162
FXUS65 KTWC 170929
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
229 AM MST Mon Nov 17 2025

.SYNOPSIS...Another storm system moving into Southeast Arizona the
middle of the this week will bring cooler temperatures, chances
for precipitation including snowfall above 7000 feet in the Sky
Islands, and breezy conditions on Tuesday. High temperatures will
run 2 to 6 degrees below normal through Tuesday, then fall 10 to
13 degrees below normal by the middle of the work week. The first
widespread freeze is expected Friday for the valleys to the south
and east of Tucson.

&&

.DISCUSSION...The upper pattern this morning is defined by an
upper-low currently ejecting NE through Utah, with an amplified
trough digging south along the coast of the Pacific Northwest. The
latest GOES water vapor imagery indicated that this trough has a
strong low center near 39N/127W, with a noticeable strong
NLY flow on the backside of this trough resulting in a split flow
and another minor low center near 30N/134W. This second feature
will assist the trough along the coast to dig south the next 24
to 36 hours as it carves out more territory in the mean through.
The upper-low will eventually make its way into southern California
and stall-out between 00Z-12Z WED. As the second vort lobe
rotates around the base of the mean trough Wednesday, it will help
elongate the main upper-low center from southern California to
the Baja Spur by 00Z THU. This feature then begins to eject E
then NE Wednesday night across Southeast Arizona as another kicker
trough moves down the West Coast.

With the upper-low initially stalling out across southern California
Tuesday and Tuesday night, it will focus a subtropical moisture
plume (with precipitable water values 1.00+ inches) from the Baja
Spur into southwest and central Arizona. This will result in a
weak AR event making its way into central Arizona with 40 percent
of the the GEFS and ECMWF ensemble members bringing up an IVT
between 250 and 500 kg/ms between 12Z TUE and 12Z WED. Given the
stalled out upper low over southern California resulting in
favorable lift/dynamics aloft over western and central Arizona,
the 17/00Z NBM indicates a 50-70 percent probability for rainfall
amounts over an inch in that area over a 24 hour span. Although the
focused subtropical moisture plume spares Southeast Arizona from
the heavier rainfall, we will start to see precipitation spread
out across the majority of the area Wednesday and Wednesday night
as the upper low ejects through the area.

The moisture available and upper dynamics associated with the
departing system will be better than this past storm system
yesterday. I would expect the majority of the precipitation to
fall Wednesday and Wednesday night, with qpf amounts in the
valleys between 0.33 and 0.67 inches NW of a line from Tucson to
Safford, and amounts between a tenth to a quarter inch SE of that
line. The Sky Islands and the White mountains will see between a
half and one inch, with the heaviest precipitation expected near
the White Mounatins. Snow levels will start out Tuesday afternoon
between 8000-9000 feet, lowering to 7500-8500 feet midday
Wednesday, then down to 6500-7500 feet Thursday morning. Storm-
total snowfall amounts between 2-5 inches will be possible above
7000 feet in the Sky Islands with 4-7 inches possible in the White
Mountains. These snowfall amounts will result in minor-to-moderate
impact for the higher elevations.

The main impact with this system will be the cooler temperatures.
The 17/00Z deterministic GFS and ECMWF both lower the 850-700mb
thickness values to around 1550m by 00Z FRI, which would
correspond to a high temperature at the Tucson International
Airport (KTUS) around 56 degrees on Thursday. That`s chilly. I
suspect that we wont be able to realize all of this cooling due to
how rapidly the upper low is ejecting, but afternoon temperature
near 60 degrees are likely none-the-less. It`s the low temperatures
Friday morning and onward that may be more impactful. It looks
like we will experience the first widespread freeze this season in
the eastern valleys. The 17/04Z NBM probability of freezing
temperatures (at or below 32 degrees); for KOLS is 51% THU, 83%
FRI, and 46% SAT, for KSAD is 37% THU, 66% FRI, and 45% SAT, and
for KDUG is 54% THU, 76% FRI, and 63% SAT.

&&

.AVIATION...Valid through 18/12Z. SCT-BKN clouds at 7k-10k ft AGL.
Increasing high clouds after 17/17Z. SFC wind less than 10 kts
and variable in direction through 17/19Z, SWLY 10-14 kts during
the afternoon and early evening. Aviation discussion not updated
for TAF amendments.

&&

.FIRE WEATHER...Another storm system will impact Southeast Arizona
late Tuesday across central and western Pima county, then the
rest of Southeast Arizona Wednesday into Thursday morning. This
feature will bring another round of precipitation including snow
above 7000 feet, well below normal temperatures and breezy south
to southwest winds (Tuesday). RH values will be well above
critical levels, with Min RH values 30-40 percent in the valleys
and 60-70+ percent in the mountains.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

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