Gleeson, Arizona Weather Forecast Discussion

831
FXUS65 KTWC 220906
AFDTWC
Area Forecast Discussion
National Weather Service Tucson AZ
206 AM MST Mon Sep 22 2025
.SYNOPSIS...Ample moisture across the region will result in
isolated to scattered showers and thunderstorms each day through
the middle of next week, with stronger storms, potentially with
heavy rainfall, impacting Southeast Arizona late in the week.
Temperatures will be running 3-6 degrees above normal through
Wednesday.
&&
.DISCUSSION...Today (September 22, 2025) we finally make our way
into astronomical autumn with the arrival of the Autumnal Equinox
occuring at 11:19 AM MST this morning. This event usually marks
one of the last chapters of a normal Monsoon season. Typically
this time of year we have already seen strong westerly flow
scour-out the surface moisture from the majority of Southeast
Arizona with the presence of a strong upper-low moving into the
region from the west. This year is no different...even if it is a
little delayed. We are starting to see the signals of the end,
but in our case, it may not scour-out the moisture right away and
we may actually see a ramp-up in activity at the very end of the
season.
The upper pattern this morning is defined by high amplitude trough
of low pressure extending SW off the California Bight and a
subtropical high centered over the Chihuahua Mexico. The GOES
precipitable water imagery this morning still shows a contrast in
moisture across the region, with drier precipitable water values
(between 1.05 and 1.25 inches) across eastern Santa Cruz, Cochise,
Graham and Greenlee counties and higher values (1.40-1.75 inches)
across the lower deserts of southeast Pinal, central and western
Pima county.
Unlike yesterday where there was plenty of lift aloft to assist in
thunderstorm development and increased coverage of storms, today
that is absent and we will likely have to just work with the
recycled moisture in place and decent surface heating. Therefore,
expect a generally isolated to low-end scattered thunderstorm
coverage east and southeast of Tucson this afternoon, with the
best chances occuring across Cochise county...closer to the weak
diffluent flow around the subtropical high in Chihuahua Mexico. In
fact, the next few days, storm coverage will be limited with a
shortwave ridge building in across the area and we see afternoon
temperatures bump-up to the century mark here in the Old Pueblo.
Things become really interesting as we make our way into the
second half of the week and into the weekend. The 22/00Z GEFS and
ECMWF ensemble members are consistent with the evolution of the
upper-low near the California Bight this morning moving north into
central CA Wednesday...then slowly ESE into the western portions
of Arizona by the end of the week. The combination of strong
divergence/deformation in the mid-upper levels over SE AZ on
Thursday and especially Friday, moderate 0-6km shear, and the
potential for strong advection of dry-air entraining into the
middle and upper-levels of storms (increasing the potential for
strong to severe downdraft/outflows due to enhanced evaporation)
will increase the potential for more widespread severe weather
across our neck of the woods. In addition to the aforementioned
mechanisms, decent moisture will remain in place for heavy
rainfall across Southeast Arizona as well. In fact, the 22/00Z NBM
Grand Ensemble probability of 24-hour rainfall exceeding an inch
is between 5-10 percent across the SE half of our forecast area as
early as Thursday, Friday and extending into Saturday. After
coordination with the WPC this morning, they have decided to put
portions of SE AZ into the slight risk category (at least 15
percent chance of rainfall exceeding flash flood guidance within
25 miles of a given point) for both Thursday and Friday.
At this point, there is still some variance on what to do with
the upper-low moving into next weekend. Most solutions are now on
track to keep the upper-low around SE AZ through the weekend. My
mental model suggests that we will see more of a severe weather
risk Thursday and especially Friday, transition to more flash
flooding risk Friday and Saturday.
Lots can still change with the intensity/track/timing of the
upper-low over the next several days...so there is caution in
jumping-on this solution with both feet this early. However, it is
still prudent to start the discussion on its potential. In any
event, the end of the Monsoon season is usually preceded by a
strong event like this...even if it is slightly different than
usual.
&&
.AVIATION...Valid through 23/12Z. SKC-SCT clouds 10k-15k ft AGL
through 18Z, then SCT-BKN clouds 6k-10k ft AGL BKN 12k-15k ft AGL
through the end of the valid period. Isolated to scattered -TSRA
this afternoon into the early evening especially to the E and SE
of Tucson. Gusty and erratic winds to around 35 kts will accompany
thunderstorms. Outside of thunderstorms, SFC winds generally less
than 12 kts, favoring NWLY-WLY direction during the afternoon
hours and ELY/SELY overnight and morning hours. Aviation discussion
not updated for TAF amendments.
&&
.FIRE WEATHER...Above normal amount moisture across the region
will result in isolated to scattered showers and thunderstorms
each day through the middle of next week. Stronger storms are
expected late in the week. Minimum RH values between 20-40
percent today, lowering to 20-30 percent by the middle of the
week. 20-foot winds will be westerly 8-12 mph with some
afternoon/early evening gusts to around 20 mph. Breezy southwest
winds 10 to 20 mph will be possible Thursday and Friday.
Temperatures will be running 3-6 degrees above normal through
Wednesday and returning to near normal by the end of the week.
&&
.TWC WATCHES/WARNINGS/ADVISORIES...None.
&&
$$
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NWS TWC Office Area Forecast Discussion