813
FXUS65 KTWC 280914
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
214 AM MST Mon Sep 28 2026

.SYNOPSIS...Southeast Arizona is still on track to be quite
busy over the next few days as a strong trough of low pressure
along the West Coast makes its way through Arizona today into
Tuesday. This system will interact with deep tropical moisture
moving into the area from the south and southwest to produce
widespread showers and thunderstorms across the region. These
storms will be capable of producing heavy rainfall that could
lead to flash flooding concerns. 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...The upper pattern this morning is defined by an
amplified trough of low pressure along the West Coast with a low
center observed early this morning near the Bay Area of
California, Hurricane Polo (a category 3 hurricane with maximum
sustained winds of 120 mph near 23.2N/114.0W or west of the tip
of the Baja, moving N at 10 kts), and a subtropical ridge across
the majority of Mexico.

The latest satellite and radar imagery indicated a band of clouds/
mainly light showers with a few embedded thunderstorms that extend
from what was left of post tropical cyclone Odalys (near 23N/123W)
into west central Arizona, with storms beginning to expand east
across central Arizona. These storms coincide with a thin axis of
deformation aloft at 500 mb between the trough and the subtropical
ridge in Mexico. The latest GOES precipitable water imagery indicated
deep tropical moisture with PWATs 2.00+ inches along the entire Gulf
of California this morning, assisted by the strong SLY flow up
the Gulf along the easterly envelope of Hurricane Polo. Across
SE AZ this morning, PWATs generally range from 1.10-1.25 inches
from Tucson southeast and 1.25-1.50 inches west of Tucson.

Expect showers and thunderstorms to expand in coverage today
(especially this afternoon) as the deep tropical moisture moves
northeast into SE AZ and the general deformation/lift aloft
expands across the area. With PWATs generally above 1.50 inches
across the entire CWA, this is an anomalously moist airmass (3.0
to 3.5 standard deviations above the norm). Precipitation
amounts will generally be light through this afternoon, with
rainfall amounts between a tenth and quarter inch (higher
amounts near a half inch south and east of Tucson and the higher
terrain).

Showers and thunderstorms really start becoming widespread with
moderate-to-locally heavy rainfall tonight as the upper-low
amplifies and moves SE to the Colorado River along the AZ/CA
state line by 29/12Z Tuesday morning. That thin axis of
deformation aloft at 500 mb shifts east overnight, positioning
itself from south-central Pima county to Tucson to Safford by
daybreak. This is where things get interesting. We begin to see
strong moisture flux convergence in the lower-levels (850-700mb)
into the same area of this deformation/lift aloft. This should
not only feed the deeper moisture into an enhanced area of storm
development, but it will allow for storms to train Tuesday
morning with newer development favored to the SW toward the
deeper moisture. This is supported by the 28/00Z HREF 3-hr QPF
PMM neighborhood probabilities in this area, with probabilities
between 70-90% for 1.00+ inch and 10+% 3.00+ inches between
29/12-18Z Tuesday. This may make the morning commute across the
Tucson Metro Area and farther NE in Safford a bit messy Tuesday
morning if very heavy rainfall amounts are focused along this
axis. This can also be complicated as we move into the afternoon
Tuesday, as the front itself moves from west to east across
Southeast Arizona resulting in another focus for heavy rainfall.

The heaviest rain will occur Monday night into Tuesday afternoon
with expected rainfall amounts during this time period between
0.75-1.25 inches with as much as 2-3" inches possible in those
areas impacted by training of storms (repeated rounds of pcpn)
and upslope regions in the higher terrain (especially the
Catalina/Rincon/Pinaleno Mountains). Rainfall runoff will result
in running water in normally dry washes and roadway dips with
significant ponding of water likely in low-lying areas and other
flood-prone locations. Isolated to scattered locations may
experience flash flooding with this event. Longer-term heavy
flows will also be possible into Tuesday night into Wednesday
within main-stem drainages due to widespread heavy rainfall.
There is a Flash Flood Watch in effect from 11 AM MST today
through 11 PM MST Tuesday evening.

