FOUS30 KWBC 010040
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
840 PM EDT Mon Aug 31 2026
Day 1
Valid 01Z Tue Sep 01 2026 - 12Z Tue Sep 01 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
FOUR CORNERS REGION...
...01Z Update...
In coordination with SLC/Salt Lake City, VEF/Las Vegas, NV, and
FGZ/Flagstaff, AZ forecast offices, the Moderate Risk was dropped
down to a Slight with this update. As is typical with the monsoon,
even on these very active days, with waning daylight, so too does
much of the convection. That said, there remains enough convection
around the 4 Corners region to retain the Slight for the rest of
the night, though the focus is really in the next couple hours.
Higher-end Slights remain for most of the Slight Risk area.
In coordination with the Gulf Coast offices, the Marginal Risk for
portions of the upper Texas Coast and Louisiana was removed with
this update. Newly upgraded T.S. Edouard is a tiny storm, and the
entirety of the rain shield remains offshore of central Louisiana.
While associated showers and a few storms may brush the southwest
Louisiana coast overnight, associated impactful rainfall is
unlikely to begin affecting the coast until the center moves closer
to the TX/LA border tomorrow. When adding the high FFG values due
to recent dry weather in that area, even some heavy rainfall
overnight is unlikely to result in any flash flooding.
The Marginal Risk in South Florida has also been removed as
remaining showers and storms are quickly dissipating.
Along the Canadian border, the Marginal was left largely intact as
much of the mostly light rainfall is expected overnight.
Wegman
...Previous Discussion...
...Desert Southwest and Four Corners...
16Z Update...The MDT risk has been expanded southward into northern
Arizona, including the Grand Canyon region, due to new model
signals for heavy rainfall. Many of the high-res CAMs indicate a
focus for convective development across northern AZ today, both due
to terrain influence but also likely in response to differential
heating boundaries developing around the cloud shield associated
with ongoing convection. In general storms across the Four
Corners/Southwest will be relatively fast moving today on 20-25 kts
on mean flow (except during initiation/organization), but
impressive shear, extremely anomalous moisture, and aligned
propagation vectors suggests repeating cells with 1+"/hr hourly
rainfall is likely. After recent heavy rain and significant flash
flooding, the region is extremely vulnerable so any additional
heavy rain today will likely lead to flash flood impacts, which
locally could be significant.
Previous Discussion:
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. Once
MLCAPE redevelops in the morning it should quickly 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.
...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, supported by PWs pooled south of
the fronts of 1.75-2.00 inches overlapping MUCAPE above 1000 J/kg.
While these rain rates could be sufficient to overwhelm FFG,
exceedance probabilities remain low and scattered at 10-15%, likely
due to the fast progression expected of any cells today. Some areas
may see repeating rounds which could still create local flood
impacts, but the MRGL risk remains, and was adjusted only slightly
for the new guidance.
...Central 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 NHC initiating advisories on Potential Tropical
Cyclone Five. The latest guidance continues to show this feature
drifting westward beneath the mid-level ridge, approaching the
Upper Texas Coast by Tuesday morning. The guidance is very well
clustered both spatially and temporally, and while the heaviest
rain is likely on D2, the accompanying plume of tropical PWs
reaching above 2.25" will support heavy rainfall in slow moving
thunderstorms along the Central Gulf Coast today. Although focused
heavy rain is progged to remain just off the coast, the MRGL risk
was maintained and adjusted to account for local exceedance
potential due to rainfall rates reaching 2-3"/hr.
...Florida Peninsula...
A MRGL risk was re-introduced for southern Florida after
coordination with the impacted WFOs. A plume of tropical moisture
remains in place across the area reflected by PWs of 2.25" on the
12Z UA sounding out of MFL. Although the TUTT to the east has
weakened, a weak impulse moving across the area from east to west
will help drive convective development this aftn, with additional
storms firing along the sea breeze/gulf breeze and residual
outflows. The deeply saturated column with thick warm cloud depths
will support 2-3"/hr rain rates, which could overwhelm soils,
especially in urban areas. For this reason the MRGL risk was added
once again where the HREF and REFS probabilities for 3-5" of rain
are highest.
Weiss
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...
Tropical Depression Five will be approaching the Upper Texas Coast
to start the period /12Z Tuesday/ and move onshore very early
during the day. While there is still some uncertainty in the
timing, placement, and intensity of this system, the guidance is
actually quite well clustered for this time range, with the system
moving onshore very close to Galveston Bay before lifting slowly NW
through Tuesday and Tuesday night.
