568
FXUS65 KPSR 010028
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
528 PM MST Sat Oct 31 2026

.UPDATE...Updated 00Z Aviation Discussion.

&&

.KEY MESSAGES...

- An Atmospheric River tonight through early Monday will result
  in a prolonged, widespread rain event for portions of the
  region, leading to gradual rises in area washes, creeks,
  streams, and rivers at a minimum.

- Already saturated soils will result in efficient runoff and
  flooding concerns. Thus, a Flood Watch is in effect for much of
  central and eastern Arizona tonight through Sunday night.

- Isolated strong to severe thunderstorms will be possible Sunday
  afternoon and evening, mostly east of the Colorado River, with
  the main threat being damaging winds.

- Conditions will quickly dry early this upcoming workweek, along
  with daytime temperatures slowly warming closer to, albeit
  still below, daily normal during the middle of the week.


&&

.SHORT TERM /TODAY THROUGH SATURDAY/...
Current objective analysis reveals broad troughing across the
western CONUS with a strong upper-level low spinning off the coast
of the Pacific Northwest. This deep wave will be the focal point for
what we can expect across the Desert Southwest through the rest of
the weekend and into the start of the next work week. Guidance shows
the broader disturbance splitting into two separate areas of
vigorous energy, with one area being the previously mentioned
coastal low, with the other setting up further south over the
sub-tropical Pacific while phasing with a strong upper-level jet.
The southern shortwave is currently helping to advect mid and
upper-level moisture over our forecast area, which will yield
considerable cloud cover through much of the afternoon and
evening while also supporting a some pop-up showers, especially
in supportive upslope areas. Current mesoanalysis indicate some
minimal CAPE across central and eastern Arizona in association
with peak day-time heating which could lead to some isolated
thunderstorms as well.

These initial showers will be just the beginning of a much longer
period of rain that could begin just prior to sunset this
evening for parts of the region. The persistent southwesterly
flow, which is typically synonymous with dry and tranquil
conditions this time of year, associated with the off-shore
disturbance will help pull a significant slug of moisture toward
Arizona and southern California. PWAT levels will approach
1.4-1.7" which actually is not out of the ordinary as values this
elevated are typically seen during monsoon season. However, this
is not July or August and seeing moisture this high during the
end of October/start of November is not normal. In fact, the
forecasted PWATs will be around 300-350% of normal, near all-time
records for this time of year compared to climatology. Projected
IVT backs up the idea of how unusual the event will be as values
will approach, and may even exceed at times, 750 kg/(ms), which
would also be an all-time record for this time of year. It is also
not as if IVTs will peak and then quickly fall off as there could
be upwards of an 12 hour timeframe (primarily between 11 p.m.
tonight through 11 a.m. Sunday) when climatological maximum are
seen, owing to a prolonged period of moderate rainfall across the
region. The most robust moisture profiles will be focused
primarily from the Colorado River Valley and eastwards, as
terrain-driven rain-shadow interference will likely result in
some degree of drying across southeastern California, but PWAT and
IVT levels there will still likely exceed the 99th percentile,
albeit for a shorter duration.

With that being said, expected rain totals will vary widely across
the Desert Southwest, with an unsurprising increase in QPF from west
to east. The greatest rainfall amounts will be focused over the
Arizona high terrain, where persistent upslope forcing could
potentially yield totals between 3-5", with locally higher amounts
around the most prominent terrain features. In the lower elevations
of Maricopa and Pinal Counties, the extended timeframe of
anomalously high moisture levels could push totals toward 0.75" to
around 2.5", with the higher end of that spread favoring
locations around foothill areas where orographic enhancement of
showers will take place. Further west toward southwest Arizona
and southeast California, totals will generally range from around
0.10" in the most heavily rain-shadowed areas on the lee side of
the Coastal Range to upwards of 0.75".

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. In turn, another Flood Watch will be in
effect for South-Central AZ starting tonight and lasting into early
Monday morning. During this time we anticipate the potential for
many of the washes filling or overflowing, and flooding in some of
the larger creeks and rivers as all of the expected rain works
through the water basins.

Something that might get lost amongst the prolonged rainfall and
flooding potential will be the very slight chance to see severe
thunderstorms late Sunday morning through the evening. Once the the
broad rain shield starts to progress further east, some minimal
destabilization of the atmosphere may take place, which could
increase the chances for convective initiation. However, hi-res
ensemble data shows CAPE values only reaching about 100-400 J/kg,
which should keep deep convection fairly isolated. However, the
environment will be very conducive for thunderstorm maintenance
with strong mid and upper-level ascent along with 0-6 km shear
profiles near 50-70 kt, more than enough to produce rotating
cells. 850-700mb winds will be consistently 40-50 kt during the
above mentioned timeframe, so strong to severe winds will likely
be the main threat, but even winds upwards of 40 mph may get
mixed down by what appear to be unassuming showers. Low-level
shear will also be quite strong, with 0-1 km SRH values rising to
150-250 m^2/s^2, levels that are rarely seen over this part of the
country. Combine that with LCL heights that will be some of the
lowest we see in a given year, and a few tornadoes cannot be
completely ruled out. With limited instability expected, hail will
not be as much of a concern, but if enough day-time heating can
take place, the threat surrounding it may increase. If this
outcome is realized, the greatest threat for strong storms will be
focused from the Colorado River and eastward. Heavy rainfall
associated with this activity could also exacerbate the flood
risk.

