418
FXUS65 KTWC 200850
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
150 AM MST Thu Nov 20 2025

.SYNOPSIS...A couple of low-pressure system moving through
Southeast Arizona today and again this weekend will bring cooler
temperatures, chances for precipitation including snowfall above
7000 feet, and breezy conditions. Showers will be scattered to
numerous this morning, decreasing early this afternoon. Afternoon
temperatures will be well below normal today, with the first
widespread freeze expected Friday morning, mainly in the valleys
to the south and east of Tucson.

&&

.DISCUSSION...It has been an active early morning across Southeast
Arizona as an upper-low that has been impacting western and central
Arizona with heavy rain and hail over the past 24 hours is in the
process to ejecting NNE though our neck of the woods. As of 2 AM MST,
radar imagery indicated scattered showers increasing in coverage with
embedded thunderstorms capable of producing small hail, as well
as high-elevation mountain snow.

The latest GOES water vapor imagery indicated the upper-low was
centered 40 miles east of Rocky Point in northwest Sonora Mexico.
The feature was beginning to accelerate NNE and should make it to
the Tucson Metro Area between 20/09-12Z this morning. Strong shear
evident ahead of this feature has been responsible for supporting
a few strong storms to develop earlier this evening across the
Tohono O`odham Nation NNE into southeast Pinal county. This area
of strong shear will shift E and NE through the early morning
hours decreasing the overall threat of hail. However, with the
orientation of the upper-low becoming more negatively tilted as it
ejects supporting an increase in mid-level lift as well as the
core of the upper low-moving overhead both support more numerous
showers across Southeast Arizona toward daybreak. The general
thinking with regards to precipitation amounts has not changed
from the previous forecasts with storm total rainfall amounts
between a quarter to as much as three quarters of an inch. This
would translate to 2-5 inches of storm total snowfall in the Sky
Islands above 7500 feet with 4-7 inches possible in the White
Mountains. In any event, this system will rapidly move out of the
area with the low-center moving into New Mexico by 20/18Z and
into central New Mexico by 21/00Z this afternoon. The prime time
for shower activity is early this morning to around noon MST.

Although the storm system will have departed the area, we will
see the cooler air settle into the valleys east and south of
Tucson Friday morning. The latest NBM probabilities of subfreezing
temperatures in the aforementioned areas remains between 60-90
percent. A Freeze Warning remains in effect from 2 AM to 9 AM MST
Friday morning for the valleys east and south of Tucson. Since
this is the first widespread freeze of the season, we have been
trying to emphasize safety messaging to remind people to protect
people, pets, pipes, and plants from freezing temperatures.

Otherwise, with the mean trough in place to our west, expect yet
another upper-low that is currently off the coast of SRN OR/NRN CA
to dive down along the West Coast into a position west of the NRN
Baja Saturday morning. Its almost like a carbon-copy of the system
we are dealing with right now, but just a little west and south
of its exact track. This upper low is expected to eject across
Southeast Arizona Saturday afternoon into Saturday night. I have
to admit, my confidence in the timing of this system to move over
us during the day on Saturday is low because of the orientation of
the upper ridge in the Eastern Pacific flopping over into the
Pacific Northwest. This would result in a pattern that will want
to tug the upper-low farther west and/or delay the timing of the
ejection by 6-12 hours. There are hints of this between the 20/00Z
ensemble members of the GEFS and ECMWF. Variance within the
models is shown to the west of the upper-low Saturday, and
generally an equal split within each model. In any event, this
system will result in another round of valley rain and mountain
snow showers to our neck of the woods Saturday night into Sunday.
It will be cool again Sunday afternoon, and freezing temperatures
in the wake of the system are expected Monday morning to the
valleys east and south of Tucson.

The pattern does quiet down after this system departs, with the
Eastern Pacific ridge strengthening and shifting the mean trough
east.

&&

.AVIATION...Valid through 21/12Z. BKN-OVC 4k-7k ft AGL with NRMS -SHRA
becoming SCT-BKN 6k-9k ft AGL SCT -SHRA after 20/15Z. Visibility reductions
and lowered ceilings with mountain obscurations may bring periods of
MVFR conditions through 20/15Z. SFC winds SWLY 7-12 kts this morning,
becoming WLY 12-16 kts in the afternoon, then light terrain driven
winds less than 12 kts overnight. Aviation discussion not updated
for TAF amendments.

&&

.FIRE WEATHER...A storm system will continue to impact Southeast
Arizona this morning with numerous showers, isolated thunderstorms,
snowfall to the mountains above 7000 feet and well below normal
temperatures. Minimum RH values will be well above critical levels
into this weekend between 35-45 percent in the valleys and 65-75+
percent in the mountains. Minimum RH values then decrease to 25-35
percent in the valleys and 35-50 percent in the mounatins.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Freeze Warning from 2 AM to 9 AM MST Friday for AZZ503-507>509.

&&

$$

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