269
FXUS65 KPSR 281055
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
355 AM MST Wed Oct 28 2026

.UPDATE...Updated Aviation Discussion.

&&

.KEY MESSAGES...

- Tranquil weather conditions will continue today and Thursday
  with temperatures within a few degrees of daily normals.

- An unsettled weather pattern will return to the region from
  Friday through Sunday resulting in much cooler temperatures and
  enhanced chances for widespread rainfall.

- Embedded thunderstorms and heavier rainfall may affect some
  areas resulting in localized minor flooding, specifically
  Friday and Sunday.

&&

.SHORT TERM /Today through Thursday/...
The current quiet weather conditions will abruptly come to an end
by Friday morning as the first of two weather systems affects the
region. Until then, other than some increasing higher level
clouds, not much will change. Our current westerly dry flow will
slowly shift more out of the southwest later today as a large
Pacific trough takes shape off the West Coast. The split flow
pattern supporting a strong upper low along the British Columbia
coast and a second much weaker disturbance well west of California
will stay distinct over the next couple of days, but that will
change by the weekend. In the near term, the air mass currently in
place will hold fast, but with some upper level moisture shifting
in from the northwest bringing some fairly thin high clouds
today. A second batch of high level moisture and clouds will move
in directly from the west on Thursday, while increasing southerly
flow in the low levels will also begin to advect subtropical low
level moisture northward into south-central Arizona. Despite the
increased cloudiness today and Thursday, daytime highs should hold
right near averages for late October.

&&

.LONG TERM /Friday through Tuesday/...
Forecast confidence is increasing for two separate weather
events, Friday through Sunday, both of which could bring impactful
rain amounts to portions of central Arizona. The increasing
moisture on Thursday into Friday is expected to peak during the
daytime hours Friday with PWATs reaching 225-250% of normals and
low level mixing ratios of 8-9 g/kg, centered over Maricopa and
Pinal Counties. Forecast soundings show the moisture will be quite
deep, extending fairly evenly throughout the column with modest
elevated MUCAPE up to around 500 J/kg. Strong jet forced ascent is
also seen moving over the area on Friday combining with the
modest southerly low level upslope ascent.

Showers are expected to begin developing a few hours after
midnight Thursday night before increasing in coverage and
intensity by around sunrise Friday. Both the GEFS and EPS show QPF
maxima centered from Phoenix to around the Tonto NF with amounts
averaging 0.50-0.75". Considering the available instability and
steep mid-level lapse rates, we should expect scattered
thunderstorms too, mostly embedded with the shower activity.
General rainfall rates are likely to mostly fall within a
0.05-0.2" range, but it would not be surprising to see very
localized rates of around 1" per hour within any thunderstorms.
These higher rainfall rates should pose a minor flood threat on
Friday, mainly affecting area washes and the typical unbridged low
water crossings. There is potential for higher rainfall amounts
with the NBM 75th percentile showing amounts between 1.5-2.0"
centered over the foothills just north of Phoenix. A Slight Risk
for excessive rainfall remains on the updated WPC ERO and based on
the model run-to-run consistency, Flood Watches are likely to be
needed for at least north-central and northeast Maricopa County.
The shower and thunderstorm activity should also extend eastward
into Gila County later on Friday before quickly winding down
Friday evening as we lose upper level support.

The large scale trough will remain positioned along and into the
Western U.S. into Saturday with the main low center near the
Pacific Northwest, but another strong shortwave diving into the
base of the trough is expected to help bring another low
developing off the coast of California Saturday night into Sunday.
Saturday is expected to be mostly a dry day, but there will still
likely be some isolated upslope type showers mostly focused over
the high terrain north and east of Phoenix. However, this will
just be the calm in between weather systems with the second one
moving in starting late Saturday night expected to be stronger
and more impactful.

By Saturday evening, guidance shows a strong atmospheric river
(AR) starting to make its way into northern Baja and far southern
California. By Sunday morning, the full force of this AR is
expected to be directed at southern Arizona and northern Sonora
Mexico with PWATs increasing to 275-325% of normal, or right near
record levels for our area this time of year. Compared to model
runs 24 hours ago, the latest depicts even better moisture and a
stronger system. Upper level forced ascent should begin to
increase over our area by Saturday night, coinciding with
increasing low and mid level upslope south southwesterly flow.

A large area of light to moderate rain is likely to develop ahead
of the approaching weather system late Saturday night into early
Sunday affecting the bulk of the region, particularly central and
eastern Arizona. Although model uncertainty is decreasing, there
is still a good deal of uncertainty in expected rainfall amounts.
This looks to be a longer duration light to moderate rain event
with limited instability keeping rainfall rates under 0.5" per
hour and even lower across the lower deserts. However, because
this may end up easily being a 6-12 hour rain event, the rainfall
could quickly add up, especially into the higher terrain
northeast of Phoenix where upslope forcing will be maximized. The
latest NBM QPF shows a wide range of potential amounts with
averages around 1" in the Phoenix area to upwards of 1.5-2" over
the high terrain northeast of Phoenix. The lower end of the
forecast shows amounts closer to 0.5" and 1-1.5" respectively, but
the higher end potential is more like 2-3". Considering this will
be the second rain event within 3 days, any additional moderate
to heavy rain could compound any flooding issues that occur
Friday. Rain chances may linger through Sunday night across
portions of central and eastern Arizona, but by mid to late Monday
morning, much of the area should be drying out as the system is
quickly pushes east of the area.

Temperatures during this unsettled weather pattern will fall below
normal starting Friday with highs dropping to between 75F-80F before
bottoming out closer to 70F highs for Sunday and/or Monday. It would
not be out of the question for some lower desert communities stuck
in prolonged rain Sunday to not even breach the 70F threshold. NBM
guidance shows fairly large forecast temperature spread for the
first half of next week as the large scale troughing may linger near
the region with depressed H5 heights for a couple more days. If this
happens, highs will very slowly creep upward through the low to mid
70s, whereas if the ridge quickly takes over, highs closer to 80F
would be more common by next Tuesday or Wednesday.

&&

.AVIATION...Updated at 1100Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT; and
Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation concerns will exist through Thursday morning
under periods of thicker high cirrus. Winds will remain light,
generally AOB 8 kts, with a tendency towards SE`rly to S`rly
directions during the daytime hours today. Confidence is low on a
full westerly wind shift at the Phoenix area terminals, though a
light W/NW component could develop for a few hours around sunset.
Otherwise, expect extended periods of variability to nearly calm
conditions.

&&

.FIRE WEATHER...
Dry conditions with near normal temperatures will persist through
Thursday under increasing cloud cover. Minimum humidity levels
will fall into a 20-40% range following fair to good overnight
recovery of 40-60%. Winds will remain unseasonably light with
minimal upslope breeziness. Beginning Friday, several days of
unsettled weather will affect the region with much higher
humidities, widespread rainfall, and cooler temperatures.

&&

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

$$

SHORT TERM...Kuhlman
LONG TERM...Kuhlman/18
AVIATION...Whittock
FIRE WEATHER...Kuhlman