Severe Thunderstorm Warning 97, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
1024 AM MST Thu Sep 25 2025

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

&&

.KEY MESSAGES...Active Monsoonal Period Today through Saturday.

-Flash Flood Watch is in effect this late morning 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.

&&

.UPDATE...The main forecast focus for today through the weekend will
be on a notably active Monsoon pattern with favorable ingredients
for heavy rainfall, wind, hail, and lightning.

This morning, cloud buildups are already brewing over mountain
peaks signifying that storms will more than likely be trying to
get going earlier in the day (between 10 am and Noon). Storms
will tend to form in the mountains in the Tucson Metro eastward
during this timeframe before growing upscale and moving into the
valleys once they tap into a more robust dynamic and thermodynamic
environments. The more favorable environment may allow for
activity to continue into the evening hours.

The primary feature responsible for the anticipated 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 will be increasing, lending a boost to individual
thunderstorm intensity into the afternoon hours. Environmental
wind shear will also be increasing as the day progresses and the
upper low nudges closer which will help to increase individual
thunderstorm longevity and intensity. Stronger storms influenced
by this ingredient 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 nudges closer, we will continue to see an
southerly flow which will help sustain moisture from the Gulf of
California. Precipitable water on yesterday afternoon's upper air
sounding at KTWC was around 1.1 inches, a bit drier than I was
anticipating. Latest GOES Total Precipitable Water shows from 0.95"
to 1.2" of precipitable water. Dryer air aloft could enhance
thunderstorm downdrafts and outflows leading to locally strong to
severe winds and blowing dust. On the flip side of that, there is a
delicate balance where we have sufficient moisture in place
supportive of heavy rainfall with rates of 0.75-1.00"+ per hour. A
Flash Flood Watch is in effect from 11 am this morning 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.

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, wind, hail, dust, and frequent lightning.

&&

.PREV DISCUSSION... /issued 251 AM MST Thu Sep 25 2025/

This will be an active monsoon pattern starting today through most
of the weekend. The low pressure center has entered California this
morning and it will drop towards Arizona through today and tomorrow.
This system will bring dynamics, mixture of upper level dry air, and
tapping into deep monsoon moisture for scattered to widespread
thunderstorms and rain showers. Expect strong afternoon
thunderstorms and overnight to early morning rain showers with
embedded thunderstorms through Saturday night. There will be three
main hazards for today, tomorrow, and Saturday: gusty winds, hail,
and heavy rainfall.

Gusty winds: Forecasted CAPE values for today and tomorrow will
generally be 1000-1600 J/kg. Friday values will be on the higher end
of the range. Plus, there will be high lapse rates and PWAT values
will be above 1.30 inches. This points to a favorable environment
for strong gusty winds today and tomorrow afternoon.
Not to mention, the low will bring necessary dynamics, diffluence
aloft, and dry air in the mid-to-upper levels to sustain organized
thunderstorms...potentially long-live storms.

Hail: Small hail to an inch size at the most will be possible today
and tomorrow. Steep lapse rates and the high CAPE values as
mentioned before will favor hail growth. Plus, our atmospheric
temperatures will be cooler than previously seen this past summer.
Shear will be 15-30kts which is pretty impressive for our neck of
the woods. Therefore, a better environment for hail to form and less
chance for substantial melting. Expect thunderstorms to grow rapidly
this afternoon with hail first, then becoming more heavy rainers by
the end of the day.

Heavy rain: We are sitting above normal monsoon moisture for this
time of year. This has been persistent for over a week, but now we
are adding more dynamics for lifting more of that deep moisture for
heavy rainfall. Tomorrow, the low pressure center will tap into a
stream of deeper moisture from the Mexico. Our IVT values will be
300-400 kg/m(s) leading to very saturated atmosphere for scattered
to widespread thunderstorms and rain showers.
Thunderstorms will be capable of producing 0.75"-1"+ of rain in a
short amount of time. Higher amounts can be expected in the
mountains. Therefore, a Flash Flood Watch is in effect today through
Saturday. The bulk of the heaviest will be today and tomorrow.
Overnight rain showers will persist on and off tonight and again
tomorrow night. If there is a little more clearing of the clouds by
the late Friday 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. Turn Around, Don't Drown!

&&

.AVIATION...Valid through 26/12Z.
SKC to FEW clouds AOA 10-12k ft AGl this morning. After 25/17Z
clouds will increase to 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. SFC winds will be south- southeasterly at 8-
15 kts this morning. By 25/18Z, south-southwest winds at 10-15 kts
with gusts up to 25 kts through the remainder of the forecast
period. Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Active monsoonal pattern today 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 today 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.
Plus, 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 from 11 AM MST this morning through Saturday evening for
AZZ503-504-506>514.

Flood Watch from Friday morning through Saturday evening for AZZ505.

&&

Update...DVS

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