068
FXUS65 KTWC 291708
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
1008 AM MST Tue Sep 29 2026

.SYNOPSIS...
A strong upper-level weather disturbance in southeast
California will make its way east through Arizona today. This
system will interact with deep tropical moisture producing
widespread showers and thunderstorms across Southeast Arizona.
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 be 10 to 15 degrees below today and Wednesday, then
warm back up to near normal late this week.

&&

.UPDATE...
Focus for today continued to be two bands of moderate to heavy
rainfall slowly translating east across southeast Arizona. The
western-most aligned from southwest to northeast, from between
serapos gate and Newfield, to north of Peridot. The second band
extends from southeast Santa Cruz county to southern Greenlee
county. Rain rates of 1 to 2 inches per hour are noted with
each band. Sabino Canyon wash is at minor flood stage and may go
to moderate flood stage with next band now moving through the
Tucson area. Flooding is increasingly likely along additional
washes in Pima, Santa Cruz, southern Pinal and Cochise
counties.

&&

.DISCUSSION...
The upper pattern this morning is defined by a
strong upper-low centered across southeast California and
Hurricane Polo (a category 2 hurricane with maximum sustained
winds of 110 mph near 25.9N/112.2W or over the southern Baja
Peninsula, moving NE at 11 kts). Polo is forecast to rapidly
weaken as it accelerates toward the NE today, moving south of
Hermosillo Mexico as a category 1 hurricane by 29/12Z this
morning, then south of the New Mexico Bootheel in NW Chihuahua
MX as a remnant low by 30/00Z this afternoon.

The latest satellite and radar imagery indicated a band of showers
with a few embedded thunderstorms that extend from Organ Pipe to
Casa Grande to Globe early this morning which coincide with a
deformation/lift axis aloft (between 500-300 mb). Elsewhere,
there are a batch of scattered showers and thunderstorms developing
across west-central and southwest Arizona that are associated
with the generalized lift out ahead of the core of the upper-
low in SE CA. The latest GOES precipitable water imagery
indicated deep tropical moisture across Southeast Arizona with
PWATs 1.50-1.75 inches extending through central Pima county
into Pinal county and PWATs 1.25-1.50 inches elsewhere across
Southeast Arizona.

Showers and thunderstorms start becoming widespread across Southeast
Arizona around daybreak this morning as the upper-low moves southeast,
elongates/amplifies along western Arizona and becomes more neutrally
oriented. This will shift that deformation band/axis east,
setting up along a line from the southern Baboquivari Mountains
to Three Points to the Catalina Mountains to Bylas.

This is where things get interesting. With that deep plume of
tropical moisture to the SW of Tucson and strengthening
pressure gradients in the low-to-mid levels as the trough re-
orients, we begin to see an area of strong moisture flux
convergence develop in the lower-levels (850-700mb). This
moisture convergence will be broad over the majority of
Southeast Arizona, but will especially enhance convection and
increase the potential for additional storms to develop and move
over the same area in that aforementioned area of
deformation/lift aloft. So, initially we will see light to
moderate rainfall in this band early this morning becoming more
moderate intensity after daybreak. Then during the mid-morning,
rainfall intensity will pick up again and become moderate to
heavy. This will then persist into the early afternoon with the
surface front itself eventually swinging from west to east
across Southeast Arizona impacting this area again.

This line is particularly concerning today. This is supported
by the 29/00Z HREF 3-hr QPF PMM neighborhood probabilities in
this area, with probabilities between 70-90% for 1.00+ inch at
the 15Z, 18Z and 21Z time intervals. In addition, there is a
10% probability for 3.00+ inches at both the 15Z and 18Z time
frames. That is a major bullseye for the HREF, and it has been
showing a line like this develop for several model-runs which
increases my confidence in something big happening. It can be
5-15 miles east or west of this line, but as it is currently
aligned, this would specifically impact the Altar/Black/Brawley
Washes that are the more flood-prone locations to the SW and W
of Tucson. If it is farther east, this will significantly impact
the NW half of the Tucson Metro Area and the Catalina Mountains
(i.e CDO, Sabino Creek, Tanque Verde Creek). At this time, the
official forecast is for 1.50-2.50 inches along and near this
line with up to 3.00-3.50 inches in the higher terrain. Given
these amounts over a widespread area, flash flooding is a major
concern.

Elsewhere, heavy rainfall will impact the entire forecast area
as widespread showers and thunderstorms occur with the general
deformation aloft associated with the approaching trough as well
as the surface frontal band that swings thru the area from west
to east this afternoon. 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 tonight into Wednesday within the main-stem drainages
due to widespread heavy rainfall. There is a Flash Flood Watch
in effect through 11 PM MST this evening.

Not to be lost in the mix, severe storms will be possible this
afternoon 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 low-top supercells possible.

Otherwise, lingering instability and moisture will be present
tonight into Wednesday morning 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 /16Z TUESDAY THROUGH 16Z WEDNESDAY/...
Valid through 30/12Z. SCT-BKN 5-8k ft AGL with BKN-
OVC 10-14k ft AGL this morning through this afternoon, with
decreasing cloud cover and storms this evening and overnight.
Widespread -SHRA/-TSRA expanding in coverage and intensity this
morning and persisting into the afternoon. Generally VFR with
brief MVFR possible under heavier showers/storms. MVFR CIGs
possible this morning with VIS down to 3-5 miles along with a
slight chance of brief IFR for KTUS/KOLS. Mtn obscuration likely
this morning through this afternoon. Gusty winds to 35 kts may
accompany stronger storms. Aviation discussion not updated for
TAF amendments.

&&

.FIRE WEATHER...
A strong upper-level weather disturbance in
southeast California will make its way east through Arizona
today. This system will interact with deep tropical moisture
producing widespread showers and thunderstorms across Southeast
Arizona. 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 be 10 to 15 degrees below today and
Wednesday, then warm back up to near normal late this week.
Minimum RH values 35-55 percent in valley locations and 50-75
percent in the mountains today through Wednesday, generally
lowering to 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
today.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Flood Watch through this evening for AZZ501>515.

&&

$$

SYNOPSIS...
UPDATE...Tripoli
DISCUSSION...Tripoli
AVIATION...
FIRE WEATHER...

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