Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Sat Aug 29 2026
Day 1
Valid 01Z Sun Aug 30 2026 - 12Z Sun Aug 30 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 UT as well as central & southern AZ. 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. The Slight Risk in AZ
required few changes -- this is the area with the heaviest rainfall
potential through 12z. Slot/box canyons, burn scars, dry
washes/arroyos and urban areas are most at risk.
...Northern Plains...
In and near the Dakotas, the threat remains for tonight with
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. The previous Marginal Risk area near the Great Lakes
was removed despite the incoming moisture, decent 850 hPa
confluence, and instability since the signal for 1"+ was modest at
best through 12z in the 12z REFS/18z HREF probabilities.
...Southeast...
Onshore flow is expected to bring the possibility of heavy rainfall
to portions of the northeast Gulf Coast, portions of the FL Gold &
Treasure Coasts, and well as the coastal Carolinas where
precipitable water values of 2-2.5" and instability from the warm
waters offshore brings the potential of 3" in an hour where cells
can manage to merge, train, or backbuild. This would be most
problematic in urban areas, given the moderate to high flash flood
guidance values regionally. The Marginal Risk was trimmed back to
these areas. It appears the heavy rainfall risk would be widely
spaced enough for a Marginal Risk to continue to cover the heavy
rainfall threat tonight into early Sunday.
Roth
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM 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
THE INTERMOUNTAIN WEST...
...2030Z Update...
...Intermountain West...
In coordination with all the forecast offices around the Four
Corners, a large expansion of the inherited Slight Risk was made
with this update. A deep longwave trough over the Pacific Northwest
will guide several upper level disturbances northeastward along its
periphery. Anomalous moisture of 2-3 sigma above normal over much
of the area will support 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. Thus, given the storms are expected
to once again be widely scattered to scattered, much like they are
in many areas currently, the Slight Risk upgrade was made.
...Elsewhere...
A few Marginal Risks in the northern Plains, Great Lakes to Mid-
Atlantic, and along the Gulf Coast. Storms in the Plains and Great
Lakes will be ridge-riders. They'll be generally fast-moving, but
capable of heavy rain with decent probability of backbuilding and
training to warrant an isolated flash flood risk. Along the Gulf
Coast and Florida, downright tropical amounts of atmospheric
moisture will fuel numerous storms once again, with a semi-tropical
low in the northern Gulf near coastal Louisiana helping focus the
convection. In Florida, daily sea breezes appear to favor either
coast for potential slow-moving storms.
Wegman
...Previous Discussion...
...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
838 PM EDT Sat Aug 29 2026
Day 3
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...
In coordination with the various forecast offices, a large
expansion of the previous Slight Risk area to include the 4 Corners
region was made with this update. Total rainfall across this region
is expected to be far more significant on Monday than on Day
2/Sunday. An upper level shortwave, trapped at the base of the
trough over southern California on Sunday night ejects out into the
Intermountain West on Monday. This could be paralleled with a
second shortwave a bit to its east. The result will be much better
forcing especially from the upper levels on Monday as compared to
Sunday. Instability will likely be the primary limiting factor, as
prior days' storms are likely to have left decent cloud cover that
will need to be eroded to allow for maximum instability Monday
afternoon. With the fast flow aloft, this could happen to some
degree. Given both prior days' rains and the flashiness of those
basins, expect more scattered instances of flash flooding, some of
which may be considerable.
...Elsewhere...
Marginal Risks remain across portions of the Northeast, with an
expansion over the UP of Michigan and small neighboring areas, as
well as along the Gulf Coast. No major changes to either area.
Ridge riding shortwaves will still contribute to areas of heavy
rain in the Northeast which could cause localized flash flooding,
particularly in urban and low-lying areas. Along the Gulf Coast,
much heavier rains with far more abundant moisture will make for
another day of heavy rains from the Florida Peninsula back through
New Orleans. High FFGs should keep any flash flooding restricted to
urban and low-lying areas.
Wegman
...Previous Discussion...
...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
838 PM 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
SOUTHWEST & NORTHERN TIER OF THE COUNTRY...
...Southwest...
A strengthening upper level trough across portions of the West and
associated front extending across the West/Great Basin/Rockies
eventually lures precipitable water values back above average late
Tuesday night through Wednesday/Wednesday night across portions of
southern AZ, while they remain ample enough for heavy rain
concerns across portions of NM which is far enough away from the
warm core ridge drifting east from the Lower MS Valley into the
Southeast for mid-level capping to not be a significant concern.
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. The
Marginal Risk level still looks most reasonable, though the areal
extent was quite reduced across the Southwest/Great Basin both
Tuesday and Wednesday given the synoptic pattern and precipitable
water values forecast from the 12z guidance.
...Northern Plains...
There's a heavy rain signal in the guidance that is strongest across
SD/southern NM Tuesday and in the vicinity of ND on Wednesday.
Its cause is a shortwave passage on Tuesday and the attempted
emergence of a strong and possibly negatively tilted vorticity lobe
out of the Western Trough into MT Wednesday night into Thursday
morning. The 12z model QPF signal still favors the Marginal Risk
category over anything higher. Precipitable water values of
1.25-1.75" are seen at times across the region during a couple
periods of return flow from the direction of the Gulf on Tuesday
and again late Wednesday night/early Thursday, which would allow
for hourly amounts to 2" should enough cell coverage occur to allow
for brief periods of cell training and cell mergers.
...Northern Mid-Atlantic and Southern New England...
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, with
Wednesday the wetter day across the region. There's enough
dispersion in the guidance to preclude a Slight Risk on Wednesday/
Wednesday night, for now, but if the guidance becomes more
agreeable, a higher risk than Marginal would become necessary,
particularly across portions of the Megalopolis between
Philadelphia and New York City.
Roth
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM 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
SOUTHWEST & NORTHERN TIER OF THE COUNTRY...
...Southwest...
A strengthening upper level trough across portions of the West and
associated front extending across the West/Great Basin/Rockies
eventually lures precipitable water values back above average late
Tuesday night through Wednesday/Wednesday night across portions of
southern AZ, while they remain ample enough for heavy rain
concerns across portions of NM which is far enough away from the
warm core ridge drifting east from the Lower MS Valley into the
Southeast for mid-level capping to not be a significant concern.
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. The
Marginal Risk level still looks most reasonable, though the areal
extent was quite reduced across the Southwest/Great Basin both
Tuesday and Wednesday given the synoptic pattern and precipitable
water values forecast from the 12z guidance.
...Northern Plains...
There's a heavy rain signal in the guidance that is strongest across
SD/southern NM Tuesday and in the vicinity of ND on Wednesday.
Its cause is a shortwave passage on Tuesday and the attempted
emergence of a strong and possibly negatively tilted vorticity lobe
out of the Western Trough into MT Wednesday night into Thursday
morning. The 12z model QPF signal still favors the Marginal Risk
category over anything higher. Precipitable water values of
1.25-1.75" are seen at times across the region during a couple
periods of return flow from the direction of the Gulf on Tuesday
and again late Wednesday night/early Thursday, which would allow
for hourly amounts to 2" should enough cell coverage occur to allow
for brief periods of cell training and cell mergers.
...Northern Mid-Atlantic and Southern New England...
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, with
Wednesday the wetter day across the region. There's enough
dispersion in the guidance to preclude a Slight Risk on Wednesday/
Wednesday night, for now, but if the guidance becomes more
agreeable, a higher risk than Marginal would become necessary,
particularly across portions of the Megalopolis between
Philadelphia and New York City.
Roth
