Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
828 PM EDT Sun Aug 30 2026
Day 1
Valid 01Z Mon Aug 31 2026 - 12Z Mon Aug 31 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
ARIZONA & THE FOUR CORNERS...
...The West...
Activity across UT, CO, NM, and the Central High Plains could hold
on for a few more hours before CIN develops and shuts off
convection for the night. AZ is another matter, as shortwaves aloft
near the base of the Western upper level trough could allow
activity to continue through the overnight hours as it feeds off of
2000 J/kg of MU CAPE. The Marginal and Slight Risk areas were pared
back on the northwest sides per radar reflectivity trends and the
18z HREF/12z REFS guidance.
...Northern Plains into the Great Lakes...
A line of convection has developed across portions of the Dakotas,
with an elevated band of showers across the Upper Midwest which
doesn't appear to be well modeled near an instability gradient
near the IA/IL/WI border. Other warm advection thunderstorm
activity is expected overnight ahead of the progressive convective
band across northern MN, portions of WI, and the Great Lakes near
another instability gradient. Since the signal for heavy rainfall
isn't exceptionally strong, a Marginal Risk continues to make
sense. Changes to continuity were made per radar reflectivity
trends and the 18z HREF/12z REFS guidance.
...Southeast Louisiana...
A low south of Louisiana is regaining central convection near a
residual MCV caused by early organized convection under an upper
level high. It lies over an area of warm sea surface temperatures,
with ML CAPE of 1500-2500 J/kg. The expectation is for late night
and early morning convection within an onshore flow regime on the
northeast side of the burgeoning warm core system to attempt to be
efficient. Precipitable water values of ~2.25" along with
antiparallel flow between the low-level flow and the flow aloft
leads to low- level inflow exceeding the mean wind, which would
lead to precipitation efficiency as the effective bulk shear tries
to rise/flirt with 25 kts. Short, training bands are the concern,
but areal coverage and coastal marsh/wetlands should keep the flash
flood risk restricted to urban areas so kept the risk as Marginal.
Included New Orleans proper as a course of least regret.
Roth
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
828 PM EDT Sun Aug 30 2026
Day 2
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...
After coordination with WFO SLC, an upgrade to a MDT risk has been
issued for portions of Utah.
Convection is expected to once again be widespread on Monday as the
region remains entrenched between an approaching trough from the
west and an anomalous ridge over the Mississippi Valley. Within
this pinched flow, 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 RRQ 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 AZ/UT and western CO. 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 UT/CO, 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. With this
activity occurring atop a naturally vulnerable area, aided by
anticipated heavy rain on D1/Sunday and noting impacts from heavy
rain that occurred Saturday, adjustments to the inherited risk
areas were significant and coordinated. This included the MDT for
Utah, an expansion of the internal 25% area surrounding the MDT,
and a secondary 25% area over far southern AZ. 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...
Complex dual-frontal structure will provide the impetus for
convective development Monday as weak impulses crest the ridge and
dig E/SE along these boundaries. Mean 0-6km winds of 20-25 kts
suggest convection will be progressive to the E/SE, but ample
30-40 kts of bulk shear will support organization into clusters to
enhance rain rates to 1-2"/hr (thanks to elevated PWs) offsetting
at least somewhat the faster motion of these storms. FFG exceedance
is modest (less than 15%) so the inherited MRGL risk remains with
only cosmetic adjustments.
...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). This system will drift
westward beneath the anomalous ridge over the MS/TN VLY, and
approach the Upper Texas coast by the end of the forecast period
/Tuesday morning/. 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. However, the overlap of elevated MUCAPE/PW will support heavy
rain within convection that occurs along the Gulf breeze, with
some enhancement possible due to convergence north of AL97. The
MRGL risk was maintained and modified for updated guidance.
Weiss
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
828 PM EDT Sun Aug 30 2026
Day 3
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...
Invest AL97 will be drifting westward along the Central Gulf Coast
Tuesday morning, with the guidance suggesting it will move onshore
the Upper Texas Coast early Tuesday morning. Despite the
uncertainty into both timing and intensity, there is an
increasing threat for heavy rainfall as this system approaches the
coast and then moves onshore. With PWs expected to be above 2.25",
and 850mb inflow increasing onshore to intensify ascent, it is
becoming more likely that several inches of rainfall will occur
across this region on Tuesday. At this point it is still too early
to discuss specific amounts and locations, but the SLGT risk was
coordinated with WFO HGX and WFO LCH to highlight the most likely
area for at least 3" of rain, with some deterministic guidance
suggesting much higher amounts are possible.
...Portions of the Southwest...
The extremely active period across the Southwest wanes drastically
on Tuesday as the mid-level trough axis pushes onto the CA coast
driving 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. There will continue to be a modest threat
for generally fast moving cells from far southeast AZ through NM
and into CO, but in general the risk is much lower Tuesday and will
be confined to any heavier rain rates that move across vulnerable
soils/terrain or regions saturated from prior rainfall. The
inherited MRGL risk was maintained and cosmetically adjusted for a
continued isolated threat.
...Great Lakes to New England...
