Severe Thunderstorm Warning 107, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
900 AM MST Fri Sep 26 2025

.UPDATE...Current radar shows a line of storms stretching from
Vail through Patagonia and into northern Sonora traveling east.
These storms have already put out wind gusts of 54 and 44 mph.
Water vapor shows a closed low over southern California slowly
moving eastward. Just ahead of it water vapor also shows some dry
mid-level air and an area of diffluence. Both will be over central
Arizona this afternoon. Surface based CAPE is around 1000 J/kg
with a some capping, but by late this morning into the afternoon
will be 1500-2500 J/kg. Plenty of moisture is also in place and
0-6 km wind shear will be 25-35 kts across the area. All the
ingredients will be in place for another round of scattered to
numerous showers and thunderstorms late this morning into the
evening from central Pima County eastward. Thanks to the shear and
dry mid-level air some storms will be severe capable of damaging
winds and hail. The HREF pinpoints Pinal County and western Graham
County as having the best chances for severe winds and hail,
which is where the best shear and diffluence will be. SPC has just
issued a slight risk for this area. Storms will also be capable
of heavy rainfall leading to localized areas of flash flooding.
The HREF again hones in on Pinal County and western Graham County
as having the best chances for heavy rainfall. 40 km neighborhood
probabilities of 1" hourly rainfall are 20-40 percent this
afternoon. In the evening the HREF is also signaling that Santa
Cruz County could see some heavy rainfall. Blowing dust may be
possible in Pinal County from outflows moving west from storms in
the eastern portion of the County. For longer term details see the
previous discussion.

&&

.PREV DISCUSSION... /issued 338 AM MST Fri Sep 26 2025/

.SYNOPSIS...A weather system will bring periods of rain showers
and thunderstorms through thIS weekend. A Flash Flood Watch in
effect through Saturday evening. Thunderstorms will be capable of
producing strong to severe gusty winds, hail, and heavy rainfall.

&&

.DISCUSSION...Doppler radar, as of 3 am, showed isolated showers
and thunderstorms mainly north and east of Tucson. These storms
were moving rather quickly, around 20 to 25 mph, with rainfall
amounts mostly <0.10". Water vapor imagery this morning showed a
closed upper low near Los Angeles, moving to the S-SE. This
feature will continue to move slowly across SE CA today and be
centered over eastern Riverside county later this afternoon, then
quasi- stationary along CA/AZ border through saturday evening
before opening up and lifting across northern AZ on Sunday.

Upper level diffluence field will increasing over the area as the
day progresses out ahead of the upper low and combined with
mid-level shear, mid-level drying and -10C 500 mb temp, isolated
to scattered severe thunderstorms are likely later today producing
severe wind gusts and hail. Can't rule out localized areas of
blowing dust, especially across Pinal county impacting I-10. With
PW values up to 1.5", locally heavy rain is likely with the
stronger storms and there could be some training of storms which
increases the flash flood threat, especially across eastern Pinal
and western Graham county where the strongest upper diffluent
field will be centered. WPC QPF has widespread 1" to 2" across the
above mentioned areas that may lead to some significant flows in
washes and creeks. See hydrology section below for additional
details. Flash flood remains in effect for the eastern half of the
forecast area through tonight.

Saturday: Not as active with drier mid-level air overspreading
the area. However any storms that do develop will be capable of
localized heavy rain. Flash flood watch in effect until Saturday
evening.

Sunday: Upper level trof moves across the area with only a slight
chance of storms mainly east of Tucson.

Below normal high temperatures today through Sunday.

Next week: Dry with a warming trend back above normal as we turn
the calendar from September to October.

&&

.AVIATION...Valid through 27/12Z.
SCT to BKN 5-10k ft AGL mainly from KCGZ-KTUS-KOLS line E today
with ISOLD -SHRA/TS thru 26/16z bcmg SCT-NMRS SHRA/TS. Lclzd MVFR
conds nr strongest storms. Gusty and erratic winds up to 45 kts
nr TS. Outside of TSs, SFC winds SLY to SWLY at 10-15 kts with
gusts up to 25 kts. Aviation discussion not updated for TAF
amendments.

&&

.FIRE WEATHER...A low pressure center will be moving through the
region and tapping into above normal moisture. Thunderstorms will
be capable of producing strong to severe level gusty winds, hail,
and heavy rainfall. Rain showers will linger through the weekend.
General winds will be south to southwest through the Saturday
with speeds up to 15 mph and strong gusts up to 25 mph. The above
normal moisture will keep min RHs above 25 percent through most of
the weekend. Temperatures will be around normal through the
weekend. Next week, temperatures will be slightly above normal
with winds less than 15 mph and min RHs 18-22 percent in the
valleys and 25-30 percent in the mountains.

&&

.HYDROLOGY...WPC QPF guidance today has 1" to 2" of rain across
much of the lower San Pedro River in eastern Pinal county and
Aravaipa Valley in western Graham county thanks to the potential
for training heavy showers and thunderstorms. We saw some river
and creek responses yesterday and likely to see more later this
afternoon into this evening. My concern is Aravaipa Creek and
creeks and washes that drain into the San Pedro. Something that
we'll need to keep and close eye on. A second area of concern will
be along the International border of Santa Cruz county where heavy
rain, possibly up to 2" near or just south of the border, may
lead to some significant flow in the Nogales Wash.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Saturday evening for AZZ503>514.

&&

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