12Z Mon Sep 07 2026 - 12Z Wed Sep 09 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHWEST INTO THE SOUTHERN GREAT BASIN, ACROSS THE UPPER LAKES
INTO THE UPPER MISSISSIPPI VALLEY AND OVER THE SOUTHEAST/NORTH
FLORIDA...
Daytime update...
With many of the models now showing tropical moisture from Marie
remaining largely offshore, PW anomalies and QPF have dropped off
notably in much the latest guidance across California into the
lower Colorado Basin on Day 5. Therefore, the Marginal Risk was
reduced to include only portions of eastern Arizona, New Mexico,
and southern Colorado with the daytime update.
Elsewhere, made only minor adjustments to the previous forecast.
Pereira
Previous discussion...
...Southwest, Southern-Central California, Southern Great Basin...
A mid to upper level trof in the mean will persist across the
western U.S days 4 and 5. A lead closed low will be ejecting
northeastward from the northern Great Basin into the Northern
Plains, while additional height falls reform the mid to upper level
trof along the West coast. The net effect will be to maintain an
axis of much above average PW values from Southern CA into the
Southwest day 4 and expanding northward into Central CA and the
Southern Great Basin by day 5 in response to the re- amplifying
West coast upper trof. A marginal risk is maintained day 4 over
eastern AZ into western to central NM/far southern CO in the model
consensus qpf axis. Scattered convection across this region will
support locally heavy rains and isolated runoff issues. With the
expansion of the anomalous PW farther to the north and west day 5
into Central CA and the Southern Great Basin, the marginal risk was
also expanded from day 4, stretching from southwest CO, western NM
into all of AZ, far southern NV and southeast CA. Increasingly
favorable right entrance region jet dynamics on the east side of
the re-amplifying upper trof will support locally heavy rains in
the widespread scattered convection, supporting isolated runoff
issues.
...Northern Plains into the Upper Mississippi Valley/Upper Lakes...
The above mentioned northeast ejection of the western trof from
the Northern Great Basin/Northern Rockies into the Northern Plains
at the beginning of day 4 will support potential for organized
convection along and to the northeast of the northwest to southeast
oriented frontal zone across the Northern Plains. Strengthening
low level southerly flow into this front in an axis of anomalous PW
values, 2-3+ standard deviations above the mean, expected across
this region, supporting potential for heavy rains and isolated
runoff issues day 4. By day 5, the associated surface frontal
boundary is expected to become more southwest to northeast
oriented along the southern edge of the westerlies. At this time
the low level inflow into this front is also expected to become
more parallel to the boundary, supporting potential for a period
of training along the front Tuesday night into early Wednesday.
...Southeast/North Florida...
Widespread scattered convection likely day 4 in the vicinity of
the slowly southward moving frontal boundary across the
Southeast/North Florida. Low confidence in qpf details in the
vicinity of this front, but with PW values 1.75-2.00", locally
heavy rains and isolated runoff issues possible.
Oravec