There has been a subtle shift W/SW with today's 12Z guidance suite,
and the QPF/ERO has been updated to reflect this adjustment. The
system is expected to remain compact as it organizes and comes
onshore, leading to a narrow corridor of heavy rainfall thanks to
PWs of over 2.25" in the accompanying tropical moisture plume.
While the heaviest rain may occur along the coast and just NE of
the center/track, there is potential for some inflow convergence to
maintain a corridor of heavy rain even behind the system as it
lifts into east-central Texas. Additionally, the forecast movement
(using latest NHC forecast positions) is progged to only be around
the 20th percentile of Gulf Coast systems, suggesting the
potential for locally excessive rainfall where training is most
pronounced and prolonged.
Both the HREF and REFS ensemble suites indicate a high chance
70%) of at least 5" in a NW oriented stripe from Galveston Bay
through near College Station, with 8"/24hr probabilities exceeding
40% near the coast. This has prompted a small shift in the SLGT
risk and internal 25% areas, but after coordination with WFO HGX,
opted to hold off on any MDT risk upgrade - both due to uncertainty
and concern about the size of the system being too small to support
MDT risk impacts. This will continue to be monitored as the storm
evolves, but either way an active day with scattered to numerous
flash floods is probable at TD5 moves onshore Tuesday.
...Four Corners and Southwest...
After several days of heavy rain and substantial impacts across
the Four Corners and Southwest, activity is likely to wane
considerable Tuesday. However, there will continue to be sufficient
forcing and moisture for another day of local impacts, especially
atop the vulnerable soils impacted the last few days, and a MRGL
risk has been expanded to cover much of the Four Corners.
Although PWs do come down, the drier air which has been progged to
push east has slowed down, leaving PW anomalies of +0.5 to +1
sigma in place on Tuesday. With significant forcing remaining in
place as a trough approaches from the west (leading to height
falls and jet level diffluence for synoptic ascent), this will
result in another day of at least scattered thunderstorm
development during peak heating, or within any enhanced lift
beneath weak impulses embedded within the flow. Mean 0-6km winds
will remain progressive at around 25 kts, but aligned propagation
vectors and continued elevated bulk shear magnitudes above 40 kts
will support organization into clusters and potentially repeating
cells. The guidance has become more aggressive with coverage
Tuesday, including the UA WRF hourly rainfall plots, and it is
possible that hourly rainfall of 0.5 to 0.75 inches will occur.
Should this rain fall atop sensitive terrain features or recently
saturated soils, additional instances of flash flooding could
occur, and because of that the MRGL risk was expanded.
...Great Lakes to the Northeast...
Another day of mid-level impulses tracking atop the ridge and
along a wavering frontal boundary will produce rounds of scattered
convection from the Great Lakes through New England and the
northern Mid-Atlantic states. As is typical with this convective
cluster setups, there is a lot of spread in both timing and
placement of the heaviest rain signatures from the available CAMs.
Some of this is due to the inherent uncertainty in timing of the
impulses, but some of it is also due to expected activity on D1
which will help reinforce boundaries and areas of greatest
convergence into D2. While the guidance is in good agreement that
many areas will receive periods of heavy rainfall exceeding 1"/hr
as storms track steadily WNW to ESE, the duration of any of this
rain may be modest due to the progressive nature of cells.
However, short duration training is likely at times thanks to
aligned propagation vectors/mean winds to the front, which could
enhance rainfall amounts locally.
In general, rainfall should be 1-2" which will only be sufficient
to create isolated impacts related to flash flooding, but if any
significant training/repeating could occur, locally higher rainfall
is possible. This is reflected by both HREF and REFS probabilities
for 3" that peak above 25%, especially across far southern New
England, Long Island, and northern PA/southern NY. However, due to
uncertainty in timing and placement, combined with generally
elevated FFG outside of the Catskills and Tri-State area, the MRGL
risk was maintained and no SLGT risk was introduced at this time.
...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. Mean flow across the Peninsula will
be very weak on Tuesday (0-6km mean winds just around 5-10 kts),
which suggests cells will drift west from the sea breeze, but may
converge just inland from the Gulf as the gulf breeze also pushes
inland. With Corfidi vectors becoming increasingly anti-parallel to
the mean flow, some short term training is possible (limited in
duration by the pulse nature of the cells) but with rainfall rates
expected to be 2-3"/hr, this could create isolated flash flood
impacts, especially within urban areas.
Weiss
Day 3
Valid 12Z Wed Sep 02 2026 - 12Z Thu Sep 03 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
NORTHEAST AND EASTERN TEXAS...
...Northern Mid Atlantic & Southern New England...
