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FOUS30 KWBC 030850
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
450 AM EDT Wed Sep 3 2025

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

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

...Southwest...
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.

...South FL...
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 2
Valid 12Z Thu Sep 04 2025 - 12Z Fri Sep 05 2025

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

...Southwest...
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.

...South FL...
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 MARGINAL RISK AREA FOR PORTIONS OF FLORIDA,
AND THE WESTERN U.S....

...Southwest...
More uncertainty with the flash flood forecast on Friday compared
to Thursday. The potential is there for more direct impacts from
Lorena across portions of AZ and NM, but confidence on the details
is low. The GFS, RRFS and GFS based hurricane model (HAFS) have
consistently been indicating a stronger system that is able to move
into northwest Mexico. This evolution would likely result in
strong enough low level forcing to drive convection and a flash
flood risk over portions of southeast AZ into southwest NM. However
the non-NCEP global models and the NAM suggest the low level
center of the system weakens and stays offshore Baja California,
with just the mid and upper level energy ejecting northeastward.
This would probably result in quite a bit of mid and upper level
moisture moving into AZ and NM, and while this could result in a
steady rainfall, the cloud cover would probably hinder
destabilization and keep rainfall rates and the flash flood risk
lower. So this could be a case where the areal averaged rainfall is
higher, but the flash flood risk is actually lower given the
decreased rates. Given this uncertainty we opted to stick with a
Marginal risk in the ERO despite our deterministic QPF showing
upwards of 1-2" of rain over southeast AZ into southwest NM. We
will continue to monitor trends and adjust as needed on future
updates.

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.

...South FL...
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