734
FOUS30 KWBC 031937
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
337 PM EDT Wed Sep 3 2025

Day 1
Valid 16Z Wed Sep 03 2025 - 12Z Thu Sep 04 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN ARIZONA...

...Southwest...

16Z Update: 12z CAMs have reasonable continuity from the previous
forecast leading to similar probs based off the latest 12z HREF.
Highest threat will be over Santa Cruz and Pima counties with the
northern edge brushing southern Pinal and Maricopa. 12z KPSR
sounding came in with a 1.47" PWAT, enough for a 90th percentile
climo with higher PWATs forecasted south of Phoenix. This
environment is plenty reason for any cell maturation to produce
locally 1-2"/hr rates that could spell problems for any of the
desert locations south of I-10. For this reason, there were only
some minor adjustments overall to the inherited SLGT risk with a
touch further north extension to match trends in QPF from the
latest CAMs.

Kleebauer

..Previous Discussion..

A broad area of instability and above normal moisture will bring a
localized flash flood risk to much of the southwestern U.S. today.
HREF guidance indicates the best convective coverage will be
across southern AZ where neighborhood probabilities of 2"+ amounts
are 30-50%. This generally looks to be where the better
instability is forecast today, and also where the heaviest rainfall
occurred yesterday. Thus maintaining the Slight risk across this
areas seems reasonable.

Chenard

...South FL...

16Z Update: Little changes necessary to the previous MRGL risk
forecast as the expected convective evolution remains on track from
the overnight issuance. Primary areas of concern are within the
urban centers from PBI down through MIA as the sea breeze
positioning should correlate with heavy convective development
within the confines of the urban setting along the southeastern FL
coast. Heavy convection will also develop over the Everglades with
some cells migrating over I-75, but the threat in these locations
is certainly on the lower end due to the high rates necessary for
flash flood prospects. HREF blended mean QPF shows spotty 2-3"
totals correlating with a 30-50% neighborhood prob for >5" in any
location across the Southern FL Peninsula from Naples to much of
Dade county. This signal coincides with a solid MRGL threat,
bordering on SLGT risk, but the threat sporadic in heavy precip
placement in that corridor leaned towards maintaining continuity,
especially with soil moisture anomalies holding near average or
even slightly below. For this case, maintained general continuity
as a result, but will monitor near term trends closely for the
urban corridor.

Kleebauer

..Previous Discussion..

A wet pattern will persist over south FL through Friday with
localized flash flooding possible each day. The overall pattern
does not change too much, with a stationary front in the low
levels, an upper jet to the north providing some right entrance
region ascent, and broad mid level troughing. It seems likely that
rounds of convection will occur each day, and with PWs around 2.3",
heavy rainfall rates are likely. There is some uncertainty with
the axis of heaviest rainfall each day, with some models favoring
the Keys, and given the location of the stationary front, this may
make sense. However, it still seems likely that heavier convection
will impact the urban corridor of southeast FL at times as well.
Uncertainty regarding both the axis and also the speed of
convection, whether it tends to propagate eastward quickly or can
hang up, dictates keeping the risk at Marginal for now. Although
could certainly see the need for a targeted Slight risk along the
southeast FL urban corridor at some point over the next 3 days
given the potential for multiple rounds of heavy rainfall.

Chenard

...Central Plains to Great Lakes...

Trough axis swinging through the northern and central CONUS will
open the door for an axis of progressive convective outputs with
the heaviest focused over the Great Lakes and Central Plains.
Vorticity maxima analyzed over the Northern Plains will rotate
southeast over the course of the period providing focused ascent
within a small diffluent axis as it loses latitude. Best prospects
will be over eastern KS into neighboring MO with some progressive
convective activity likely to develop along the front once it
reaches latitude south of I-70. Relatively high probs for >1" exist
in this area, but there`s a notable drop in probs for higher precip
total thresholds. This is likely due to the progressive nature of
the precip, but some inference of training is hinted within a few
of the CAMs across eastern KS meaning perhaps an isolated flash
flood concern could arise. The threat is lower than the MRGL risk
threshold, but it`s still non-zero overall, so wanted to make
mention to note the potential, albeit small.

