485
FXUS65 KTWC 282207
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
307 PM MST Mon Sep 28 2026

.SYNOPSIS...
Southeast Arizona remains on track to be quite busy over the
next couple days as a strong trough of low pressure along the
West Coast makes its way through Arizona into Tuesday. This
system will interact with deep tropical moisture moving into the
area from the south and southwest to produce widespread showers
and thunderstorms across the region. These storms will be
capable of producing heavy rainfall that could lead to flash
flooding concerns. Thereafter, dry conditions are forecast for
the second half of next week. High temperatures will lower to
5-10 degrees below normal Monday through Wednesday, then warming
back up to near normal late next week.

&&

.UPDATE...
Updated the Hydrology Seciton as of 300 PM.

&&

.DISCUSSION...
Overall the forecast remains on track with only
minor differences in placement of features and timing. Flash
Flood Watch remains in effect until 11 PM Tuesday.

As for the MesoA...the fire hose of mid and upper level
moisture continues to stream into the region with a steady climb
in SFC TDs. The only remaining layer of dry air is situated
over portions of south-central AZ, roughly several hundred feet
off the SFC, to 2-3k ft AGL. However, this is now rapidly
moistening as a weak SFC low is situated across SEAZ with
increasingly favorable upper level dynamics overspreading the
region. 500mb height falls are also evident, with cooling mid
level temps to support an increase in elevated instability this
evening into the overnight. This is leading to an overall upward
trend in coverage, with more lightning beginning to be evident.
MUCAPE roughly 100-500 j/kg over much of SEAZ with pockets of
500-1000 j/kg over portions of eastern AZ. With the better
instability and increasingly favorable upslope flow, we are
seeing an increase in rainfall rates for the White Mtns.

Widespread 1-2" of rain remains likely area wide (exception of
Western Pima County 0.25-0.75"), with a continued signal of
2-4"+ for the Sky Islands NE of Tucson and the White Mtns. In
fact, some 75th/90th percentile REFS/RRFS guidance suggest 4"+
is possible in the White Mtns, possibly extending SW into the
Sky Islands. Given the duration, possible training, and threat
for thunderstorms, this seems reasonable. Another thing of note,
is the EFFI and Shift of Tales, which supports the potential for
these higher end totals. Over the past 24 hrs, this signal has
only grown. While some of the CAMs have been bouncing around a
bit this morning and early afternoon, there are no changes in
forcing mechanisms and forecast rainfall amounts from the
EPS/GEFS/AIGFS/AIECMWF..etc. All of the guidance continue to
paint robust rainfall totals across much of SEAZ.

Now zooming into the Tucson Metro, generally 0.75-1.75" is
expected for the event. The 75th/95th percentile (reasonable
worst case) continues to paint totals of 2-3"+. This includes
the 12Z HREF, showing similar amounts and continued 10%+ probs
for 3" in a 6hr period Tuesday morning and/or afternoon. Timing
continues to show a gradual increase in coverage this evening
into Tuesday early morning with peak potential Tuesday morning
into Tuesday afternoon. While these higher end totals are an
increasingly distinct possibility, it will favor upslope regions
near the Catalina/Rincon Mtns and/or repeated rounds (training)
of heavier showers/storms. Another thing to consider is this
will not be your classic Monsoonal thunderstorms. It will be
deep tropical moisture with widespread showers and isolated to
scattered thunderstorms, which may persist off and on for a
longer duration into Tuesday. With every event, there will be
areas that under perform and over perform, dependent on where
these heavier bands of rainfall setup. WPC maintains a Slight
Risk over much of the area for today and has a Moderate Risk for
areas from Sells east, including Tucson, to the NM border on
Tuesday.

One side-note, severe storms will be possible Tuesday...but
should be considered a secondary threat compared to the risk of
flash flooding. As the upper-level low tracks across the area,
decent forcing, marginal CAPE and abundant shear will be
present. Therefore, expect strong to perhaps isolated severe
storms...with brief transient low-top supercells also possible.

Otherwise, lingering instability and moisture will be present
Wednesday for additional shower activity, but drier air will
quickly lower precipitation chances through the day. A ridge of
high pressure then builds into the Desert Southwest the second
half of the work week, shutting down thunderstorm activity.