One side-note, severe storms will be possible Tuesday...but
should be considered a secondary threat compared to the risk of
flash flooding. As the upper-level low tracks across the area,
decent forcing, marginal CAPE and abundant shear will be
present. Therefore, expect strong to perhaps isolated severe
storms...with brief transient low-top supercells also possible.

Otherwise, lingering instability and moisture will be present
Wednesday for additional shower activity, but drier air will
quickly lower precipitation chances through the day. A ridge of
high pressure then builds into the Desert Southwest the second
half of the work week, shutting down thunderstorm activity.

&&

.AVIATION...Valid through 29/12Z. SCT-BKN 5-8k ft AGL BKN-OVC
10-14k ft AGL with isolated -SHRA northeast of Tucson through
28/19Z, then -SHRA/-TSRA expanding in coverage and intensity
this afternoon and overnight tonight. Generally VFR through the
early evening hours, with brief MVFR possible under heavier
showers/storms. MVFR CIGs possible by late Monday evening into
Tuesday morning with VIS down to 3-5 miles. There is even a
slight chance of brief IFR for KTUS/KOLS tonight into Tuesday.
Mtn obscuration likely by Monday evening into Tuesday. Gusty
winds to 35 kts may will accompany stronger storms. Aviation
discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Southeast Arizona is still on track to be quite
busy over the next few days as a strong trough of low pressure
along the West Coast makes its way through Arizona today into
Tuesday. This system will interact with deep tropical moisture
moving into the area from the south and southwest to produce
widespread showers and thunderstorms across the region. These
storms will be capable of producing heavy rainfall that could
lead to flash flooding concerns. 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.
Minimum RH values 35-55 percent in valley locations and 50-75
percent in the mountains today through Wednesday, generally
lowering 15-25 percent the second half of the week as drier air
moves into the region. 20-foot winds will be less than 15 mph
into mid-week, then gusty east to southeast winds possible the
second half of the week. Breezy southwest to west winds will be
possible along the surface front as it moves across the area
Tuesday.

&&

.HYDROLOGY...Rainfall to date in September has been above
average for portions of SEAZ with many creeks/washes/rivers
showing above to much above average flows for this time of the
year via the USGS stream gauge page.

This creates antecedent conditions that are primed for heavy
rainfall with quick response times for SEAZ waterways. Recent
coordination with the Colorado Basin River Forecast Center
suggest some locations could reach or exceed Action Stage.
Comparing some of the more recent rises in the Pantano, Brawley,
Santa Cruz etc, this would flood flood-prone low water crossings.
So it is reasonable to expect similar conditions with this
event. For example, at 6 feet, the Pantano @ Vail results in low
water crossings flooded downstream.

Sabino Creek near Sabino Dam could reach Minor Flood Stage if
the basin receives some of 75th/90th percentile probabilistic
rainfall scenarios.

For the Gila River Basin, there is potential for heights to
approach Action Stage or exceed at Duncan and lower confidence
downstream toward Solomon. It would take nearly double the
forecast rainfall for the Gila Basin upstream or near the Duncan
Gauge to reach Minor Flood Stage. So while it is unlikely,
75th/90th Percentile rainfall amounts from the REFS/RRFS could
start to bring this scenario into play. At this time, there is
low confidence. With a more steady, at times heavy rainfall, we
will be able to see these rises and adjust the forecast
accordingly with the RFC.

Elsewhere, Flash Flooding is possible at any of the known flood
prone locations and persons that rely on these know problem
areas for commuting should be prepared for alternate routes or
delays Monday evening, Tuesday, possibly into Wednesday morning.
The morning commute across the Tucson Metro Area may especially
be problematic Tuesday morning.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...

Flood Watch from 11 AM MST this morning through Tuesday
evening for AZZ501>515.

&&

$$

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