Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Mon Aug 31 2026
Day 1
Valid 12Z Mon Aug 31 2026 - 12Z Tue Sep 01 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
UTAH...
...Desert Southwest and Four Corners...
A Moderate Risk for excessive rainfall and flash flooding was
maintained for portions of Utah for this period as widespread
convection is expected. Dual shortwaves are progged to lift
northward (one from Mexico, one from California), bringing enhanced
ascent to a region already experiencing pronounced synoptic lift
through impressive diffluence in the right rear quadrant of a
strengthening and meridionally advancing jet streak. This deep
layer lift will act upon extreme thermodynamics characterized by
PWs above 1" lifting as far north as the Utah Swell, with +2 sigma
PWs pooled across much of Arizona, Utah and western Colorado.
While there is some uncertainty as to how quickly convection will
develop due to morning CINH (owing from overnight thunderstorms
moving north from AZ), once MLCAPE does re-develop it should reach
500-1000 J/kg to support another day of widespread convective
activity.
Within the extreme thermodynamics, the robust lift will generate
thunderstorms which have a 20-30% chance of producing hourly
rainfall above 1 inch. This will be despite generally progressive
storms on 0-6km mean winds of 15-20 kts, faster over Utah and
Colorado, and it is important to distinguish that convection over
southern AZ will move much more slowly. However, bulk shear of
30-40 kts will help organize storms, and aligned propagation
vectors indicate a strong threat for repeating cells, enhancing the
rainfall risk. Impacts will be most significant over sensitive
features like dry washes, slot canyons, and burn scars, but intense
rain rates repeating anywhere on Monday could cause excessive
rainfall related impacts.
...Upper Midwest/Lower Great Lakes/Northeast...
Showers and thunderstorms are expected fire in the proximity of two
frontal boundaries that will be draped across the region during
this period. Individual storms will have the potential to produce
rain rates of 1-2 inches/hour. FFG exceedance is modest (less than
15%) so maintained the Marginal Risk are that spans from the
Minnesota Arrowhead to southern New England.
...Gulf Coast...
A mid-level closed low that has been meandering around the
northern Gulf the past few days is beginning to organize as
reflected by an NHC invest area (AL97). The latest guidance
continues to show this feature drifting westward toward the upper
Texas coast by Tuesday morning coast under the ridge centered over
the Mississippi and Tennessee Valleys. The accompanying plume of
tropical moisture noted via a large pool of PWs of 2-2.25" will
help support slow moving thunderstorms along the Gulf Coast through
Monday, although any focused heavy rain is progged to remain off
the coast at this time. A Marginal Risk area was maintained from
the upper Texas coast to the just east of the Alabama/Florida
state line.
Campbell/Weiss
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Mon Aug 31 2026
Day 2
Valid 12Z Tue Sep 01 2026 - 12Z Wed Sep 02 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE UPPER
TEXAS COAST AND FAR SOUTHWEST LOUISIANA...
...Central Gulf Coast...
Guidance varies in timing and intensity however there is consensus
for Invest AL97 to move onshore the upper Texas coast on Tuesday
morning bringing an increasing threat for heavy rainfall. Several
inches of rain may materialize given PW values greater than 2.25
inches and an increasing low level flow to intensify ascent across
this region. A Slight Risk was maintained although shifted to
reflect the latest guidance and WPC QPF placement.
...Portions of the Southwest...
The multiday, heavy rainfall event over the Southwest/West will
drastically reduce coverage and intensity during this period in
response to the mid-level trough moves onshore the California
coast. This will lead to height falls well into the Southern
Rockies and Central Plains. This not only will shift the favorable
corridor for ascent, but also push the greatest moisture anomalies
farther east and out of the Southwest. For portions of Arizona, New
Mexico and souther Colorado a modest threat will continue for this
period although cell motion will likely be more progressive in
nature thus limiting the threat for flash flooding. A Marginal Risk
remains in effect.
...Great Lakes to New England...
Multiple mid-level disturbances will track through the flow from
the Great Lakes to the Northeast and in tandem with a stationary
frontal boundary draped over the region, will bolster ascent. The
resulting lift will act upon PWs pooled south of the boundary that
will approach 2" (+1.5 sigma) to support fast moving thunderstorms
(0-6km mean winds 25-35 kts) but that will contain 1+"/hr rain
rates. The latest suite of guidance suggests multiple rounds of
convection on Tuesday due to the weak mid-level impulses, and as
bulk shear climbs to 30-40 kts, repeating rounds of convective
clusters could produce as much as 3" of rain. A Marginal Risk area
remains in effect for this region. Should models trend higher with
amounts/repeating rounds cause the heaviest rain to fall atop the
more vulnerable soils around the Tri-State area of Connecticut, New
York and New Jersey, an upgrade to a Slight risk may be needed.
...Gulf Coast and Florida...
Persistent easterly flow across the Florida Peninsula will once
again supply tropical PWs around 2.25" to the region to support
scattered afternoon convection. This setup should allow for a rapid
expansion of thunderstorms on the westward advancing sea breeze
before pinning storms along the western/Gulf side of the Peninsula
later in the aftn. While coverage is a but uncertain, a weak mid-
level impulse progged to move across the area may act to enhanced
diurnal activity, and with rainfall rates likely reaching 2-3"/hr
at times, this could produce excessive rainfall within urban areas
of the Sun coast therefore the Marginal Risk area was maintained.
