301
FXUS65 KTWC 262100
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
200 PM MST Sat Sep 26 2026

.SYNOPSIS...Dry conditions areawide through tonight. 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 through Sunday, lowering to 4-9
degrees below normal Monday through Wednesday, before warming back
up to near normal late next week.

&&

.DISCUSSION...Very little change versus 24 hours ago. The large
scale pattern has a storm system moving across the northern tier of
states and high pressure aloft centered over northern Mexico and the
western Gulf. A trough starts to dig south along the Pacific Coast
on Sunday, as Hurricane Polo moves northward off Mexico to west of
the Baja tip. Monday into early Tuesday, an area of low pressure
forms over central California within the deepening trough over
California, as Polo starts to make an eastward curve into Gulf of
California south of the Baja Spur. Tuesday into midday Wednesday,
the upper low moves from west to east across central Arizona
dragging a frontal boundary/trough axis across the state, with the
remnants of Polo tracking northeast across central Sonora and
northern Chihuahua.

Dry conditions will continue through tonight across southeast
Arizona. PWATS start to increase on Sunday as the trough begins to
dig to our west and we start to tap moisture to our south/southwest.
PWATs come up into the 1.0 to 1.3 inch range by late in the day
Sunday. Much of this moisture increase will be in the mid/upper
levels, resulting in significant mid and high clouds along with a
slight chance of showers and thunderstorms Sunday afternoon and
Sunday night.

Monday and Monday night PWATs jump up into the 1.2 to 1.6 inch range
as the trough deepens to our west and Hurricane Polo curves east
across the southern Baja Peninsula, the Gulf of California and then
makes landfall near Ciudad Obregon. The flow across southeast
Arizona at this time will turn southerly tapping this deep moisture,
as the upper low and associated trough begin to push in from the
west. Precipitation chances increase across the forecast area during
the day Monday into the 40-50 percent range across much of the area,
with 70 percent POPs in the White Mountains. POPs then increase
significantly Monday evening into Tuesday morning into the 65-85
percent range across most of southeast Arizona, with 90+ percent
POPs in the White Mountains.

Tuesday into midday Wednesday, the upper low and trough axis will
shift eastward across the state and precipitation chances will
gradually wind down from west to east. Across western Pima County
the precipitation chances are expected to end during the early
evening hours Tuesday and then during the morning hours Wednesday
across eastern portions of the forecast area.

Behind the upper low and trough Wednesday afternoon/evening, there
will be a slight chance of showers and thunderstorms along the New
Mexico state line, with the rest of the forecast area remaining dry.
High pressure aloft then builds over the eastern Pacific Ocean and
southeast Arizona dries out significantly under westerly flow
Thursday into next weekend.

Given the deep moisture across southeast Arizona Monday and Tuesday
and the strong dynamics associated with the upper low and frontal
boundary/trough axis there will likely be isolated severe
thunderstorms over this time frame, along with the potential for
flash flooding. Low end probabilistic guidance (25%) for storm total
rainfall shows around 0.50 inches in the Tucson metro, lesser
amounts west of Tucson and values of 1.0-1.5 inches along the New
Mexico state line. High end storm total probabilistic guidance (90%)
shows 2-3 inch totals across much of southeast Arizona, with a
pocket of 4+ inches in the Greenlee County. Our official forecast
has 1.0-1.5 inches in valley locations from Sells eastward and 1.5-
2.0 inches in the mountains. WPC has all of southeast Arizona
highlighted in a SLGT Risk of excessive rainfall Monday and the
eastern half of southeast Arizona in a MDT Risk Tuesday.

Given the synoptic setup and the potential for heavy rainfall/flash
flooding, have elected to issued a Flash Flood Watch from 11 am
Monday through 11 pm Tuesday. For more information on this product,
please refer to PHXFFATWC. While this time frame incorporates most
of the expected heavy rainfall across southeast Arizona, some main
stem rivers may have significant rises past the ending time of this
watch due to response times.

High temperatures will be a couple of degrees above normal through
Sunday, lowering to 4-9 degrees below normal Monday through
Wednesday, before warming back up to near normal late next week.

&&

.AVIATION...Valid through 28/00Z.
SKC-SCT 20k ft AGL (high clouds) through early this evening, then
increasing mid and high clouds overnight 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...Dry conditions areawide through tonight. 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 then 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 through Sunday, lowering to 4-9
degrees below normal Monday through Wednesday, before warming back
up to near normal late next week. Minimum RH values in the valley
locations will be in the 20-30 percent range through Sunday (higher
in the mountains). RH values then increase 15-30 percent on Monday
through Wednesday of next week, gradually lowering through the end
of next 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 late Monday morning through Tuesday evening for
AZZ501>515.

&&

$$

Zell

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