359
FXUS65 KPSR 202000
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
100 PM MST Tue Oct 20 2026

.KEY MESSAGES...

- A Pacific low pressure system will bring multiple rounds of
  showers and thunderstorms, mainly across central and eastern
  Arizona, Wednesday and Thursday.

- A Flood Watch is in effect across much of south-central and eastern
  Arizona from Wednesday afternoon through Thursday evening for
  periods of moderate to heavy rainfall leading to flash flooding.

- Quieter weather is expected beginning Friday and lasting through
  the upcoming weekend.

&&

.SHORT TERM /Today through Thursday/...
A very active period is shaping up during the middle portion of
the week as a strong Pacific low pressure system affects the
Desert Southwest, bringing multiple rounds of showers and
thunderstorms. Latest water vapor satellite imagery as well as
objective analysis show the aforementioned low pressure system
centered off the northern CA coast this morning. The low pressure
system is expected to continue to dive southward and set up off
the southern CA coast by this evening. In the meantime, an upper-
level shortwave that was responsible in sparking scattered
convection yesterday across the southeastern half of AZ continues
to lift away from the region with weak subsidence overhead.
Therefore, conditions for this afternoon should remain relatively
quiet with dry conditions prevailing across the region; the only
exception being near the vicinity of the White Mountains where the
combination of sufficient mid-level moisture and orographic lift
will likely spark a few afternoon showers and thunderstorms. With
upper-level heights rising ahead of the digging Pacific low,
afternoon high temperatures today will be a few degrees above
normal as many of the lower desert communities top out around 90
degrees.

As the Pacific low positions itself off the southern CA coast later
this evening, the flow aloft will become established more out of the
south. This will result robust moisture advection with PWAT values
by early Wednesday morning expected to approach 250-300% of normal
and will be near record levels for mid-October. There will also be a
shortwave out ahead of the main Pacific trough that will traverse
the region early Wednesday morning, which may spark areas of
showers, especially across the higher terrain areas. Otherwise, the
main period of impactful weather is likely to occur from Wednesday
afternoon through Thursday. Latest guidance continues to show
moderate instability building across much of south-central and
eastern AZ Wednesday with MLCAPE values approaching 1000 J/KG. This
amount of instability combined with abundant moisture will yield a
favorable environment for thunderstorms to produce heavy rainfall
rates in excess of 1-2"/hr with training of activity likely to occur
and thus heightening the potential for flash flooding. Latest hi-
res guidance shows activity developing Wednesday afternoon across
higher terrain areas east of Phoenix before activity develops into
the south-central AZ lower deserts heading into Wednesday evening
and continuing into the overnight and morning hours of Thursday
as the main upper-level jet streak energy approaches, resulting in
strong divergence aloft. The period between Wednesday evening
through Thursday afternoon will likely be the timeframe with the
greatest threat for widespread heavy rainfall and thus flash
flooding. Latest WPC QPF depicts total rainfall amounts of
1.50-2.50" across the foothills and higher terrain areas north and
east of Phoenix while across the lower deserts, average rainfall
amounts will remain under an inch. However, even across the lower
deserts, especially with any training of activity, there will be
the potential for areas to receive locally higher rainfall amounts
in excess of 1". In coordination with the neighboring offices, a
Flood Watch has been issued across much of south-central and
eastern AZ.

In addition to the heavy rainfall and flash flood threat, another
threat to watch out for will be severe weather. Given the moderate
levels of instability in place combined with strong 0-6 km vertical
shear in excess of 50 kts, the thunderstorms that do develop will
likely have some organization to them with the potential to acquire
supercellular characteristics. In fact, the Storm Prediction Center
has highlighted much of the higher terrain areas east of Phoenix in
a slight risk (Level 2 out 5) for severe weather for Wednesday, with
a marginal risk (Level 1 out 5) across the rest of south-central AZ,
including the Phoenix metro area. The thunderstorms that do develop
will be capable of producing damaging wind gusts, large hail, and
even an isolated tornado given that there will be about 10-20 kts of
low-level shear. The threat of severe weather will also exist into
Thursday as well, especially Thursday morning, as the instability
and shear parameters will still be sufficient.

