FOUS30 KWBC 290946
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
546 AM EDT Sat Aug 29 2026

Day 1
Valid 12Z Sat Aug 29 2026 - 12Z Sun Aug 30 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND SOUTHERN ARIZONA...

...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 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 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
 
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