Kleebauer


Day 2
Valid 12Z Thu Sep 04 2025 - 12Z Fri Sep 05 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST CALIFORNIA AND CENTRAL AND SOUTHERN ARIZONA...

...Southwest...

20Z Update: Guidance continues to depict a very active period
beginning Thursday afternoon, carrying through the first half of
the evening with convective development across southern AZ
precluding to a large outflow moving westward out of the southern
deserts through the south-central and western portions of AZ. The
signal is clear within all CAMs leading to a strong probability
assessment for 3 and 6hr FFG exceedance intervals, each carrying
50+% over a large area extending from Tuscon all the way to the
Colorado River basin between the 18-06z time frame in the period.
Rates of 1-2"/hr will be plausible in this setup as PWATs remain
~2 deviations above normal, a signal favoring locally heavy
rainfall and heightened flash flood concerns over flashy
soil/terrain features. This beginning to shift closer to a higher
end SLGT risk scenario, but the mesoscale evolution of the pattern
leans towards generally progressive, so this would be a detriment
for an upgrade potential. For now, this is sufficient for a SLGT
risk forecast and will monitor the setup closely for any changes in
the spatial coverage of the risk, and the eventual magnitude of
impacts that could necessitate an upgrade.

Kleebauer

..Previous Discussion..

Thursday is looking like a more active day across portions of AZ
into southeast CA as the area begins to feel the impacts of a
stronger moisture surge ahead of Lorena. An uptick in moisture and
low level flow should help drive a more organized convective
threat. Rather impressive instability for this part of the country
is forecast, with values upwards of 2000-3000 J/KG, and this
combination of above average PWs and instability should support
intense rainfall rates. Always worry about cloud cover impacting
instability, which could happen in spots, but the overall model
signal certainly favors areas of robust instability. In a part of
the country where model QPF struggles, CAPE and PW overlap often
ends up being a better flash flood potential indicator. Given the
ingredients in place, we went ahead and upgraded to a Slight risk
for much of central and southern AZ into southeast CA. Additional
expansion into more of southeast CA are possible going forward.

Chenard

...South FL...

20Z Update: Little change was necessary from the previous forecast
as the repeating pattern of scattered heavy convection across
South FL will continue through D2 with the best prospects for flash
flooding likely tied to more urban zones. Current indications are
for areas of southwest FL and the Lower Keys will have the greatest
potential, albeit those areas are tougher to flood due to the sandy
soil composition. The MRGL was sufficient for the forecast.

Kleebauer

..Previous Discussion..

A wet pattern will persist over south FL through Friday with
localized flash flooding possible each day. The overall pattern
does not change too much, with a stationary front in the low
levels, an upper jet to the north providing some right entrance
region ascent, and broad mid level troughing. It seems likely that
rounds of convection will occur each day, and with PWs around 2.3",
heavy rainfall rates are likely. There is some uncertainty with
the axis of heaviest rainfall each day, with some models favoring
the Keys, and given the location of the stationary front this may
make sense. However it still seems likely that heavier convection
will impact the urban corridor of southeast FL at times as well.
Uncertainty regarding both the axis and also the speed of
convection, whether it tends to propagate eastward quickly or can
hang up, dictates keeping the risk at Marginal for now. Although
could certainly see the need for a targeted Slight risk along the
southeast FL urban corridor at some point over the next 3 days
given the potential for multiple rounds of heavy rainfall.

Chenard


Day 3
Valid 12Z Fri Sep 05 2025 - 12Z Sat Sep 06 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST
ARIZONA AND SOUTHWEST NEW MEXICO....

...Southwest...

