Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sat Aug 29 2026
Day 1
Valid 16Z Sat Aug 29 2026 - 12Z Sun Aug 30 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND SOUTHERN ARIZONA AS WELL AS FOR PORTIONS OF THE
INTERMOUNTAIN WEST...
...16Z Update...
...Intermountain West...
A Slight Risk upgrade was introduced with this update for portions
of the Intermountain West from central Utah through the Yellowstone
region of southwest Montana. A deep positively tilted trough over
the Pacific Northwest will swing across the Intermountain West
today. Anomalous moisture to 1 inch PWATs, or 3 sigma above normal
for this time of year over portions of the region will be drawn
northeastward with the incoming upper level shortwave. This will
support scattered shower and thunderstorm development across much
of the region this afternoon, not including the ongoing showers and
storms ongoing across northern Utah and southwest Wyoming. The
storms will be fast moving to be sure given the fast upper level
flow. However, this will be offset by likely backbuilding, so that
light to moderate rainfall persists for multiple hours in some
areas. Of course, with the many mountain ranges in the area, it's
likely some of the storms will be locally enhanced by upslope flow,
which will slow their movement locally somewhat as they get "hung
up" on the mountains. Guidance suggests that MUCAPE values across
this region will range from 500 to 1,000 J/kg. This too will
support at least widely scattered convective development.
...Arizona...
The Slight Risk area is largely the same across this region, though
the threat into metro Phoenix is high enough to include the Valley
of the Sun into the Slight Risk area, given the urban concerns
there. The heaviest rains will be in the mountains surrounding
Phoenix, but they are likely to still affect the valley in spots.
...Northern Plains and Great Lakes...
The Marginal Risk was split with this update with the ongoing MCS
expected to evolve to more of a widely scattered thunderstorm
threat over the next few hours. Much of Minnesota is behind these
storms and clear of any flash flooding threats. Into the Dakotas,
the threat is more into tonight with scattered convection with
localized areas of backbuilding posing and isolated flash flooding
threat.
...Southeast...
The entire Florida Peninsula was added to the Marginal Risk area as
numerous showers and storms are tracking west towards the East
Coast. That convection will reform over the Peninsula later this
afternoon. An urban and small stream flooding threat remains. See
MPD 1141 for more details on the near-term flooding threat.
Further north into the Carolinas. A stalled out front is supporting
nearly stationary thunderstorms mostly into eastern North
Carolina. See MPD 1140 for more details.
Wegman
...Previous Discussion...
...Southwest, Great Basin, and the Rockies...
A Slight Risk for excessive rainfall was maintained for
southern and central Arizona where HREF probabilities of locally heavy
rainfall totals greater than 2 inches, as well as 20-30% 3 hour
flash flood guidance exceedance in central Arizona and 40-60% 3
hour flash flood guidance exceedance across Tucson's CWA. An uptick
in moisture transport is expected ahead of the upper trough and
subtropical jet. Much of the area will have PW values nearing 1.5
to 1.75 inches which will help support locally heavy rainfall and
scattered areas of flash flooding.
A broad Marginal Risk remain in effect for the Great Basin and
Rockies for diurnally driven convection with above average
monsoonal moisture. This will lead to widely scattered to
scattered convection across Nevada, New Mexico, and the Colorado
Front Range. The pattern would support isolated flash flooding,
with favored terrain, urbanized areas, box/slot canyons,
arroyos/dry washes, and burn scars being the most prone to
flooding. Additionally, areas farther north in the Great Basin and
Rockies from Utah to Wyoming and Montana may see some locally heavy
rainfall as anomalous moisture streams in from the Pacific ahead
of a potent upper-level trough. Model guidance is indicating
heavier QPF totals of 1-1.5 inches across northern Utah, eastern
Idaho, and western Wyoming. Sensitive terrain would be favored for
flash flooding.
...Southeast and Gulf Coast...
A stalled frontal boundary along with diurnal heating will support
convection capable of producing rainfall totals of 2-3 inches with
a 20% chance for exceeding FFG exceedance according to the
afternoon HREF. Closer to the Gulf Coast, a combination of a moist,
tropical environment and a mid-level low /surface trough may bring
locally heavy rainfall and some flash flood concern as well,
therefore the Marginal Risk area was maintained for this period.
...Northern Plains into the Upper Midwest...
Model guidance continues to indicate rounds of storms that form
overnight in the Plains to head eastward towards Minnesota and
Wisconsin. Generally, anomalous moisture and sufficient
instability with lift from a shortwave should bring a general 1-2
inches of rain with embedded locally heavy rain rates that may
exceed flash flood guidance. Renewed convection is possible ahead
of an upper-level trough Saturday afternoon and evening across
eastern Montana and western North Dakota that may also bring a
localized flash flood threat.
Campbell
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sat Aug 29 2026
Day 2
Valid 12Z Sun Aug 30 2026 - 12Z Mon Aug 31 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
ARIZONA AND SOUTHERN UTAH...
...Southwest/Great Basin/Intermountain West...
A persistent Monsoon environment characterized by PWATS in the
1.5-1.75 inch range and CAPE around 1000-1500 J/Kg, as well as
increasing synoptic support as an upper-trough enters the
Southwest, will support increasing thunderstorm coverage and
periods of heavy rainfall from Arizona to southern Utah. During
this time portions of Arizona and southern Utah will have
increasing coverage of showers and thunderstorms and thus have a
Slight Risk for excessive rainfall in effect.
Elsewhere, a Marginal Risk of excessive rainfall remains for above
average moisture fueling diurnally driven widely scattered
convection across portions of the Rockies, and parts of the Great
Basin. The degree of moisture expected could lead to hourly rain
amounts to 2" wherever cells sit, merge, or train. This could lead
to isolated flash flooding...with favored terrain, urbanized areas,
box/slot canyons, arroyos/dry washes, and burn scars being the
most prone to flooding.
..Southeast...
During this period an upper level disturbance will be retrograding
near the central Gulf in proximity to a dying frontal boundary. With
precipitable water values of 2"+ expected, hourly rain amounts to
3" are possible where cells merge or train, thus maintaining an
elevated threat for isolated flash flooding. The Marginal Risk was
maintained across the Gulf Coast/Southeast.
...Upper Midwest/Northern Plains/Great Lakes...
Upper level ridge axis will continue to shift to the east during
this period which will increase monsoonal moisture reaching to the
Plains and Midwest. Precipitable water values are expected to be
in the 1.5-2" range. Where cells train or merge, hourly rain
amounts to 2.5" are possible. Marginal Risk areas remain in
effect.
Campbell
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sat Aug 29 2026
Day 3
Valid 12Z Mon Aug 31 2026 - 12Z Tue Sep 1 2026
...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS SOUTHERN AND
CENTRAL ARIZONA...
...Southwest/Four Corners/Eastern Great Basin/Rockies...
A surge of Pacific moisture will be directed to the Southwest that
will reach/exceed 90th climatological IVT values over the Desert
Southwest. Meanwhile, multiple days of accumulating rainfall will
likely increase soil saturation, particularly for Arizona. The
unusually strong synoptic- scale forcing, increasing IVT and
lingering 90th climatological percentile PWs, and increasingly
sensitive soils will continue to support a Slight Risk for
excessive rainfall and flash flooding for this part of the region.
A broad Marginal Risk spans from Arizona/New Mexico northward to
Utah and Colorado.
...Upper Midwest/Lower Great Lakes/Northeast...
Organized showers and thunderstorms expected thanks to upper-level
forcing for ascent supporting local enhancement. PW values above
the 90th climatological percentile will revolve around the northern
flank of the ridge and reach the Northeast by evening. The
environment will be supportive for at least localized areas of
excessive rainfall across much of New York, northern Pennsylvania,
and as far east as the I-95 corridor from northern New Jersey to
southern New England. A Marginal Risk remains in effect for
portions of the Northeast.
...Eastern Gulf Coast...
Shortwave energy moving westward underneath the closed 500 mb High
will be the focus for potentially more numerous (organized)
convective clusters during peak diurnal heating, especially when
interacting with the Atlantic and Gulf sea breezes. A Marginal Risk
remains in effect for the Gulf Coast/Florida.
Campbell/Mullinax
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sat Aug 29 2026
Day 4 and Day 5
Valid 12Z Tue Sep 01 2026 - 12Z Thu Sep 03 2026
...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE
WEST AND FROM THE NORTHERN PLAINS AND MIDWEST TO THE NORTHEAST...
...Southwest/Four Corners/Easter Great Basin/Rockies...
For Day 4 (Tue-Tue night), have pared the elongated, largely N-S
Marginal Risk area. Favorable cool-season dynamical forcing ahead
of the full-latitude trough along the West Coast (upper level
difluence/DPVA and thus deep-layer Qs vector convergence) will
favor more widespread showers and embedded thunderstorms over the
northern portions of the outlook area (ID-MT and western WY),
however the degree of deep-layer instability and thus the short-
term rainfall rates remain questionable to support anything more
than a Marginal (isolated) flash flood risk. Across the Southwest
into southern Great Basin (AZ-NM into UT), somewhat more favorable
CAPE and PWAT profiles (PWAT anomalies closer to 1 standard
deviation above normal) will make for better thermodynamic support,
however large-scale forcing southeast of the upper jet streak
traversing the Central Great Basin will be much weaker. Therefore
the risk here remains Marginal as well.
