355
FXUS65 KTWC 270954
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
254 AM MST Sun Sep 27 2026

.SYNOPSIS...A moisture increase is expected from the southwest
Sunday 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.
Dry conditions are then forecast for the second half of next
week. High temperatures will be a couple of degrees above normal
today, lowering to 5-10 degrees below normal Monday through
Wednesday, then warming back up to near normal late next week.

&&

.DISCUSSION...Complex weather scenario developing over the next
few days will result in an active start to the work week for
Southeast Arizona. This primarily revolves around the tropical
activity across the eastern Pacific and an amplified trough off
the West Coast. The first tropical feature is defined by the
NHC at 8 PM MST SAT as Hurricane Odalys, which is a category 1
system with maximum sustained winds 80 mph near 21.1N/123.6W
moving N at 6 kts. The second tropical feature defined by the
NHC at 11 PM MST SAT is Hurricane Polo, which is a category 3
system with maximum sustained winds of 120 mph near 19.8N/113.4W
moving NW at 9 kts.

Odalys will continue to move slowly north over cooler waters
today, weakening to a tropical storm by 28/00Z this afternoon
near 22.4N/123.1W. The latest water vapor imagery is showing the
middle to upper portions of this feature already shearing NE
into southern California this morning along the SERN periphery
of the upper trough along the West Coast. The latest GOES water
vapor imagery indicated a dry airmass across SE AZ, with PWATs
generally between 0.50-0.90 inches. Deeper moisture (PWATs 1.50+
inches) observed on this imagery along and west of the NRN Baja
MX associated with the sheared off mid and upper portions of
Hurricane Odalys. As the trough digs to the SE today, it will
shunt this moisture into SW by 28/00Z this afternoon with the
plume extending into central AZ by 28/06Z this evening. We will
start to see significant mid and high clouds expand from west to
east as the day progresses, with a few light showers west of
Tucson late this afternoon into tonight. With the lower-levels
still dry today, virga will be evident with a few gusts to 30
mph near the evaporating downdrafts.

Polo is much stronger and closer than Odalys to our neck of the
woods so it will have a more significant impact of moistening-
up SE AZ. Polo is progged to move northwest today but will
quickly recurve to the NE as the upper trough moves closer. Polo
will weaken to a category 1 between the Baja Spur and tip of
the Baja by 5 PM MST Monday afternoon. This track will help
push tropical moisture northward up the Gulf of California
today and across all of southern Arizona Monday and Tuesday.
The 27/00Z GEFS and ECMWF ensemble means still indicate a plume
of moist PWATs 2.5-3.5 standard deviations above normal across
Southeast Arizona by 29/00Z Monday afternoon. The GEFS ensemble
mean is less with regards to the moisture than the ECMWF
ensemble mean. Polo itself is still are NOT expected to impact
us directly. The official track by the NHC has Polo becoming a
remnant low in east-central Sonora Mexico near the Sonora/Chihuahua
border at 5 PM MST Tuesday afternoon.

Our main event is still expected to behave more like a fall-like
synoptic system utilizing the tropical moisture available across
our area. The upper pattern will focus a deformation axis out
ahead of the trough this afternoon/tonight, oriented SW-NE from
the northern Baja into central Arizona. Originally, weak
vorticity aloft will move through this axis resulting in weak
showers. However, as we move into the evening, a modest lower-
level (850-700 mb) moisture flux convergence will feed moisture
into this area increasing the coverage and intensity of the
showers, but it will also tend to allow showers the train as new
storms will form toward the deeper moisture despite individual
storms moving rapidly toward the NE. The impact of this will
generally avoid SE AZ tonight, but will start to move into the
western portions of the forecast area Monday morning.

The trough tries to become neutrally oriented Monday,
establishing a more broad diffluence area aloft across the
eastern portions of the CWA (east of Tucson) Monday afternoon.
This will increase the coverage of showers and thunderstorms.
Deformation across all of Southeast Arizona will then increase
Monday night and eventually phase with the remnants of Polo
moving NE through Sonora/Chihuahua Mexico Tuesday. Strong
dynamics and deep moisture in place will result in widespread
showers and thunderstorms Monday night into Tuesday. Strong
moisture flux convergence will continue to allow for storms to
train, increasing the potential for heavy rain along the band of
precipitation that develop. Strong 0-6km bulk shear Tuesday
will bring about the potential for mini-supercells to be
embedded within the widespread storm activity, especially to the
SE of Tucson.

Although rainfall rates may not be as impressive as we see
during the heart of the Monsoon season, this rainfall may be
more widespread and become problematic with time as rainfall
runoff struggles to drain into normally dry washes, roadway dips
and other low-lying areas. Training storms will result in
localized flash flooding issues. The WPC has all of Southeast
Arizona highlighted in a SLGT Risk of excessive rainfall Monday
in their Excessive Rainfall Outlook (ERO) and the eastern half
of Southeast Arizona in a MDT Risk Tuesday.

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

&&

.AVIATION...Valid through 28/12Z. Increasing mid and high clouds
today and tonight with SCT-BKN 12k-15k ft AGL BKN-OVC 20k ft
AGL. SFC wind less than 12 kts thru the valid period and SWLY
thru NWLY in direction during the afternoon/early evening hours
and variable in direction at other times. Aviation discussion
not updated for TAF amendments.

&&

.FIRE WEATHER...A moisture increase is expected from the southwest
Sunday 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.
Dry conditions are then forecast for the second half of next
week. High temperatures will be a couple of degrees above normal
today, lowering to 5-10 degrees below normal Monday through
Wednesday, then warming back up to near normal late next week.
Minimum RH values in the valley locations will be in the 20-30
percent range today (higher in the mountains). RH values then
increase 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. Gusty and erratic winds will be
possible with any thunderstorms that do develop.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...

Flood Watch from Monday morning through Tuesday evening for
AZZ501>515.

&&

$$

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