749
FXUS65 KPSR 291103
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
403 AM MST Thu Oct 29 2026

.UPDATE...Updated Aviation Discussion.

&&

.KEY MESSAGES...

- A Flood Watch has been issued for portions of south-central and
  eastern Arizona for Friday.

- Showers and thunderstorms, some producing moderate to heavy
  rainfall, are expected on Friday centered over south-central
  Arizona. Localized rainfall amounts of 1 to 2 inches are likely.

- A second weather system will result in a rainy Sunday for the
  entire area with light to moderate rainfall much of the day.

- Sunday will again bring flooding concerns as additional rainfall
  amounts of 1 to 2 inches are expected from Phoenix and areas to
  the north and east of Phoenix.

&&

.SHORT TERM /Today through Friday/...
Wet weather in on its way with two weather events Friday through
Sunday, both of which could produce heavy rainfall. Current
objective analysis shows a complex synoptic set up with a deep
closed low centered off the coast of Washington and several
shortwaves along a near climatological record strong west to east
subtropical jet stretching from just northeast of Hawaii to just
west of Baja. This jet will play a pivotal role in how much
rainfall we can expect over the next several days, especially on
Sunday as it will drive a strong atmospheric river into our
region.

For today, the large scale troughing along and just off the West
Coast will gradually get more organized with a weak shortwave
moving through California into the Great Basin, followed by
another shortwave farther to the southwest which will eventually
track through our region on Friday. The passing disturbance to our
northwest will bring some thicker higher level clouds today while
also helping to shift our flow out of the south. Guidance shows
strong southerly moist advection throughout the day today and
into Friday with PWATs quickly rising from just above normal
currently to 200-250% of normal by Friday morning. Forecast
soundings and the latest HREF guidance also show good elevated
instability developing across south-central Arizona tonight into
Friday, potentially leading to MUCAPEs of over 1000 J/kg in a
small corridor from central Pima County through western Pinal into
northern Maricopa Counties. Modestly strong jet forced ascent
will also develop over our area within the left exit region of the
subtropical jet by Friday morning and last through the afternoon
hours. This upper level forcing will combine with 25-30 kts of
H8-H7 upslope southerly flow pointed directly at south-central
Arizona.

Hi-res CAMs are showing showers and isolated thunderstorms
developing somewhere over or near the Phoenix area as early as 4-6
AM Friday morning, but coverage is likely to remain fairly
scattered through a good portion of the morning. By late morning
or early afternoon, shower and thunderstorm coverage is expected
to become more widespread across south-central Arizona with the
morning elevated storms potentially becoming rooted within the
boundary layer. Given the forecast MUCAPE may stay near 1000 J/kg
into Friday afternoon and 0-6km shear will still be a respectable
25-30 kts, a few strong to severe storms look possible. Even if
storms do not turn severe, any storms will be capable of producing
heavy rainfall on Friday and training of storms will be possible.
Average forecast rain amounts on Friday are still hovering
between 0.5-1.0" centered over and just northeast of Phoenix, but
localized amounts of up to around 2" are definitely possible. A
Flood Watch has been issued for the bulk of south-central Arizona
on Friday with the peak potential of heavy rainfall from late
morning through the afternoon. As the second shortwave trough
moves to the east into New Mexico late Friday evening, any
lingering showers or thunderstorms across eastern Arizona should
come to an end.

Temperatures today will be very similar to the last few days with
highs mostly in the mid 80s across the lower deserts, despite the
periods of thicker high clouds. The increased clouds and the
periods of rain on Friday should hold daytime highs to at most 75
degrees in the Phoenix area, while the western lower deserts will
only dip down to around 80 degrees for highs.

&&

.LONG TERM /Saturday through Wednesday/...
A short drying period will move in on Saturday, but higher level
moisture and clouds will be increasing throughout the day. We
can`t rule out a few light showers on Saturday, but with little to
no instability and drier air in the mid levels little if any
additional accumulating rainfall is expected through at least the
afternoon. Some guidance is showing shower activity to begin
already in the evening, but for most areas it should hold off
until after midnight.

Attention then turns toward one of our stronger ARs that can
affect the Desert Southwest moving in Saturday night and lasting
through much of Sunday. The AR will also coincide with a fairly
strong shortwave trough developing west of northern Baja on
Saturday before tracking right through the southern half of
Arizona during the daytime hours Sunday. This weather event is
shaping up to be a classic El Nino season AR event which would
normally occur from late December through February. All of the
ingredients for a prolonged moderate rainfall event look to be
there on Sunday with an impressive 18 hours of 35-50kt
southwesterly upslope H8-H7 flow and at least 12 hours of strong
jet forced ascent. These impressive forcing mechanisms will also
line up with record or near record moisture levels for this time
of year with PWATs rising to 300-350% of normal. Instability will
be lacking with MUCAPEs at most 200-300 J/kg, but the long
duration of the rain will make up for the lack of higher rainfall
rates.

