575
FXUS65 KPSR 281929
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1229 PM MST Wed Oct 28 2026

.KEY MESSAGES...

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

- An unsettled weather pattern will return to the region 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/...
Semi quasi-zonal aloft continues over the region today and will
shift out of the southwest starting Thursday. Both directions
will keep calm weather, dry conditions, and near normal
temperatures over the region today and Thursday, while the next
weather system is at the doorstep of the region and is expected to
impact the region as early as Friday morning. Until then, as
previously mentioned, quiet weather is expected. Heights aloft are
near the NAEFS climatological average for late October, resulting
in near to slightly above afternoon highs, with today`s highs in
the mid 80s, and tomorrow the western lower deserts will remain in
the mid 80s, while south- central AZ will be 1-2 degrees warmer,
in the mid to upper 80s. Ahead of the aforementioned weather
system, PWATs will begin to gradually increase as subtropical
moisture will begin to be advected northward into the region,
increasing PWATs from 100-130% of normal to to 200-220% of normal
by Thursday night. This will lead to increasing in cloud coverage
over the course of Thursday. This increase in cloud coverage in
combination of the warmer temperatures will lead to Friday
overnight and morning`s low temperatures 5-7 degrees above
normal.

&&

.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 0.05-0.20" 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 1.50-2.00" 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 0.50" 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 1.50-2.00" over the high terrain northeast of
Phoenix. The lower end of the forecast shows amounts closer to 0.50"
and 1.00-1.50" 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 1725Z.

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

No major weather issues will exist through Thursday afternoon under
periods of increasing thick cirrus decks. Wind behavior will be
similar to the past 24 hours including an extended period of
mid/late afternoon south cross runway winds (140v240) around the
Phoenix metro before briefly becoming NW in the evening. Confidence
in a full westerly wind shift at the Phoenix area terminals is low.
SE California aerodromes should continue with a general diurnal S/SE
component and nocturnal W/NW component. Extended periods of nearly
calm conditions will be common throughout the region.

&&

.FIRE WEATHER...
Dry conditions with near normal temperatures will persist through
Thursday under increasing cloud cover. Minimum humidity levels
through Thursday will fall into a 20-40% range following fair to
good overnight recovery of 40-60%. Starting Friday. and through
the weekend, MinRH`s will increase substantially with widespread
50-80%, due to several days of unsettled weather, bringing
widespread rainfall, and cooler temperatures. Otherwise, winds
will remain unseasonably light with minimal upslope breeziness.

&&

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

$$

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

NWS Phoenix (PSR) Office