771
FXUS65 KPSR 311042
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
342 AM MST Sat Oct 31 2026

.KEY MESSAGES...

- A followup weather system tonight through Sunday night 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.

- Since rain will fall on already saturated soils, particularly
  for locations from Phoenix north and east into the Central
  Arizona high terrain, runoff will be very efficient and lead to
  flooding concerns. Thus, a Flood Watch is in effect for much of
  Central and Eastern Arizona tonight through Sunday night.

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

&&

.SHORT TERM /TODAY THROUGH SUNDAY/...
Lingering elevated instability, persistent southerly to
southwesterly upslope flow, and some modest 850 mb convergence
continues to trigger spotty convective showers to the east and
north of Phoenix early this morning, despite the area falling
under subsidence on the backside of a quick-moving shortwave as it
ejects northeastward over the Four Corners. On the backside of
this feature, some modest midlevel drying has also been realized,
with RAP analyzed PWATs dropping by 0.1-0.2" compared to Saturday
afternoon and partial/temporary clearing of cloud decks seen in
longwave IR satellite imagery (evidence of only a couple shallow,
near-saturated layers). This has set the stage for a lull in rain
chances/shower activity through the daytime hours today, although
areas of spotty light to moderate showers are likely to continue
generating throughout the day in these upslope favored areas, or
where transient pockets of weak instability are released by
increasing upper level ascent. Daytime highs in the middle to
upper 70s will be common across the lower deserts today, a few
degrees below normal for Halloween. It would be a good idea to
wrap up trick or treating and other outdoor activities on the
earlier side this evening, as conditions begin to align for a
longer duration and more widespread rainfall event across the
region.

A subtropical jet streak which stretched from near Hawaii to Baja
California has begun to cyclonically curve as it phases with an
impulse from higher latitudes and a broad closed low off the
Washington Coast. As these features phase together and begin to
progress inland along the West Coast, the cyclonically curved
subtropical jet will pull abundant, deep moisture into the Desert
Southwest. NAEFS/ENS mean PWATs will surge in excess of
climatological maxima for this time of year tonight into Sunday,
between 1.00-1.50" over Southern AZ, and forecast IVT values are
advertised at upwards of 250-750 kg/(ms), consistent with an
inland penetrating atmospheric river. Ahead of this system, a
period of moist isentropic ascent is likely to take place late
today into Sunday morning as almost the entire tropospheric
column becomes saturated. This will result in an initial shield of
precipitation developing/filling in across the area and pushing
northeastward this evening, affecting much of the area by 7-9 PM
MST. Scattered to numerous showers may also begin to fill in as
the nose of the subtropical jet punches into the Desert Southwest
late this afternoon into the evening. While scattered/embedded
thunderstorms cannot be ruled out through the duration of this
event, instability will be quite a limiting factor. REFS mean
MUCAPEs, backed up by GFS bufr soundings, will peak generally
between 200-400 J/kg, and these pockets of instability will be
transient and relatively short lived. Midlevel lapse rates will
also be quite poor at between 4.5-6 C/km. If it weren`t for the
poor lapse rates and low instability, the very high shear
environment would cause concern for some organized convective
modes. As it stands, the main concern with any scattered/embedded
convective elements late today through Sunday night will be
locally heavier rainfall and possibly some lightning. The
convective showers could also help mix higher momentum air down to
the surface and result in locally strong wind gusts.

Throughout this entire event, southwesterly upslope/orographic
enhancement will be a source for higher rain rates and result in the
most persistent areas of precipitation. As a result, rainfall
totals will show a strong gradient towards the central AZ high
terrain, north and east of Phoenix, and drop off quickly into the
lower deserts. NBM/WPC QPF over Phoenix seems to be biased a bit
high and will rely on likely the training of convective elements.
Average forecast rain totals for the Phoenix area range between
0.75-1.75", again, with a strong gradient towards the north and
east, and totals over the foothills/higher terrain should range
between 2-4" and locally higher over the most prominent terrain
features. Much lower rainfall totals of 0.25-0.5" are expected
over the lower deserts of southwest Arizona and southeast
California due to 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. 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.

Rain chances will quickly deteriorate by late Sunday morning for
Southeast CA and by the late afternoon for much of Southwest AZ as
the winds aloft abruptly shift westerly, then northwesterly behind
the trough axis. Moisture will quickly be scoured out by this,
except in the lower levels for South-Central AZ. The aforementioned
closed low off the Washington Coast 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. Activity 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 0530Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT;
SHRA activity and low MVFR to IFR CIGs/VIS starting Saturday
evening will be the main aviation weather issues throughout the
TAF period. In the meantime, the TSRA activity that has affected
portions of the area through the early evening hours has now
dissipated with mostly SCT cloud cover with bases as low as 4-5
kft prevailing through Saturday afternoon. Some hi-res guidance
brings in additional SHRA activity at times through Saturday
afternoon, however, confidence is low of this scenario
materializing. Much better confidence of more widespread SHRA
activity moving in Saturday evening with CIGs/VIS rapidly
deteriorating to MVFR to at times IFR conditions. Winds will
generally maintain an easterly component throughout the TAF period
with light speeds generally under 10 kts.

Southeast California/Southwest Arizona including KIPL and KBLH:
SHRA activity and sub VFR CIGs/VIS starting Saturday evening will
be the main aviation weather issues throughout the TAF period. At
KIPL, winds will generally maintain a westerly component through
the overnight period with mostly variable conditions expected
early Saturday morning. At KBLH, winds will generally fluctuate
between the southeast to south with a period of light variability
expected during the morning and afternoon hours. SCT-BKN high-
level cloud decks will persist through the overnight period before
low to mid-level cloud coverage increases during the day
Saturday. More widespread light SHRA activity will move in by the
late afternoon/early evening hours with CIGs/VIS rapidly
deteriorating to sub VFR conditions.

&&

.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 this evening through late Sunday night for
     AZZ534-537>563.

CA...None.
&&

$$

SHORT TERM...Whittock
LONG TERM...Whittock
AVIATION...Lojero
FIRE WEATHER...Whittock