As the upper trough gradually pushes inland through the southern
Great Basin during the day on Thursday, strong dry air advection
will gradually spread from west to east with rain chances ending
late Thursday afternoon across the south-central AZ lower deserts
and likely take until late Thursday night into early Friday
morning for rain chances to end across the higher terrain areas.

With the main trough axis still remaining far enough west on
Wednesday, afternoon high temperatures will still be near normal as
readings top out in the mid to upper 80s. As the main trough axis
pushes inland through the southern Great Basin on Thursday,
temperatures will be noticeably cooler with afternoon highs topping
out in the low 80s across the south-central AZ lower deserts to low
to mid 80s across the western deserts. Depending on the amount of
cloud cover and rainfall activity, there is the potential that
temperatures in south-central AZ turn out to be cooler than what is
currently forecasted.

&&

.LONG TERM /Friday through Monday/...
Much quieter weather is expected starting Friday with much drier
air in place behind the departing trough. Another follow-on
shortwave is likely to dive in from the northwest late Friday into
Saturday with guidance indicating that this shortwave will become
a cutoff low over AZ as it detaches from the main jet stream
energy. At this time, this feature is likely to be inconsequential
given that there will be an abundance of dry air in place
limiting any additional rainfall potential. Under the general
troughing influence, afternoon high temperatures through the
upcoming weekend will remain below normal. Coolest day is expected
on Friday when much of the lower deserts struggle to reach 80
degrees for afternoon highs.

&&

.AVIATION...Updated at 1825Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
The primary aviation weather concern will be the potential for
showers and thunderstorms Wednesday. Guidance continue to suggest
the development of showers and a few isolated thunderstorms
between 06Z-12Z overnight tonight. Uncertainty in this activity
impacting the terminals remains, with PROB30s in place. The
primary impacts with this would be briefly lowered visibilities
and erratic wind directions. Expect SCT-BKN cloud bases to lower
to around 6-8 kft and may stick around throughout the morning and
afternoon Wednesday. Thunderstorm chances increase at the end of
the period with some potentially becoming strong to severe.

Outside of any thunderstorm impacts, winds will favor a southerly
crosswind component this afternoon, becoming light and variable
this evening through the overnight hours. Speeds will mostly
remain aob 10 kts.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation weather concerns are anticipated through the next 24
hours. Winds at KIPL will favor the S-SE, while directions at KBLH
favor the S-SW. Speeds will mostly remain aob 10 kts with light
and variable winds occurring throughout the period. Cloud bases,
becoming SCT-BKN at times, will remain aoa 7 kft.

&&

.FIRE WEATHER...
Wind speeds today will generally remain light with limited
afternoon gustiness before increasing towards the middle of the
week ahead of an approaching weather system. Mainly dry conditions
will prevail this afternoon before rain chances increase
substantially Wednesday and Thursday with significant rainfall
amounts, in excess of 1.00", likely across the higher terrain
areas of the eastern districts. MinRHs today will range between
30-50% before increasing above 50-60% across the eastern districts
Wednesday and Thursday along with good to excellent overnight
recoveries. Temperatures will be in a near normal range tomorrow
before falling into a below normal category by the end of the week
and lasting through the weekend.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flood Watch from Wednesday morning through Thursday evening for
     AZZ545-547-552-556>558-560>563.

     Flood Watch from Wednesday evening through Thursday afternoon
     for AZZ534-537-538-540>544-546-548>551-553>555.

CA...None.
&&

$$

SHORT TERM...95/Lojero
LONG TERM...Lojero
AVIATION...Smith
FIRE WEATHER...95/Lojero

NWS Phoenix (PSR) Office