&&

.AVIATION /21Z MONDAY THROUGH 21Z TUESDAY/...
SCT-BKN 5-8k ft AGL with BKN-OVC 10-14k ft AGL, lowering into
the evening and overnight. -SHRA/-TSRA expanding in coverage
and intensity this afternoon and overnight. Generally VFR
through the early evening hours, with brief MVFR possible under
heavier showers/storms. MVFR CIGs possible by late Monday
evening into Tuesday morning with VIS down to 3-5 miles. There
is even a slight chance of brief IFR for KTUS/KOLS tonight into
Tuesday. Mtn obscuration likely by Monday evening into Tuesday.
Gusty winds to 35 kts may accompany stronger storms. Aviation
discussion not updated for TAF amendments.

&&

.FIRE WEATHER...
Southeast Arizona is still on track to be quite busy over the
next couple of days as a strong trough of low pressure along the
West Coast makes its way through Arizona into Tuesday. This
system will interact with deep tropical moisture moving into the
region to produce widespread showers and thunderstorms. These
storms will be capable of producing heavy rainfall that could lead
to flash flooding. Thereafter, dry conditions are forecast for
the second half of next week. High temperatures will lower to 5-10
degrees below normal through Wednesday, then warming back up to
near normal late next week. Minimum RH values 35-55 percent in
valley locations and 50-75 percent in the mountains today through
Wednesday, generally lowering to 15-25 percent the second half of
the week as drier air moves into the region. 20-foot winds will
be less than 15 mph into mid-week, then gusty east to southeast
winds possible the second half of the week. Breezy southwest to
west winds will be possible along the surface front as it moves
across the area Tuesday.

&&

.HYDROLOGY...
As of 245pm, there are no responses on area stream gages.
However, moderate to locally heavy rain continues to stream into
portions of eastern AZ, especially for the White Mtns. Totals
in the higher elevations are generally 0.1-0.6" with lesser
amounts in the lower elevations.

Recent coordination with the Colorado Basin River Forecast
Center suggest some locations could reach or exceed Action
Stage. Comparing some of the more recent rises in the Pantano,
Brawley, Santa Cruz etc, this would lead to issues for flood-
prone low water crossings. So it is reasonable to expect similar
conditions with this event. For example, at 6 feet, the Pantano
@ Vail results in low water crossings flooded downstream.

Sabino Creek near Sabino Dam is forecast to remain below Action
Stage, but Minor Flood Stage is possible if the basin receives
some of 75th/90th percentile probabilistic rainfall scenarios.
With these higher amounts, flows from the Pantano, Tanque Verde,
Sabino would result in rises down the Rillito flooding any low
water crossings.

For the Gila River Basin, there is potential for heights to
approach Action Stage or exceed at Duncan/Solomon/Calva. For
Solomon, the forecast is just below Minor Flood Stage and it is
reasonable to see Minor Flood Stage exceeded. For Duncan, It
would take nearly double the forecast rainfall for the Gila
Basin upstream or near the Duncan gage to reach Minor Flood
Stage. So while it is unlikely, 75th/90th Percentile rainfall
amounts from the REFS/RRFS/Other CAMs could start to bring this
scenario into play. With a more steady, at times heavy rainfall,
we will be able to see these rises and adjust the forecast
accordingly with the RFC.

The Santa Cruz River should see elevated flows, but the current
forecast generally keeps areas below Action Stage. The one
exception is the Santa Cruz River at Trico. This is where the
gage could approach Action Stage, especially if some of the
higher totals materialize. This would result in many low water
crossings flooded along the Santa Cruz near Trico and
where/upstream of the Btawley Wash west of the Tucson Mtns down
to Highway 86.

Elsewhere, Flash Flooding is possible at any of the known flood
prone locations and persons that rely on these know problem
areas for commuting should be prepared for alternate routes or
delays Tuesday, possibly into Wednesday morning. The morning
commute across the Tucson Metro Area may especially be
problematic Tuesday morning.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Tuesday evening for AZZ501>515.

&&

$$

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