We expect showers to blossom first over the high terrain northeast
of Phoenix sometime in the evening before filling in across most
of south-central Arizona around or just after midnight. Showers
will stay somewhat scattered through the overnight hours before
becoming widespread over much of Arizona by mid morning Sunday. As
the main shortwave begins to track through southern California
later Sunday morning, a fairly solid shield of light to moderate
rain should move through southern California and then through
Arizona Sunday afternoon. Upslope showers should persist over
portions of south-central Arizona into the high terrain into
Sunday evening, but the activity should be gradually winding down.

Expected rainfall rates on Sunday are not all that high, but rain
totals will add up over time as this event will last longer than
our typical weather events. Based on the record moisture levels,
strong forcing, but little instability rainfall rates will
generally be in the 0.1-0.2"/hr range in the Phoenix area to as
high as 0.2-0.3"/hr in the high terrain north and east of the
Phoenix area. Fairly steady light to moderate rainfall is expected
from just before sunrise through the afternoon in the
aforementioned areas with rainfall totals averaging 1-1.5" in
Phoenix to 2-3" over the higher terrain. Believe it our not, the
NBM is also suggesting the possibility (25%) of rainfall totals
over 2" in the Phoenix to as much as 3-4" in the high terrain.
Much lower rainfall totals of 0.25-0.5" are expected over the
lower deserts of southwest Arizona and southeast California due to
some rain-shadowing effects and the lack of upslope forcing.

Even if the average forecast rainfall totals come to fruition on
Sunday, the bulk of the rain will runoff as it will be falling on
already saturated soils. We anticipate a more expansive Flood
Watch will be needed for Sunday with many of the area washes
filling or overflowing, potentially for a good chunk of time. We
may also have to worry the flooding of some larger creeks and
rivers later Sunday into Monday as all of the expected rain works
through the water basins.

Dry air will begin to filter into the region from the northwest
later on Sunday, overspreading the bulk of the area by Monday
morning. We may have some lingering showers across the eastern
1/3rd of Arizona Sunday night into Monday morning, but additional
rainfall amounts should be fairly light. The rest of Monday will
bring decreasing clouds as the drier air works into the region
while another follow-on shortwave trough moves southeastward into
northern New Mexico. Temperatures for both Sunday and Monday will
be fairly cool with highs as low as the lower 70s for the Phoenix
area to maybe the mid to upper 70s across the western lower
deserts. Upper level ridging is then forecast to mostly take over
across the Desert Southwest going into the middle part of next
week, but temperatures will be fairly slow to rise with readings
staying below normal.

&&

.AVIATION...Updated at 1100Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT;
No major weather issues will exist through early Friday morning.
Wind behavior will be similar to the past 24 hours. Another
extended period of mid/late afternoon south cross runway winds
(140v240) is expected around the Phoenix metro before becoming
light W in the evening. Extended periods of light and variable to
nearly calm conditions will be common.

Dry conditions are expected through the next 24 hours. Around or
shortly before sunrise on Friday morning showers and storms are
expected start popping up in/around the Phoenix Metro. Due to
this VCSH/SHRA has been introduced starting around 10Z Friday,
with a PROB30 for TSRA at KPHX starting mid morning. While cloud
bases will drop to between 4-7K ft AGL, lower CIGs will likely not
fill in completely until late morning/Friday afternoon. Any heavy
rainfall could also lead to briefly lowered VIS.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major weather issues will exist through Friday morning under
periods of increasing thick cirrus decks. Wind behavior will be
similar to the past 24 hours. KIPL will see winds fluctuate
between S/SE and W. Winds at KBLH will become S late this morning
or early this afternoon and generally fluctuate between SW and SE
through the remainder of the period. Winds speeds will be AOB 8
kt. Extended periods of variable winds to nearly calm conditions
will continue to be common.

&&

.FIRE WEATHER...
After another overall dry day today, unsettled weather conditions
are expected starting Friday. Showers and thunderstorms will
impact much of the eastern districts on Friday with heavy rainfall
likely in some areas. Minimum humidity levels today will improve
slightly, but still fall into a 20-40% range before rising
substantially on Friday with widespread 45-65%. The unsettled
weather and cooler temperatures will last through the weekend with
widespread rainfall on Sunday. Outside of any thunderstorms,
winds will remain on the lighter side through the weekend.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...     Flood Watch from Friday morning through Friday afternoon for
     AZZ534-537-538-540>551-553>555-559.

     Flood Watch from Friday afternoon through Friday evening for
     AZZ552-556>558-560>563.

CA...None.
&&

$$

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