Small adjustments were made to the inherited SLGT risk to account
for the latest guidance. While the setup Wednesday is at least
synoptically similar to both Monday and Tuesday, a more impressive
shortwave tracking atop the region to interact with a slowly
northeast advancing warm front will result in scattered to
widespread convection. There is still some uncertainty as to the
placement of the heaviest rainfall, but an improving warm sector
with favorable thermodynamics along a sharp gradient should support
training of clusters of thunderstorms with 1-2"/hr rain rates. The
guidance focuses this heaviest rainfall across portions of Upstate
New York, and while soil moisture is generally below normal
according to NASA SPoRT, this should support up to 3" of rain in
some areas leading to scattered instances of flash flooding, with
more isolated responses elsewhere.
...Eastern Texas and Southwest Louisiana...
Tropical Depression 5 will be well inland by the start of the
forecast period, but with slow movement expected to continue,
additional heavy rainfall is likely even as it begins to dissipate.
PWs of 2-2.25" will support continued heavy rain, and as 850mb
inflow only slowly weakens it will drive enhanced convergence to
support 2-3"/hr rain rates. While most of the rain should be around
the center of the system which will be lifting slowly northward
towards north Texas, there is potential for a locally enhanced
inflow band on its south side which could prolong and enhance the
rain across areas that receive heavy rain on Tuesday as well. The
small size of the storm should limit the footprint and potential
impacts, but ECENS/GEFS probabilities still indicate that at least
an additional 3" of rain may fall, with locally higher amounts,
leading to instances of flash flooding. Some further refinements in
the SLGT risk area are expected with future updates, both to
capture the latest guidance track, but also better resolve where
any additional rainfall will occur on top of heavy rain from
Tuesday.
...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 Great Lakes.
...Florida...
Another day where the tropical moisture plume remains entrenched
across Florida, reflected by PW anomalies of +1.5 sigma stretching
across most of the state. The biggest change from prior days, at
least as modeled in guidance, is weaker overall flow leading to
slower storm motions, and a better chance for both coasts to
experience slow moving storms with 2+"/hr rain rates, as well as
across the inland portions of the peninsula where storm
mergers/collisions can occur. After coordination, the inherited
MRGL risk was expanded to cover the entire Peninsula south of
Jacksonville, with the greatest risk for impacts occurring atop
urban areas.
Weiss/Campbell
Day 4 and Day 5
Valid 12Z Thu Sep 03 2026 - 12Z Sat Sep 05 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL THURSDAY AND FRIDAY
ACROSS PORTIONS OF ARIZONA...
...2030Z Update...
The signal for the return of heavy rain into portions of Arizona
with the monsoon remains strong on both Thursday and Friday. As
before, the signal on Thursday remains lower than on Friday, but
enough moisture, forcing, and instability move into southern
Arizona from off the Gulf of California to allow for scattered
thunderstorms to develop once again Thursday afternoon. Despite the
"break" of the prior days, soils may remain a bit more saturated
than normal by Thursday afternoon, and the area around I-19 remains
highly susceptible to flooding caused by heavy rains. Thus, have
opted to upgrade to a Slight from Tucson south.
Moisture off the Gulf of California will continue to spread
northward across southern Arizona on Friday, while a nearly
stationary positively tilted trough remains parked over the
Pacific Northwest. This will allow for some upper level energy,
combined with the thermal trough, to allow storms to blossom over
much of the area. The Slight Risk area was expanded north and west
a bit up the Mogollon Rim on Friday.
Marginal Risk areas were added in portions of the Northwest,
particularly the area surrounding the northern Idaho Panhandle.
This area will be much closer to the upper level trough, so forcing
should be much better than in the Southwest. However, moisture and
instability will be much lower, resulting in more likelihood of
stratiform rainfall. Despite this, the rainfall should still have
modest amounts of instability to work with in a generally dry, and
therefore flash flood prone, area of the country.
Wegman
...Previous Discussion...
...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
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt__;!!DZ3fjg!_eVEFqt7aAGayL1BFrBWuLF7LgTW4qiduRForbmxNPrZ8_Lyx0R9zKXxrFZxpaTYI59LHuPRDjb3wXBJtWufsvvSa3c$
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt__;!!DZ3fjg!_eVEFqt7aAGayL1BFrBWuLF7LgTW4qiduRForbmxNPrZ8_Lyx0R9zKXxrFZxpaTYI59LHuPRDjb3wXBJtWuf38Hygtw$
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt__;!!DZ3fjg!_eVEFqt7aAGayL1BFrBWuLF7LgTW4qiduRForbmxNPrZ8_Lyx0R9zKXxrFZxpaTYI59LHuPRDjb3wXBJtWufG0MX0HU$
$$
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