257
FXUS65 KPSR 270600
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1100 PM MST Sat Sep 26 2026

.UPDATE...Updated 00Z Aviation Discussion.

&&

.KEY MESSAGES...

- Enhanced moisture aloft will surge over the region today and
  Sunday resulting in increasing clouds and eventual chances for
  light rain during the latter half of the weekend.

- Rain is likely across much of Arizona to start off the workweek
  with the possibility of some strong to severe thunderstorms by
  Tuesday. Moderate to heavy rainfall will also be possible,
  particularly across the Arizona high terrain.

- Temperatures will cool to below normal during the first half of
  next week with highs only in the eighties for much of the lower
  deserts.

&&

.SHORT TERM /Today through Sunday/...
Relatively flat ridging is currently in place over the Desert
Southwest as the sub-tropical high sits over Southern Texas and
Northern Mexico. Meanwhile, Hurricanes Odalys and Polo continue
their residence in the Eastern Pacific. WV imagery shows a sheared
off moisture plume from the former of these Tropical Cyclones
currently progressing toward the Baja Peninsula and Southern
California. This enhanced moisture aloft will eventually spread
over the Desert Southwest this evening into tonight, leading to a
noticeable increase in cloud cover. This will be the first
indication of a drastically changing pattern for the region as we
go through the next few days. Before we get to those changes
though, mostly sunny skies and temperatures in the upper 90s to
around 105 degrees will be common across the lower deserts this
afternoon.

By Sunday morning, saturated moisture profiles above the 600mb
level and ascent associated with an approaching Pacific cold-
core will support the presence of light shower activity across
the region, beginning in Southeast California before spreading
eastward toward South-central Arizona later in the day. However,
a decently dry sub-cloud layer will be in place, with dew point
depressions near the surface around 20-25 degrees, so pinpointing
exactly where rain may reach the ground will be easy task. Not to
mention the added uncertainty of knowing exactly where bands of
rain will setup and progress. Nonetheless, the region is blanketed
in PoPs between 10- 30% as moisture flux will be strong and
persistent enough that seeing shower activity appears all but
certain. As mentioned above, the spatial uncertainty of what
locations will actually see measurable rainfall is why PoPs are
not any higher. For the areas that do measure, it likely will not
amount to much. QPF projections indicate that few hundredths is
the most likely outcome, with some isolated higher amounts around
0.10" at most if showers track over the same areas. The increase
in clouds will drop temperatures compared to what is expected this
afternoon, but a high degree of exists that lower elevations
readings will max out near normal in the middle to upper 90s
regardless.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Model guidance is still pointing toward the potential for a
significant weather event occurring early next week as a strong
Pacific low pressure system moves into the region at the same time
moisture from TC Polo is forecast to advect northward into
Arizona. There have been some changes within the model runs over
the last 24 hours, noticeably with the low deepening further on
Monday and potentially becoming a slow moving closed low for
Tuesday into early Wednesday. The latest NHC track of Polo still
keeps it well to our south and southeast with its eventual remnant
circulation missing Arizona. However, it is still expected to
heavily influence our low and mid level moisture which will be one
of the key ingredients needed for a big rainfall event.

Model clusters heavily favor a closed low forming to our northwest
going into Monday with further strengthening possible as it moves
into our region on Tuesday. This deepening would result in a
slower moving system which has pushed back the timing of the best
rain chances, now most likely occurring late Monday night and/or
on Tuesday. Scattered showers and isolated thunderstorms are still
expected during the first half of Monday, but they should favor
the central and eastern Arizona high terrain. Pockets of moderate
rainfall will also be possible Monday, but the heavy rainfall
threat seems to have pushed back to Monday night and Tuesday. WPC
still maintains a Slight Risk for excessive rainfall for Monday
and Monday night for most of Arizona, minus the southwestern
portion.

Assuming no big changes with the timing and the strength of the
low, Tuesday now looks to hold our best chances for widespread
rainfall, likely moderate to heavy at times, and potentially a few
discrete severe thunderstorms. As the Pacific low slowly moves
into our region, strong 35-55 kt mid level south to southwesterly
flow will support very good upslope forced ascent into central and
eastern Arizona. At the same time, we are expecting to fall under
strong difluence aloft from as early as Monday evening through
Tuesday evening. One of the more uncertain ingredients for the
severe storm potential is the amount of instability that will be
available. Although model guidance suggests CAPE will struggle to
top 500 J/kg, it seems very possible a narrow corridor of better
instability closer to 1000 J/kg may develop later on Tuesday. If
all the ingredients come together, a few severe storms producing
damaging winds and large hail will be possible.

