532 
FXUS65 KPSR 311300
AFDPSR

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

.UPDATE...Updated 12Z Aviation Discussion.

&&

.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 1300Z.

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

Considerable aviation weather impacts are expected starting
tonight and lasting through at least Sunday morning, 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. Confidence in sub-VFR conditions is high. Conditions will 
degrade by 4-6Z tonight and likely reach lowest flight categories 
starting around 8-10Z with arrival of SHRA. The low flight 
categories may persist through Sunday afternoon. A few
thunderstorms may also develop Sunday afternoon.

In the shorter term, today, SCT to at times BKN clouds around 5-6K
ft are expected through this afternoon and spotty VCSH/-SHRA may
develop at any time. Winds will favor an E-SE direction through
this evening with speeds aob 10 kts.

Southeast California/Southwest Arizona including KIPL and KBLH:

Significant aviation weather impacts are expected starting tonight
into Sunday morning with a prolonged period of SHRA and sub-VFR 
CIG/VIS. In the interim, light winds with gradually increasing 
and lowering clouds are expected through this evening at both 
terminals. VCSH/-SHRA is forecast to start later 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.

&&

.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...Benedict
FIRE WEATHER...Whittock