FOUS30 KWBC 300822
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Sun Aug 30 2026

Day 1
Valid 12Z Sun Aug 30 2026 - 12Z Mon Aug 31 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR
PORTIONS OF UTAH & ARIZONA...

...The West...

A Slight Risk of excessive rainfall continues in and near central
& northern Utah as well as central & southern Arizona for this
period. A deep positively tilted trough swinging across
Intermountain West today has lured anomalous moisture into the
region. Scattered shower and thunderstorms have formed and there
have been signs of cell training and occasional backbuilding. The
18z HREF and 12z REFS guidance suggest that the highest potential
of heavy rainfall beyond 01z is where the Slight Risk remains.
Although cell coverage should reduce from here on out, convection
could bubble overnight into early Sunday morning as shortwaves
interact with residual MU CAPE, which in spots remains 2000+ J/kg.
Slot/box canyons, burn scars, dry washes/arroyos and urban areas
are most at risk.

...Northern Plains into the Northeast...

Shortwave energy tracking through the northern tier will trigger
scattered convection with localized areas of backbuilding posing
and isolated flash flooding threat near an area of 850 hPa
confluence and precipitable water values of 1.5-2". There is a
decent signal for 2-3" of rainfall, which could fall quickly near
the central border of the Dakotas, as quickly as within a couple of
hours where cells merge or manage to train. Localized Slight Risk
impacts cannot be ruled out should this heavy rainfall impact urban
areas. A Marginal Risk area spans from northeast Montana to
southern portions of the Northeast.

...Southeast...

Heavy rainfall possible across the Gulf Coast, Florida and for
the coastal Carolinas with the onshore. PW values across the region
are in the 2-2.5" range and instability from the warm waters
offshore brings the potential of 3" in an hour where cells can
manage to merge, train, or backbuild. Accumulations and/or rainfall
rates of this amount would be most problematic in urban areas,
given the moderate to high flash flood guidance values regionally.
The Marginal Risk area remains in effect.

Campbell/Roth
 

Day 2
Valid 12Z Mon Aug 31 2026 - 12Z Tue Sep 01 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE INTERMOUNTAIN WEST...

...Intermountain West...

A broad Slight Risk for excessive rainfall and elevated threat for
flash flooding remains in effect from Arizona northward to Utah
and Colorado. During this period a series of upper level
disturbances will be traveling through the deep longwave trough
over the Pacific Northwest. Meanwhile Pacific moisture will be
pooled over the region along a steady influx leading to anomalous
moisture of 2-3 sigma above normal. This environment will be
supportive of scattered showers and thunderstorms developing with
the monsoon. Much of this area is very prone to flash flooding
should heavy enough rains occur, which is likely given the history
of afternoon showers and storms in this same region under the same
pattern. The degree of moisture expected could lead to hourly rain
amounts to 2" wherever cells sit, merge, or train especially near
favored terrain, urbanized areas, box/slot canyons, arroyos/dry
washes, and burn scars being the most prone to flooding.

...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 from the
Minnesota Arrowhead to portions of the Northeast.

...Eastern Gulf Coast...

The Marginal Risk for excessive rainfall remains in effect. 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.

Campbell/Mullinax/Wegman
 

Day 3
Valid 12Z Tue Sep 01 2026 - 12Z Wed Sep 02 2026

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE
SOUTHWEST & NORTHERN TIER OF THE COUNTRY...

...Southwest...

Above average PW values expected to return to portions of Arizona
as the upper level trough strengthens over the West during this
period. Hourly rain amounts up to 2" are possible given the
moisture and instability expected, which would be most problematic
in burn scars, slot/box canyons, dry washes/arroyos, and urban
areas. Therefore a Marginal Risk was maintained for portions of
Arizona, New Mexico, West Texas and southeast Colorado.

...Northern Plains, Southern New England and northern Mid-
Atlantic...

A shortwave passing within the flow will trigger scattered
convection across the northern tier states capable of producing
hourly rates up to 2 inches/hour should cell training and mergers
occur. Through this period PW values will be near 1.25-1.75 inches
as Gulf moisture advects northward within the low-level flow.

A moist west to west-northwest low-level flow imports precipitable
water values of 1.5-2" across the region on the northeast side of a
warm core ridge as upper level disturbances near a front lead to
heavy rainfall concerns. Hourly rain amounts to 2.5" with local
totals to 5" are possible given the ingredients expected. A
Marginal Risk spans from eastern Montana/Wyoming to southern New
England. There may be a need for an upgrade in the future particularly
across portions of the Megalopolis between Philadelphia and New
York City should guidance trend higher.

Campbell/Roth
 
$$
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