621
FXUS65 KTWC 312123
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
223 PM MST Sat Oct 31 2026

.SYNOPSIS...Scattered showers will be possible this afternoon,
increasing in coverage from Tucson west this evening as a
weather system approaches. Sunday into Monday morning as the
weather system moves through showers become widespread, with
heavy rainfall at times and embedded thunderstorms. A Flood
Watch is in effect for all of southeast Arizona from 11 PM
Saturday through 5 AM Monday. Expect dry conditions Tuesday into
Friday.

&&

.DISCUSSION...Scattered showers have been ongoing this morning
and afternoon as moisture begins to stream in as part of an
atmospheric river ahead of a Pacific low. Rainfall amounts have
been relatively light, generally 0.01"-0.4" with no lightning.
Instability is fairly limited, but can`t rule out a lightning
strike or two this afternoon while the sun is up. Hi-res
guidance has struggled with this shower activity, but expect it
to continue on and off through the afternoon.

Precipitable water will continue to increase this evening and
overnight ranging from 1.6" in western Pima County to 1.25"
along the AZ/NM border by Sunday morning. The Pacific low will
dig south this evening and overnight, then move inland Sunday.
This will lead to widespread moderate to heavy showers with embedded
thunderstorms tonight into Monday.

Over the past couple model cycles hi-res guidance shows a first
wave of moderate showers this evening and part of the overnight
hours from Tucson westward. Shower activity is expected to
increase west of Tucson between 4-6 pm then increase in coverage
through the evening. General rainfall totals through midnight
look to be 0.25"-0.75". Sunday morning showers will likely still
mainly be from Tucson west then increase in coverage to the
east late morning into the afternoon as the core of the trough
moves through Arizona. A strong jet associated with the low will
aid shower and thunderstorm development. Heavy rainfall leading
to flash flooding and wash/river flooding will be the main
threats. However the jet will also help create an environment
conducive to strong to maybe even severe wind gusts in stronger
thunderstorms. SPC has southeast Arizona in a marginal risk for
severe weather Sunday. However the main limiting factor in this
will be instability which may be lacking, at least in Arizona.
Better instability will be in place over New Mexico. Regardless
convection within the widespread showers will likely be
responsible for the heaviest rainfall rates. Activity is
expected to decrease from west to east Sunday night into Monday.
A Flood Watch is in effect from 11 PM this evening through 5 AM
Monday. QPF hasn`t fluctuated by much over the past 12-24 hours
with Pinal County, northern Graham and Greenlee Counties, and
the Sky Islands north of I-10 expected to see the heaviest rain.
The REFS shows the Catalinas, Pinalenos, and White Mountains
having a 20-60 percent chance to see 3+ inches through Monday
morning. Below is a look at the expected totals and high/low end
totals.


-Expected: Valleys 0.50"-1.75", Mountains 1"-3+"

-Low end (9 in 10 chance):
 Valleys 0.25"-1.25", Mountains 0.50"-1.5"

-High end (1 in 10 chance):
 Valleys 1.75"-3.25", Mountains 2"-4" locally higher

With the expected rain on Sunday into early Monday we will see
a response in area rivers, creeks, and washes across SE AZ. See
hydrology section below for further details.

Monday morning will see lingering showers, but no significant
rainfall is expected. In the wake of the trough, brief ridging
will be in place through Tuesday. Then a couple shortwaves moves
through the region during the latter half of the work week. The
atmosphere dries out fairly quickly Monday leading to dry
conditions for the work week. High temperatures will be 4-7
degrees below normal through the work week.

&&

.AVIATION...Valid through 02/00Z.
Expect intermittent MVFR cigs at all terminals through the
forecast period with brief IFR cigs possible at KDUG and KOLS
tonight through the end of the forecast period. Scattered
-SHRA will be possible at KOLS and KTUS through 01/00Z, then
 more numerous SHRA with slight chc -TSRA mainly from KTUS/KOLS
 westward through the overnight hours. SHRA and isolated TSRA
 increase eastward encompassing all terminals after 01/15Z. Vis
 down to 1-2 miles will be possible in SHRA or TSRA. SFC winds
 mainly SELY/SLY 5-10 kts thru Sunday morning becoming SWLY
 12-20 kts and gusts 25-30 kts after 01/18Z. Near stronger TSRA
 wind gusts of 30-40 kts will be possible on Sunday. Aviation
 discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Scattered showers this afternoon then precipitation
chances increase tonight with widespread showers and isolated
thunderstorms Sunday into Monday. Moderate intensity rainfall
with pockets of heavy rain is expected Sunday into Sunday night.
The heaviest rain will be over the higher terrain. Afternoon
high temperatures will lower below normal through the forecast
period. Breezy to locally windy southwest winds expected Sunday,
turning northwest Monday. Minimum RH levels values into early
next week will be 40+ percent.

&&

.HYDROLOGY...As mentioned in the discussion section above, with
the expected moderate to heavy rain on Sunday into early Monday
we will see a response in the area rivers, creeks and washes
across SE AZ. There is the potential for significant rises on
the San Francisco River, Blue River, Gila River, Eagle Creek,
and Sabino Creek with Minor to Major flood stages possible. This
will continue to be evaluated over the next day and through the
event.

&&

.CLIMATE...Below is a listing of probability of exceedance of
36-hour rainfall totals from 5 pm Saturday Oct 31 to 5 am
Monday Nov 2.

Chance of seeing more than    1"    2"    3"
Tucson                       76%   52%    4%
Marana                       82%   55%    5%
Mount Lemmon                100%   88%   55%
Ajo                          67%   19%    2%
Picacho Peak                 95%   56%    4%
Oracle                       96%   63%   19%
Hannagan Meadow             100%   94%   63%
Safford                      65%   28%    3%
Clifton                      89%   46%    9%
Willcox                      52%    4%    0%
Douglas                      27%    3%    0%
Sierra Vista                 44%    4%    0%
Nogales                      68%   26%    3%

Tucson rainfall records
October 31st: 0.44" in 1926
November 1st: 0.42" in 1987
November (one-day): 2.09" on November 22, 1931

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Flood Watch from 11 PM MST this evening through late Sunday
night for AZZ501>515.

&&

$$

Hardin

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

NWS Tucson (TWC) Office