Pinched flow around the periphery of a ridge centered over the TN
VLY will serve as a corridor through which mid-level impulses will
track WNW to ESE from the Great Lakes through the Northeast. At the
same time, a stationary front will waver within this pinched flow,
upon which these shortwaves will act to enhance 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 guidance suggests that there may be 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. At this
point the MRGL risk remains, but should any repeating rounds cause
the heaviest rain to fall atop the more vulnerable soils around the
Tri-State area of CT/NY/NJ, an upgrade to a SLGT risk may be
needed.
...Gulf Coast of the Florida Peninsula...
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 Tuesday, 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, prompting the addition of this MRGL
risk area.
Weiss
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
828 PM EDT Sun Aug 30 2026
Day 4 and Day 5
Valid 12Z Wed Sep 02 2026 - 12Z Fri Sep 04 2026
...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE
NORTHERN MID-ATLANTIC STATES WEDNESDAY/WEDNESDAY NIGHT...
...Northern Mid Atlantic & Southern New England...
The pattern will remain quite unsettled Wednesday and Thursday, as
numerous showers and thunderstorms ahead of mesoscale/convectively
enhanced shortwaves move across the area on the northern side of a
warm core ridge. The activity should wind down by Friday as the
surface front pushes south of the area. Thermodynamics will be
favorable, especially considering the precipitable water values of
1.75-2" available for portions of Wednesday and Thursday. The
guidance heavy rain signal shows rather good agreement for
Wednesday/Wednesday night considering the forecast range, so added
a Slight Risk to portions of NY, PA, & CT. Hourly rain amounts to
2.5" with local totals to 5" are possible in this environment. As
the wet ECMWF trended down this cycle, removed the Marginal Risk
area from much of ME Thursday into Thursday night, which was
coordinated with CAR/the Caribou ME forecast office.
...Northern Plains and Midwest...
Lingering MCS activity in possible early on Day 4 across the Upper
Midwest, though it is unclear if it would lie close to the IA/MN
border or across southern MI, based on the 12z guidance. Warm air
advection should be occurring in earnest Thursday into Thursday
night, allowing precipitable water values and a low-level focus to
waver across the region mid-week. 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. Until we
get a stronger and more consistent heavy rainfall signal from the
guidance, the excessive rainfall category will remain at Marginal
both days.
...Southeast TX...
A convective low in the Gulf of Mexico is expected to move into
southeast TX early to mid-week. 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. Marginal Risk
areas have been added to southeast TX for both days 4-5 to cover
this system's heavy rainfall prospects, and later shifts can
determine if we need to step up to a Slight Risk or higher, which
is a real possibility/concern. It should be wet in its vicinity
whether or not it develops into a tropical cyclone. See NHC
Tropical Weather Outlooks for more information on its chances of
TD/TS formation.
...In and near FL...
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. Marginal Risks of excessive rainfall have been drawn for both
days, 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.
Roth
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
828 PM EDT Sun Aug 30 2026
Day 4 and Day 5
Valid 12Z Wed Sep 02 2026 - 12Z Fri Sep 04 2026
...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PARTS OF THE
NORTHERN MID-ATLANTIC STATES WEDNESDAY/WEDNESDAY NIGHT...
...Northern Mid Atlantic & Southern New England...
The pattern will remain quite unsettled Wednesday and Thursday, as
numerous showers and thunderstorms ahead of mesoscale/convectively
enhanced shortwaves move across the area on the northern side of a
warm core ridge. The activity should wind down by Friday as the
surface front pushes south of the area. Thermodynamics will be
favorable, especially considering the precipitable water values of
1.75-2" available for portions of Wednesday and Thursday. The
guidance heavy rain signal shows rather good agreement for
Wednesday/Wednesday night considering the forecast range, so added
a Slight Risk to portions of NY, PA, & CT. Hourly rain amounts to
2.5" with local totals to 5" are possible in this environment. As
the wet ECMWF trended down this cycle, removed the Marginal Risk
area from much of ME Thursday into Thursday night, which was
coordinated with CAR/the Caribou ME forecast office.
...Northern Plains and Midwest...
Lingering MCS activity in possible early on Day 4 across the Upper
Midwest, though it is unclear if it would lie close to the IA/MN
border or across southern MI, based on the 12z guidance. Warm air
advection should be occurring in earnest Thursday into Thursday
night, allowing precipitable water values and a low-level focus to
waver across the region mid-week. 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. Until we
get a stronger and more consistent heavy rainfall signal from the
guidance, the excessive rainfall category will remain at Marginal
both days.
...Southeast TX...
A convective low in the Gulf of Mexico is expected to move into
southeast TX early to mid-week. 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. Marginal Risk
areas have been added to southeast TX for both days 4-5 to cover
this system's heavy rainfall prospects, and later shifts can
determine if we need to step up to a Slight Risk or higher, which
is a real possibility/concern. It should be wet in its vicinity
whether or not it develops into a tropical cyclone. See NHC
Tropical Weather Outlooks for more information on its chances of
TD/TS formation.
...In and near FL...
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. Marginal Risks of excessive rainfall have been drawn for both
days, 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.
Roth
