Debris Flow in Arizona, September 25, 2025

Area Forecast Discussion
National Weather Service Tucson AZ
138 PM MST Thu Sep 25 2025

.SYNOPSIS...A weather system will bring periods of rain showers and
thunderstorms this afternoon through the 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.

&&

.KEY MESSAGES...Active Monsoonal Period This afternoon through
Saturday.
-Flash Flood Watch is in effect through Saturday evening.
-Thunderstorms will be capable of producing strong to severe gusty
winds, hail, and heavy rainfall. The strongest of storms will be in
this afternoon and Friday afternoon.

&&

.DISCUSSION...The main forecast focus for this afternoon through
the weekend will be on a notably more active Monsoon pattern with
favorable ingredients for heavy rainfall, wind, hail, and frequent
lightning. Storms have been forming mainly from the Tucson Metro
eastward and have been quickly growing upscale after tapping into a
more robust dynamic and thermodynamic environment. The more
favorable environment may allow for showers and a few thunderstorms
to continue into the evening hours. Most of the activity so far has
been just east of the Tucson Metro, but we have also been seeing an
uptick in activity on the west to northwest side of Tucson as well.
Storm motions have been fairly steady pace so far, but more intense
cores have been responsible for high rainfall rates between 1 to 1.5
inches per hour. At rainfall rates over 1 inch per hour, urban
and rural flood prone locations have a much higher likelihood of
experiencing impacts from flash flooding.

The primary feature responsible for the uptick in Monsoonal
thunderstorm activity is a strong upper low which will be nudging
further inland from the California coast. As this transition
occurs, the air will have a tendency to rise over southeast
Arizona. Strong dynamic support in the form of diffluence aloft is
increasing, lending a boost to individual thunderstorm intensity.
Environmental wind shear is also increasing as the upper low
nudges closer which will continue to help increase individual
thunderstorm longevity and intensity. Stronger storms influenced
by these ingredients are more likely to produce heavier rainfall
rates, hail, strong winds, and more frequent lightning.

The secondary feature responsible for the anticipated uptick in
Monsoonal thunderstorm activity is a plume of atmospheric moisture.
As the aforementioned low continues to nudge closer, we will see a
gradual uptick in deep southerly flow to help sustain above normal
moisture from the Gulf of California. While precipitable water on
yesterday afternoon's upper air sounding at KTWC was around 1.1
inches, a bit drier than I was anticipating, this morning's 18Z
sounding had around 1.4 inches. Dry air aloft mixing into the upper
part of the storms could enhance thunderstorm downdrafts and
outflows leading to locally strong to severe wind gusts and blowing
dust. On the flip side of that, we have sufficient moisture in place
supportive of heavy rainfall with rates over 1 inch per hour. A
Flash Flood Watch is now in effect through 8 pm Saturday evening.
Expect water rises in rivers and washes with low water crossings
becoming dangerous and impassible. Remember: Turn around, don't
drown!

Tomorrow, the low pressure center will tap into a stream of deeper
moisture from northern Mexico and the Gulf of California. IVT values
will be in the 300-400 kg/m(s) range resulting in a very saturated
atmosphere supporting scattered to widespread showers and
thunderstorms. Stronger storms will again be capable of producing
0.75"-1"+ of rain in a short amount of time. Higher amounts can be
expected in the mountains. Overnight rain showers will persist on
and off again tomorrow night. If there is a little more clearing of
the clouds by the late tomorrow morning, we can expect a rapid and
strong thunderstorm growth due to heating (increasing the CAPE).
Rain showers and thunderstorms will be waning on Saturday, but with
how saturated the soils will be, it will continue to pose flooding
issues through Saturday.

Overall, this is looking to be a surprisingly active period for the
Monsoon with multiple hazards on the table over the next few days
including flash flooding, strong winds, blowing dust, hail, and
frequent lightning.

&&

.AVIATION...Valid through 27/00Z. SCT to BKN mainly KTUS eastward
AOA 5-10k ft AGL with scattered to widespread TSRA and SHRA
through most of the forecast period. Gusty and erratic winds to
around 35-45+ kts will accompany thunderstorms. Outside of
thunderstorms, SFC winds SLY to SWLY at 10-15 kts with gusts up to
25 kts. Scattered to numerous storms are expected again tomorrow
afternoon as well. Aviation discussion not updated for TAF
amendments.

&&

.FIRE WEATHER...Active monsoonal pattern this afternoon through the
weekend, bringing thunderstorms and wetting rain showers. 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
southerly to southwesterly this afternoon 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.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Saturday evening for AZZ503-504-506>514.

Flood Watch from Friday morning through Saturday evening for
AZZ505.

&&

DVS

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