By Day 5, the Marginal Risk shrinks a bit towards the Four Corners
area, given the weakening dynamical support/flat upper ridging.
Model QPFs (including AI guidance) aren't overly impressive nor
clustered very well, even across the Southwest where the moisture
anomalies remain most impressive.
...Northern Plains and Midwest...
On Day 4, an upper level jet streak traversing far northern MN into
southern Manitoba will help fire convection across eastern portions
of the Northern Plains within the right-entrance region Tuesday
afternoon-evening. While the spread in the QPF guidance is still
quite high, kinematic and thermodynamic profiles do support some
upscale growth (potential MCS) Tuesday night farther downstream
across the Midwest as the resultant meso/convective enhanced
shortwave flattens the upper ridge within this corridor. Until we
get a better (more consistent) heavy rainfall signal from the
guidance, for now the ERO will remain at Marginal.
...Upper Mid Atlantic and Northeast...
Both Days 4 and 5 (Tuesday through Wednesday night) look to provide
multiple opportunities for at least locally heavy rainfall given
periodic transient upper jet streaks passing to the north. The
ensuing favorable right-entrance region upper forcing and resultant
convectively-enhanced shortwaves rippling eastward along the outer
perimeter of the upper ridge will provide at least a couple of
opportunities for more organized convection (one Tue and the other
later Wed-Wed night). The areal spread in the model and ensemble
mean QPFs at this point is still rather high, even on Day 5 (Wed-
Wed night) when the globals show a better signal for heavier
rainfall across Upstate NY into New England. For this reason, for
now have opted to not hoist a Slight Risk in the Day 5 ERO across
the Northeast, however suspect at some point one will be needed
somewhere within the broad Marginal Risk area, pending on the
guidance trends.
Hurley
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sat Aug 29 2026
Day 4 and Day 5
Valid 12Z Tue Sep 01 2026 - 12Z Thu Sep 03 2026
...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE
WEST AND FROM THE NORTHERN PLAINS AND MIDWEST TO THE NORTHEAST...
...Southwest/Four Corners/Easter Great Basin/Rockies...
For Day 4 (Tue-Tue night), have pared the elongated, largely N-S
Marginal Risk area. Favorable cool-season dynamical forcing ahead
of the full-latitude trough along the West Coast (upper level
difluence/DPVA and thus deep-layer Qs vector convergence) will
favor more widespread showers and embedded thunderstorms over the
northern portions of the outlook area (ID-MT and western WY),
however the degree of deep-layer instability and thus the short-
term rainfall rates remain questionable to support anything more
than a Marginal (isolated) flash flood risk. Across the Southwest
into southern Great Basin (AZ-NM into UT), somewhat more favorable
CAPE and PWAT profiles (PWAT anomalies closer to 1 standard
deviation above normal) will make for better thermodynamic support,
however large-scale forcing southeast of the upper jet streak
traversing the Central Great Basin will be much weaker. Therefore
the risk here remains Marginal as well.
By Day 5, the Marginal Risk shrinks a bit towards the Four Corners
area, given the weakening dynamical support/flat upper ridging.
Model QPFs (including AI guidance) aren't overly impressive nor
clustered very well, even across the Southwest where the moisture
anomalies remain most impressive.
...Northern Plains and Midwest...
On Day 4, an upper level jet streak traversing far northern MN into
southern Manitoba will help fire convection across eastern portions
of the Northern Plains within the right-entrance region Tuesday
afternoon-evening. While the spread in the QPF guidance is still
quite high, kinematic and thermodynamic profiles do support some
upscale growth (potential MCS) Tuesday night farther downstream
across the Midwest as the resultant meso/convective enhanced
shortwave flattens the upper ridge within this corridor. Until we
get a better (more consistent) heavy rainfall signal from the
guidance, for now the ERO will remain at Marginal.
...Upper Mid Atlantic and Northeast...
Both Days 4 and 5 (Tuesday through Wednesday night) look to provide
multiple opportunities for at least locally heavy rainfall given
periodic transient upper jet streaks passing to the north. The
ensuing favorable right-entrance region upper forcing and resultant
convectively-enhanced shortwaves rippling eastward along the outer
perimeter of the upper ridge will provide at least a couple of
opportunities for more organized convection (one Tue and the other
later Wed-Wed night). The areal spread in the model and ensemble
mean QPFs at this point is still rather high, even on Day 5 (Wed-
Wed night) when the globals show a better signal for heavier
rainfall across Upstate NY into New England. For this reason, for
now have opted to not hoist a Slight Risk in the Day 5 ERO across
the Northeast, however suspect at some point one will be needed
somewhere within the broad Marginal Risk area, pending on the
guidance trends.
Hurley
