812
FXUS65 KTWC 272206
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
306 PM MST Sun Sep 27 2026

.SYNOPSIS...
Moisture continues to increase from the southwest this evening
resulting in mid and high level cloud cover and a slight chance
of showers and thunderstorms. Deep tropical moisture will move
into the area from the south Monday and Tuesday. A low pressure
system moving in from the West Coast will interact with this
moisture, resulting in widespread showers and thunderstorms.
Heavy rain could lead to flooding across portions of southeast
Arizona. Thereafter, dry conditions are forecast for the second
half of next week. High temperatures will lower to 5-10 degrees
below normal Monday through Wednesday, then warming back up to
near normal late next week.

&&

.DISCUSSION...
Forecast remains on track with little to no changes overall. Any
changes are related to the details and scenarios, which are
coming into better focus as much of the event is captured in the
CAMs. Flash Flood Watch remains in effect with no planned
changes to the time.

The first change is a possible earlier start time to some of
the precip. Some CAMs are showing scattered showers with
isolated thunderstorms by Monday morning, increasing in coverage
through the day. The second change is an upward trend in the
forecast rainfall. While widespread 1-2" of rain is likely area
wide (exception of Western Pima County 0.25-0.75"), there is an
increasing signal of 2-4" for the Sky Islands and the White
Mtns. In fact, some 75th/90th percentile REFS/RRFS guidance
suggest 4"+ is possible in the White Mtns, possibly extending SW
into the Sky Islands. Given the duration, possible training,
and threat for thunderstorms, this seems reasonable. Another
thing of note is the EFFI and Shift of Tales, which supports the
potential for these higher end totals.

Now zooming into the Tucson Metro, generally 0.5-1.5" is
expected for the event. The 75th/95th percentile (reasonable
worst case) is pointing to totals of 2-3"+. While this is a
distinct possibility, it will favor upslope regions near the
Catalina/Rincon Mtns and/or require repeated rounds (training)
of heavier showers/storms. Some of the CAMs are showing
variations of this possibility and it bears watching. Another
thing to consider is this will not be your classic Monsoonal
thunderstorms. It will be deep tropical moisture with widespread
showers and isolated to scattered thunderstorms, which may
persist off and on for a longer duration Monday into Tuesday.
With every event, there will be areas that under perform and
over perform, dependent on where these heavier bands of rainfall
setup. We will make adjustments and updates to better depict
these scenarios as we move into Monday and confidence increases.
However, we want to reiterate there will be significant
rainfall for portions of SEAZ and at least isolated to perhaps
scattered instances of Flash Flooding. WPC maintains a Slight
Risk over much of the area for Monday and has a Moderate Risk
just east of Tucson to the NM border on Tuesday. Moderate Risk
Excessive Rainfall Outlooks for the Desert Southwest from WPC
are fairly rare.

As the upper level low moves across Arizona into Tuesday, the
threat for flooding and heavy rain persist into Tuesday. Given
the forcing, marginal CAPE, and abundant shear, strong to
perhaps an isolated severe storm will be possible. Even brief
transient low top supercells will be possible.

Activity shifts east by Wednesday, with the possibility for
lingering flooding on area washes. Easterly gusty winds may be
possible by Thursday and Friday, but confidence is low at this
time.

&&

.AVIATION /21Z SUNDAY THROUGH 21Z MONDAY/...
VFR through tomorrow morning for all TAF locales with increasing
mid and high level cloudiness. VCSH/SHRA with VCTS by Monday
morning increasing in coverage through the day. Brief MVFR
possible under heavier showers/storms. MVFR CIGs possible by
late Monday into Tuesday with VIS down to 3-5 miles. There is
even a slight chance of brief IFR for KTUS/KOLS Mon night into
Tuesday. Gusty winds winds from TS at VRB to 35 kts possible,
locally higher. Mtn obscuration likely by Monday evening into
Tuesday.

&&

.FIRE WEATHER...
Deep tropical moisture will move into the area from the south
Monday and Tuesday. A low pressure system moving in from the West
Coast will interact with this moisture, resulting in widespread
showers and thunderstorms. Dry conditions are then forecast for
the second half of the week. High temperatures will lower to 5-10
degrees below normal Monday through Wednesday, then warming back
up to near normal late week. Minimum RH values in the valley
locations will increase to 15-30 percent Monday through
Wednesday, gradually lowering through the end of the week. 20-foot
winds will be less than 15 mph into early next week. Breezy
southwest to west winds will be possible along the surface front
as it moves across the area Tuesday. By next Thursday, gusty east
to southeast winds will be possible.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Flood Watch from Monday morning through Tuesday evening for
AZZ501>515.

&&

$$

Visit us on Facebook, X, YouTube, and at weather.gov/Tucson