The latest QPF amounts have risen over the south-central Arizona
lower deserts with average amounts closer to 1.0" in the Phoenix
area to anywhere from 0.15-0.5" across the western lower deserts.
The highest storm total rainfall amounts are expected over the
high terrain north and east of Phoenix with current forecast
average amounts of 1-2". Locally higher amounts will occur due to
training of showers and storms with some areas potentially
reaching 3" for the two day period. The latest NBM guidance shows
probabilities of 2" as high as 55% for the higher terrain north
and east of Phoenix to 20% within Phoenix. NBM probabilities of
getting 3" of rainfall lower to 10-20% for higher terrain areas.
We will have to wait and see if these rainfall probabilities hold,
although even at this point it seems quite likely Flood Watches
will be needed for later Monday and Tuesday.

Due to the likely deeper and slower progression of the Pacific
low, rain chances now extend through Tuesday night for the eastern
half of Arizona and we anticipate PoPs to increase on Wednesday
if the timing holds. Any rainfall Tuesday night and Wednesday
should be more scattered, but we can`t rule out a few
thunderstorms producing additional localized moderate rainfall.
If the cold core of the low hangs out over our area into Wednesday
afternoon, we could easily see some afternoon storms that produce
small hail. For the western portions of our area on Wednesday,
drier air is expected to quickly move in leaving clearing skies
and quiet conditions.

Temperatures during the first part of next week will definitely
cool off with highs falling into the 80s for the south-central
Arizona lower deserts to the upper 80s and lower 90s for the
western lower deserts. After the Pacific low finally ejects to the
east late week, upper level ridging is favored to move in from the
west leading to gradual warming with highs potentially reaching
100 degrees again by next weekend.

&&

.AVIATION...Updated at 0600Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major weather concerns are expected through most of tomorrow
afternoon. VCSH/SHRA will start to move in around or after 23Z
tomorrow and has been included in all four TAFs with this package.
Overall winds will tend to follow their typical diurnal tendencies
with extended periods of light and variable winds during the
diurnal transitions through tomorrow afternoon. Wind speeds will
generally remain aob 10 kt through tomorrow afternoon. BKN high
clouds will be common through the period with increasing mid level
clouds (with bases aoa 10-15 kft) moving in starting tomorrow
morning. VCSH/SHRA will start to move in around 23Z tomorrow, with
a TEMPO for -SHRA from 00-04Z for KPHX and KIWA and 01-05Z at KSDL
and KDVT. Periodic light showers are expected to continue into the
overnight hours. While there will be light showers around the
Phoenix Metro tomorrow night, a lot of the activity may fall as
virga, especially initially. Some periodic gusts into the upper
teens are also possible with the showers.

Southeast California/Southwest Arizona including KIPL and KBLH:
The possibility of VCSH/SHRA will be the main aviation concern in
the upcoming TAF period. Current SW`rly winds at KIPL will go
S`rly during the overnight hours. At KBLH, winds will continue to
fluctuate between S`rly and SW`rly, with a brief period of NW`rly
possible early tomorrow morning. Wind speeds at both terminals
will generally be aob 10 kt through the period. VCSH/SHRA will
start to move into KIPL`s area by mid-morning and therefore VCSH
has been introduced into this TAF package beginning at 15Z.
Confidence in -SHRA is low (less than 30%) so that has been left
out of the TAF for now, in fact most rain may fall at virga or
remain SE of the terminal. At KBLH, any rain tomorrow is expected
to remain to the south and east of the terminal with better
chances for VCSH/SHRA at the terminal arriving after 06Z
tomorrow. BKN/OVC high clouds will continue with FEW-SCT mid level
clouds moving in tomorrow morning.

&&

.FIRE WEATHER...
Dry conditions will prevail today before higher level moisture
moves in on Sunday resulting in chances for mostly light rain.
MinRHs today will bottom out today at 15-20% across the lower
deserts to 20-30% over the eastern Arizona high terrain. Slow
improvement in RHs is expected on Sunday before much higher
moisture levels move in on Monday as rain chances quickly
increase and expand. Winds will generally follow the typical
diurnal tendencies with limited to no afternoon gusts this
weekend. Rain and much cooler weather are expected for Monday and
Tuesday with heavy rainfall a decent possibility across the
eastern districts. Conditions should eventually dry out later
next week with temperatures returning to near normal.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

SHORT TERM...RW
LONG TERM...Kuhlman
AVIATION...Berislavich
FIRE WEATHER...Kuhlman