Campbell/Weiss
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Mon Aug 31 2026
Day 3
Valid 12Z Wed Sep 02 2026 - 12Z Thu Sep 03 2026
...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE
NORTHEAST AND EASTERN TEXAS...
...Northern Mid Atlantic & Southern New England...
Showers and thunderstorms will focus along the frontal boundary
sprawled across the region. PW values of 1.75-2" expected to be
pooled over the region to help support heavy rain, with hourly
rates up to 2.5 inches with total accumulations nearing 5 inches
for some locations. A Slight Risk is in effect for much of
central/southeast New York, northeast Pennsylvania, northern New
Jersey, western Massachuesetts and western Connecticut.
...Northern Plains and Midwest...
Lingering MCS activity possible early in the period across the
Upper Midwest. PWAT anomalies remain elevated at times mid- week,
with values of 1.5-2" forecast, however the spread in the
deterministic QPF is higher than average. A broad Marginal Risk
spans from Minnesota/Iowa eastward to the Northeast.
...Southeast TX...
After landfall, the guidance QPF signal moves minimally, which is
concerning, as random pieces of guidance drop 6"+ per day.
Precipitable water values of 2-2.5" would certainly support hourly
amounts to 3" with additional local totals to 6". The low-level
inflow could exceed the mean wind and could also be antiparallel to
the flow aloft if the system were to become a tropical cyclone. If
enough effective bulk shear is present, local totals could end up
substantially higher than listed above, especially if the system
stalls/dawdles. A Slight Risk was raised for this period as it
overlaps a majority of the D2 QPF footprint, thus reduces the FFGs
over that part of the state. See NHC Tropical Weather Outlooks
for more information on its chances of TD/TS formation. Should it
develop into a tropical system there may be a need to further
upgrade categories with time.
...In and near FL...
Heavy rainfall possible due to a combination of onshore flow from
a warm and unstable subtropical Atlantic and Gulf and
moisture/remnant energy from what was tropical cyclone Dolly.
Coverage is expected to be spotty therefore a Marginal Risk best
highlights the level of threat for flash flooding.
Campbell/Roth
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Mon Aug 31 2026
Day 4 and Day 5
Valid 12Z Thu Sep 03 2026 - 12Z Sat Sep 05 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FRIDAY-FRIDAY NIGHT
ACROSS MUCH OF SOUTHERN ARIZONA...
...Southwest/Four Corners/Central-Southern Rockies...
After a brief respite on Day 3 (Wed-Wed night), moisture returns
Thursday and especially by Friday and into next weekend per the
guidance. The 00Z GFS was not utilized during this forecast cycle,
given the the deeper, more neutrally-tilted full-latitude trough
along the West Coast, which draws any potential tropical moisture
farther east-northeast closer to northern Baja and the Southwest.
Nevertheless, the ECMWF/CMC/UKmet show deep-layer moisture
returning by Friday and especially by the weekend, and the non-GFS
consensus QPF (including the NBM) has come up. Per collaboration
with WFO Tucson, Phoenix, and Flagstaff, have hoisted a Slight Risk
on Day 5 across much of southern AZ.
...Middle-Upper TX Coast and East-Central TX to Southwest LA...
Maintained the Marginal Risk on Day 4 (Thu-Thu night), which would
be dependent on the track and evolution of the current low pressure
system over the northern Gulf. NHC currently has a high (70%)
chance that this feature would develop into a short-lived tropical
depression or tropical storm before reaching the Upper TX or
southwestern LA Coast by Tuesday morning. However, the big question
would be how long this feature will linger as a rainfall threat
inland before dissipating. The 00Z ECMWF and especially CMC are
notably slower with the inland progression and dissipation, and as
such have maintained the Marginal Risk on Day 4, with the emphasis
of it being early in the period (i.e. Thursday morning).
...Upper Midwest/Ohio Valley/Appalachians/Parts of the Northeast...
Mid/upper level trough (initially closed H5 low) pivoting through
eastern Ontario and Quebec late this week will enhance the upper
level confluence south of this feature towards the upper ridge.
This will result in at least a couple of 90-100kt jet streaks
traversing the along/near the U.S.-Canadian border, providing the
necessary dynamical support (deep-layer ascent) for more organized
convection (potentially growing upscale into an MCS), again
tracking along the outer periphery of the flattened upper ridge,
ahead of the surface frontal boundary that will be dipping
southward Thursday through early Saturday across the OH Valley and
central Appalachians. For now, we have maintained the Marginal
Risk on Day 4 (inherited from yesterday's Day 5 ERO), while
shifting the outlook area a bit farther south on the new Day 5 ERO
(12Z Fri-12Z Sat). Deep-layer moisture anomalies ahead of the front
remain high, with with PWATs around 2 standard deviations above
normal per the 00Z GEFS and ECENS.
...Central Gulf Coast and Florida...
