661
FXUS65 KPSR 272325
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
425 PM MST Tue Oct 27 2026

.UPDATE...Updated 00Z Aviation Discussion.

&&

.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 late in the
week resulting in much cooler temperatures and enhanced chances for
widespread rainfall through early next week.

- Embedded thunderstorms and heavier rainfall may affect some areas
resulting in localized minor flooding, primarily Friday and Sunday.

&&

.SHORT TERM /Today through Thursday/...
A distinctive split flow pattern remains across the country with
progressive, low amplitude features poised to gradually buckle
towards a more blocked pattern over the next week. Broad northern
stream troughing extends from the Pacific NW into the plains early
this afternoon with a pronounced southern stream jet approaching the
southern California coast incorporating a deep negative height
anomaly near 33N 140W. For the remainder of the week, the slow
amplification phase will allow the northern stream troughing over
the Pacific NW to partially absorb the enhanced energy in the
southern stream forcing highly anomalous u-component jet energy and
a notable atmospheric river towards the Southwest. This scenario is
very reflective of a mature El Nino phase and its atmospheric
influence towards lower latitude shortwave intensification and
moisture transport.

Over the next 48 hours, quasi-zonal midlevel flow across the
forecast area will steadily become more cyclonic with the approach
of the aforementioned southern stream troughing. Other than a marked
increase in mid/upper level moisture profiles and thickening cloud
cover, pressure falls along the coast will allow backing low level
winds aiding in the advection of a plume of a higher theta-e airmass
from northern Mexico into south-central Arizona, and setting the
stage for increased rainfall chances into the medium range (beyond
Thursday). Otherwise, H5 heights in a 575-579dm range will hold
nearly steady, and narrow multi-model ensemble output yields
excellent confidence of temperatures within a few degrees of daily
normals.

&&

.LONG TERM /Friday through Tuesday/...
Friday is still looking like a fairly wet day across the bulk of
central Arizona as moisture levels rise to near 200-250% of normal
under strong low level moisture advection, while simultaneously
situated under strong difluence aloft in the left front quadrant of
the incoming near record strength (for this time of year)
subtropical jet. Guidance continues to heavily favor showers and
isolated thunderstorms developing Friday morning centered over south-
central Arizona, then expanding north and east throughout the day.
Rainfall rates may not be particularly heavy with the bulk falling
at less than 0.25"/hour, however embedded heavier showers and/or
thunderstorms with rates 0.50-1.00"/hour may be possible.

GEFS and EPS ensemble mean QPF for the Friday event shows an average
of around 0.25"-0.75" from Phoenix and into the higher terrain north
and northeast of Phoenix to 0.10-0.25" over much of the rest of
southern and central Arizona. The higher end of guidance (NBM 75%
percentile) still shows potential for heavy rainfall with amounts of
up 1.00"-1.50" across south-central Arizona, albeit more tied to
convective elements, and likely not widespread. As a result, WPC`s
ERO continues to depict a Slight Risk for excessive rainfall on
Friday. Given the propensity for these higher end NBM QPF
probabilities to subjectively be overzealous in mid range forecasts
over lower elevations, the ensemble means would be more favored over
larger area. Nevertheless, some higher end amounts of over 1"
should be expected in more convectively dominated areas, but these
type of totals are likely to be fairly localized.

As one portion of the subtropical jet and embedded shortwave lifts
northeast late Friday, the best ascent will shift out of Arizona
leaving lesser rainfall chances, and tied more towards isolated
upslope driven shower activity Friday night and Saturday. Models
continue to highlight a distinct break in rainfall Saturday before
the unsettled pattern returns Saturday night and Sunday with an
atmospheric river approaching the region. The majority of modeling
suggests the brunt of this expected modest atmospheric river
impacting areas across southern Arizona and northern Sonora. While
the overall trend in ensembles continues to depict mean troughing
advancing through the SW Conus, uncertainty remains quite high
regarding some very important details regarding the track, strength,
phasing, and orientation of the southern stream shortwave trough
yielding substantial differences in the amount of moisture that may
be thrust inland by the atmospheric river.

Large differences in output continue to be exhibited among each
iteration of operational and ensemble members with the key being the
ability for deep southwest flow at lower latitudes to become
juxtaposed with the atmospheric river in order for moisture to
breach the terrain gap in the Baja peninsula. Given the continued
wide range in solutions, it may be a couple more days to reach a
better consensus. Overall, both GEFS and EPS members continue to
show wide ranges from nearly no accumulations across lower
elevations to well over 2 inches. Realistically, the GEFS mean 0.25-
0.50" for south-central Arizona looks to be a bit too low
considering the potential amount of moisture, but the EPS mean 0.75-
1.50" may be a bit too high and a symptom of grid spacing bleeding
greater high terrain amounts into lower elevations. Regardless, the
bulk of the rain should fall throughout the day Sunday (which could
be a rare "wash-out" day for some locations), but as stated above,
how much total rain will fall is still quite uncertain. 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 eastern
Arizona, but by Monday, 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 2325Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No aviation weather concerns are anticipated through the forecast
period. Winds will tend to follow their typical diurnal patterns,
with an extended period of southerly winds expected tomorrow
afternoon before the full westerly shift occurs. Speeds should
remain aob 6 kts at all terminals. Clear skies will continue
through the overnight with increasing high clouds moving in
beginning around sunrise tomorrow.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation weather concerns are expected at the SE California
terminals through the forecast period. Winds will fluctuate
between SE and SW at KIPL and remain predominantly S`rly at KBLH.
Speeds at both sites will remain around 7 kts or less. Extended
periods of light and variable winds are also expected. Skies will
be clear through this evening before SCT high cirrus begins to
move into the region late tonight.

&&

.FIRE WEATHER...
Warm and dry conditions will persist through at least the middle
part of the week before a much cooler and wetter pattern sets up
Friday into this weekend. Minimum humidity levels will bottom out
today at 15-30% Tuesday before gradual moisture improvement arrives
starting Wednesday. Overnight recoveries will remain good at 40-65%.
Winds will remain light each day favoring typical diurnal patterns
and minimal upslope breeziness.

&&

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

$$

SHORT TERM...18
LONG TERM...18/Kuhlman
AVIATION...Berislavich/Salerno
FIRE WEATHER...Kuhlman