457
FXUS65 KTWC 190903
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
203 AM MST Wed Nov 19 2025

.SYNOPSIS...A low pressure system will cross the region over the
next several days bringing cooler temperatures, chances for
precipitation including snowfall above 7000 feet, and breezy
conditions. Showers will be isolated to scattered through early
this afternoon, becoming more numerous this evening into Thursday
morning. Temperatures will become well below normal Thursday with
the first widespread freeze expected Friday morning, mainly in
valleys to the south and east of Tucson.

&&

.DISCUSSION...The upper pattern impacting Southeast Arizona this
morning is defined by an upper-low over southern California and
another trough near the Gulf of Alaska. The latest GOES upper-
level water vapor band imagery indicated a strong vortmax (near
28.0N/122.5W) rotating around the base of the upper-low, which
should force the upper-low to elongate down to the Baja Peninsula
toward the Baja Spur by 20/00Z this afternoon. This feature has
consistently been progged the past several days to begin to eject
NE across Southeast Arizona this evening into Thursday, with the
southern low center of the upper-low (ensemble mean 500 mb height
5640m) overhead by 20/12Z Thursday morning...then exiting the area
over central New Mexico by 21/00Z Thursday afternoon.

The latest GOES TPW imagery indicated a modest amount of moisture
across our CWA, with precipitable water values between 0.65-0.80+
inches west and north of Tucson, and 0.65 to 0.50 inches from
Tucson SE into Cochise county. A secondary plume of 0.80+ PWAT is
observed moving NNE through eastern Sonora Mexico early this
morning and should clip SE Cochise county later this afternoon.
With falling heights, we will see the GEFS and ECMWF ensemble mean
1000-500mb thickness as low as 5460-5490m pushing across our area
this evening into Thursday morning. I mention this because the
saturation thickness helps define the most PWAT the atmosphere can
hold before it starts to wring out the excess moisture. In this
case the max PWAT value would be between 0.55-0.63 inches. This
suggests that the best precipitation will occur during this time
period, with those areas W and N of Tucson and the extreme SE
corner of Cochise county capable of a higher precipitation
efficiency. The current forecast/NBM QPF amounts have been stable
the last several runs indicating this storm system will produce
between 0.33 and 0.67 inches in the valleys of Southeast Arizona,
with the Sky Islands and the White Mountains expecting between
0.67-1.25 inches. The heaviest precipitation is expected near the
White Mounatins.

Snow levels this afternoon will range between 8500-9500 feet, lowering
to 6500-7500 feet Thursday morning. Storm-total snowfall amounts
between 2-5 inches will be possible above 7500 feet in the Sky
Islands with 4-7 inches possible in the White Mountains. These
snowfall amounts are below winter weather advisory criteria but
will still result in minor-to-moderate impacts for the higher
elevations.

Otherwise, the main impact with this system will be the reinforcement
of cooler temperatures across Southeast Arizona. With the low-center
moving over our neck of the woods at 20/12Z Thursday morning, the
19/00Z deterministic GFS and ECMWF 850-700mb thicknesses are near
1560m, which would correspond to a high temperature at the Tucson
International Airport (KTUS) around 60 degrees Thursday. Although
this is chilly, the minimum temperatures Friday morning will be
more impactful. It still looks like we will experience the first
widespread freeze this season in the eastern valleys. The 19/06Z
NBM probability of freezing temperatures (at or below 32 degrees)
Friday morning; 60 percent for KOLS, 46 percent for KSAD and 58
percent for KDUG.

Otherwise, with the mean trough in place to our west, expect that
upper-low currently in the Gulf of Alaska to dive along the same
path toward SRN CA/NRN Baja by Saturday morning, then ejecting
across our neck of the woods this weekend. This would bring another
round of valley rain/mountain snow and reinforce the cold
temperatures in place across the area.

&&

.AVIATION...Valid through 20/12Z. Increasing BKN-OVC 6-9k ft AGL
today with ISOLD-SCT -SHRA across much of southeastern Arizona
becoming BKN-OVC 4-7k ft AGL with NMRS -SHRA this evening and
overnight with a slight chance for -TSRA. MTNS TOPS may be OBSCD
at times tonight aft 20/03Z. SFC winds SLY less than 12 kts this
morning through 19/17Z, then SWLY 14-18 kts with gusts 25 kts.
Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...A storm system will begin to impact Southeast
Arizona this morning across central and western Pima county, then
the rest of Southeast Arizona this evening into Thursday. This
system will bring numerous showers with a slight chance of
thunderstorms, snowfall to the mountains above 7000 feet, well
below normal temperatures and breezy south to southwest winds.
Minimum RH values will be well above critical levels over the next
week, with values generally 35-45 percent in the valleys and
65-75+ percent in the mountains.

&&

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

&&

$$

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