A combination of onshore flow from a warm and unstable subtropical
Atlantic and Gulf and Dolly's remnants moving into the region are
a recipe for heavy rainfall. However, any heavy rain signal is
spotty and not showing any real run to run model continuity at the
present time. The Marginal Risk of excessive rainfall from yesterday's
Day 5 ERO was maintained on the current Day 4 outlook, but if
Dolly ends up more organized than currently expected, a higher risk
threshold could be necessary.
...Southwest/Southern Rockies...
The combination of a nearby front at the leading edge of a strong
western trough for this time of the year and a monsoon surge up
the Gulf of CA caused by a future tropical cyclone is expected to
cause a substantial rise in the precipitable water values across
Thursday and Thursday night, ending up near 2" by Thursday morning.
Any shortwave moving through the base of this trough could cause a
convective uptick, so the real question is whether or not any
outflow cirrus from the as-yet-to-form tropical cyclone moves into
the area to inhibit daytime heating. There are more than enough
ingredients available for a Marginal Risk area, which was added in
this update. Hourly rainfall up to 2.5" would be problematic in
urban areas, burn scars, dry washes/arroyos, and box/slot canyons.
Hurley/Roth
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Mon Aug 31 2026
Day 4 and Day 5
Valid 12Z Thu Sep 03 2026 - 12Z Sat Sep 05 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FRIDAY-FRIDAY NIGHT
ACROSS MUCH OF SOUTHERN ARIZONA...
...Southwest/Four Corners/Central-Southern Rockies...
After a brief respite on Day 3 (Wed-Wed night), moisture returns
Thursday and especially by Friday and into next weekend per the
guidance. The 00Z GFS was not utilized during this forecast cycle,
given the the deeper, more neutrally-tilted full-latitude trough
along the West Coast, which draws any potential tropical moisture
farther east-northeast closer to northern Baja and the Southwest.
Nevertheless, the ECMWF/CMC/UKmet show deep-layer moisture
returning by Friday and especially by the weekend, and the non-GFS
consensus QPF (including the NBM) has come up. Per collaboration
with WFO Tucson, Phoenix, and Flagstaff, have hoisted a Slight Risk
on Day 5 across much of southern AZ.
...Middle-Upper TX Coast and East-Central TX to Southwest LA...
Maintained the Marginal Risk on Day 4 (Thu-Thu night), which would
be dependent on the track and evolution of the current low pressure
system over the northern Gulf. NHC currently has a high (70%)
chance that this feature would develop into a short-lived tropical
depression or tropical storm before reaching the Upper TX or
southwestern LA Coast by Tuesday morning. However, the big question
would be how long this feature will linger as a rainfall threat
inland before dissipating. The 00Z ECMWF and especially CMC are
notably slower with the inland progression and dissipation, and as
such have maintained the Marginal Risk on Day 4, with the emphasis
of it being early in the period (i.e. Thursday morning).
...Upper Midwest/Ohio Valley/Appalachians/Parts of the Northeast...
Mid/upper level trough (initially closed H5 low) pivoting through
eastern Ontario and Quebec late this week will enhance the upper
level confluence south of this feature towards the upper ridge.
This will result in at least a couple of 90-100kt jet streaks
traversing the along/near the U.S.-Canadian border, providing the
necessary dynamical support (deep-layer ascent) for more organized
convection (potentially growing upscale into an MCS), again
tracking along the outer periphery of the flattened upper ridge,
ahead of the surface frontal boundary that will be dipping
southward Thursday through early Saturday across the OH Valley and
central Appalachians. For now, we have maintained the Marginal
Risk on Day 4 (inherited from yesterday's Day 5 ERO), while
shifting the outlook area a bit farther south on the new Day 5 ERO
(12Z Fri-12Z Sat). Deep-layer moisture anomalies ahead of the front
remain high, with with PWATs around 2 standard deviations above
normal per the 00Z GEFS and ECENS.
...Central Gulf Coast and Florida...
A combination of onshore flow from a warm and unstable subtropical
Atlantic and Gulf and Dolly's remnants moving into the region are
a recipe for heavy rainfall. However, any heavy rain signal is
spotty and not showing any real run to run model continuity at the
present time. The Marginal Risk of excessive rainfall from yesterday's
Day 5 ERO was maintained on the current Day 4 outlook, but if
Dolly ends up more organized than currently expected, a higher risk
threshold could be necessary.
...Southwest/Southern Rockies...
The combination of a nearby front at the leading edge of a strong
western trough for this time of the year and a monsoon surge up
the Gulf of CA caused by a future tropical cyclone is expected to
cause a substantial rise in the precipitable water values across
Thursday and Thursday night, ending up near 2" by Thursday morning.
Any shortwave moving through the base of this trough could cause a
convective uptick, so the real question is whether or not any
outflow cirrus from the as-yet-to-form tropical cyclone moves into
the area to inhibit daytime heating. There are more than enough
ingredients available for a Marginal Risk area, which was added in
this update. Hourly rainfall up to 2.5" would be problematic in
urban areas, burn scars, dry washes/arroyos, and box/slot canyons.
Hurley/Roth