When showers and thunderstorms begin to clear the region will depend
heavily on the aforementioned convective timeframe, but at the very
least, the bulk of activity will move off to the east by late
Sunday night. The Pacific closed low off the Washington and Oregon
coasts will serve as a quick followup behind the subtropical jet
and initial trough axis, bringing a brief period of WAA and
continued upslope flow for more westward-facing slopes to the east
of Phoenix through the overnight hours into Monday morning,
potentially yielding some further scattered showers. Beyond this,
rainfall will gradually trail off as even the lower level
moisture is scoured out, and flow shifts northwesterly through
essentially the entire column, shutting off any upslope component.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Through the middle of the upcoming work week, following the
departure of this dynamic weekend system, we can expect dry
conditions, and transient upper level ridging, resulting in
temperatures slowly warming closer to seasonal levels (despite
daytime highs likely remaining a few degrees below the daily
normals). By late week, ensembles diverge considerably with
respect to another upper low/positively tilted shortwave trough
possibly impacting the region. Some solutions show quasi-zonal
flow taking shape over the Southwest US after the transient
ridging shifts east of the area, while others show a shortwave
disturbance capable of diving well south of the region and
resulting in a brief moisture return. Regardless of the outcome,
fairly benign conditions seem to be the most likely outcome. The
trajectory of the late week system is not favorable for rain
chances, its residence time too short for better quality moisture
to be pulled into the area, and if it remains an open wave, its
positive tilt does not bode well for providing upper level support
for precipitation. NBM PoPs become introduced to the eastern AZ
high terrain on the order of 15-25% Friday as a result, but we
should not put too much stock in this, as the forecast remains
highly uncertain at this time.

&&

.AVIATION...Updated at 0027Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT;

Considerable aviation weather impacts are expected to continue
through the majority of Sunday, with long duration SHRA and IFR-
MVFR CIG & VIS. A few terminals may near alternate minimums and
even airport minimums due to the low CIG & VIS by early Sunday
morning. Confidence in sub-VFR conditions is high with cloud decks
falling to MVFR around 03Z-06Z tonight and likely reaching IFR
flight categories starting around 08Z-10Z with arrival of heavier
bands of SHRA. The low flight categories may persist through
Sunday afternoon. A break in the RA is expected tomorrow afternoon
but VCTS/VCSH will hang on into the evening. Best time to see TSRA
at the terminals will be between 00-04Z tomorrow evening. Winds
will predominately be easterly into the overnight with a westerly
shift possible overnight as the heaviest rain bands progress
through the region. Winds are then expected to go S/SE tomorrow
morning and continue through the remainder of the period. By
tomorrow afternoon gusts of 25-35 kt are expected to develop and
continue into the evening.


Southeast California/Southwest Arizona including KIPL and KBLH:

Significant aviation weather impacts can be expected tonight into
Sunday morning with a prolonged period of SHRA and sub-VFR
CIG/VIS. VCSH/-SHRA is forecast to start this evening with
conditions rapidly degrading below VFR starting around midnight
tonight. MVFR CIG/VIS is favored, however periods of IFR may
develop at times Sunday morning, mainly at KBLH. Showers are
expected to clear by late tomorrow morning with skies quickly
scattering out behind the rain. Westerly wind gusts of 20-30 kt
are expected to develop tomorrow afternoon at both terminals.

&&

.FIRE WEATHER...
Spotty showers and possibly a few thunderstorms will impact
primarily the higher terrain of the eastern districts today before
rain chances increase areawide tonight. Widespread wetting
rainfall is expected across the area tonight through Sunday night.
Minimum humidities will range between 30-70% areawide today and
and improve further on Sunday, following excellent overnight
recoveries. Outside of any thunderstorms, winds will remain
generally between 10-20 mph and favor southeasterly to southerly
directions through the weekend. Conditions will gradually dry out
next week as the unsettled pattern departs and temperatures
moderate closer to seasonal normals, however, humidities will
remain above 25% areawide, and temperatures will still remain in a
slightly below normal category.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flood Watch from 11 PM MST this evening through late Sunday
     night for AZZ534-537>563.

CA...None.
&&

$$

SHORT TERM...RW/Whittock
LONG TERM...Whittock
AVIATION...Salerno/Berislavich
FIRE WEATHER...Whittock

NWS Phoenix Office