20Z Update: The latest 12z guidance continues to lend uncertainty
in the exact track of the circulation from Lorena as the storm
makes approach into the CA Baja before sliding into Sonora. Some of
the model guidance remains particularly aggressive with holding the
center of circulation to the International border, just south of
southeastern AZ leading to multiple solutions (GFS/RRFS/NAM/HAFS
suite/HWRF) indicating a swath of heavy rainfall impacting at least
portions of southeast AZ into southwest NM. ECMWF/CMC and mean
ensemble output remains not as aggressive as the previous models
referenced putting a little more emphasis on the low degrading
prior to reaching northern Sonora leading to the remnant moisture
field not advecting further north.

Assessment of the guidance compared to reality is important in
this scenario as major discrepancies early on would very well
inhibit model outputs through the course of the run. As of now, the
cluster with the greater outputs over AZ/NM are handling the
system a bit better which trends more in favor of the wetter
solutions as we move forward in time. Ensemble bias corrected QPF
was also particularly wet aiding to a little better consensus on
what could transpire across the high deserts near the border.
Ensemble totals between 1.5-3" with locally higher are being
forecast in those more substantial outputs, a range that would
cause some flash flood concerns, especially when you add in the
more effective warm rain processes being depicted when looking at
forecast soundings over parts of the aforementioned area. PWAT
anomalies closer to 3 deviations above normal would be common in
the current expected evolution, plentiful for more locally
significant impacts. Considering the increasing concern and
marginally favorable consensus out of the 12z suite, a SLGT risk
was implemented over southeast AZ into southwest NM for the threat.
There is the potential for an even higher risk overall, but didn`t
want to make commitment until we have a stronger consensus on the
potential as higher risks necessitate more agreement at these
leads. Will be monitoring the situation closely as even smaller
deviations in track/evolution can play a significant role on the
overall forecast. The SLGT will suffice, for now given the recent
trends.

Elsewhere across the West the large Marginal risk area stretches
northward into OR and as far east as CO. The overall synoptic
pattern favors isolated to scattered convection over this entire
area, and the forecast PWs and instability would support locally
heavy rainfall rates. At the moment not seeing enough coverage to
justify any Slight risk areas, but we will continue to monitor.
Sometimes we can get a more organized flash flood risk develop on
the edge of the deeper tropical moisture, where cloud cover is low
enough to allow for destabilization, but moisture is still high
enough for heavy rates.

Kleebauer/Chenard

...South FL...

20Z Update: Very little deviation in the previous forecast as the
12z model suite remains consistent in its interpretation of
convection expected across southern FL. Locally heavy precip
between 2-4" are forecast in the period, however its location will
certainly be important with the urban centers the most favorable
for at least scattered flash flood prospects. As we move forward in
time, the output via CAMs will provide better insight on the
threat, so wanted to hold off on any upgrades until we have a look
at the high-res suite that could resolve smaller mesoscale factors
a bit better than the global deterministic and smoothed ensemble
mean. MRGL risk remains.

Kleebauer

..Previous Discussion..

A wet pattern will persist over south FL through Friday with
localized flash flooding possible each day. The overall pattern
does not change too much, with a stationary front in the low
levels, an upper jet to the north providing some right entrance
region ascent, and broad mid level troughing. It seems likely that
rounds of convection will occur each day, and with PWs around 2.3",
heavy rainfall rates are likely. There is some uncertainty with
the axis of heaviest rainfall each day, with some models favoring
the Keys, and given the location of the stationary front this may
make sense. However it still seems likely that heavier convection
will impact the urban corridor of southeast FL at times as well.
Uncertainty regarding both the axis and also the speed of
convection, whether it tends to propagate eastward quickly or can
hang up, dictates keeping the risk at Marginal for now. Although
could certainly see the need for a targeted Slight risk along the
southeast FL urban corridor at some point over the next 3 days
given the potential for multiple rounds of heavy rainfall.

The large scale pattern becomes more meridional by Friday, with
upper level flow becoming more southerly over south FL and the
upper jet orientation favoring a bit more upper level divergence.
Thus could potentially see a pattern that may favor more of a
stationary or training convective threat along the urban corridor
of southeast FL. However the location of the front by then will
play a role in this, and so not confident enough to go with a
Slight risk at this point.

Chenard


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt