999
FXUS65 KPSR 012035
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
135 PM MST Mon Sep 1 2025

.KEY MESSAGES...

- Above normal temperatures early this week will steadily cool into
a below normal range by the end of the week

- A favorable weather pattern and increasing moisture will foster
growing thunderstorm and rainfall chances through the middle of the
week

- Gusty winds, localized blowing dust, and isolated instances of
heavy rainfall will be common with any thunderstorms throughout the
week

&&

.SHORT TERM /Today through Tuesday/...
Expansive ridging covers much of the western Conus early this
afternoon with objective analysis depicting an anti-cyclone center
pivoting over NW Arizona. Despite the amplification of this high
pressure system, midlevel height and thermal profiles are not
terribly anomalous for early Sept with H5 readings only approaching
594dm and H7 temperatures just marginally above +10C. Meanwhile, a
notable MCV/northern inverted trough extension was evident in
satellite imagery over northern Sonora trapped on the southern
fringes of the anti-cyclonic circulation to the north. Convective
complexes over northern Mexico/southern Arizona have encouraged a
shallow moisture surge up the lower Colorado while outflow
boundaries last evening have resulted in more efficiently mixed
boundary layer moisture.

Despite this favorable moisture intrusion, overall boundary layer
mixing ratios at lower elevations are only marginally supportive for
storm development this afternoon/evening with CinH hovering around
50 J/kg. The preponderance of high resolution model output indicates
mountain storms producing another pronounced easterly outflow
sweeping into south-central Arizona this evening, but with
convective elements decaying as they propagate off higher terrain
(albeit likely advancing closer to populations centers than last
evening). Should multiple deep outflow develop, this would be
conducive for better storm chances into lower elevations, but little
model evidence exists suggesting this outcome. More likely, gusty
winds upwards of 30-40 mph with patchy dust (and eventually lofted
dust) would be the most likely weather impact before activity
shifts westward overnight.

A convincing trend in both high resolution and global scale modeling
suggests the aforementioned northern Sonoran MCV progressing
westward and becoming absorbed into southerly flow associated with a
jet streak entering southern California. Aligning within the favored
left front jet quadrant, large scale synoptic ascent should become
juxtaposed with deep moisture profiles and modestly steep lapse
rates. Forecast BUFR soundings indicate widespread saturation within
the H7-H5 layer tapping MUCape nearing 1000 J/kg which should result
in numerous showers and embedded thunderstorms through Tuesday
morning across much of the western CWA. Given this scenario, have
actually boosted POPs in the western CWA into a 50-70% range through
the early afternoon.

Forecast confidence begins to deteriorate somewhat Tuesday afternoon
with a greater dependence on antecedent convective activity and
locations best suited towards destabilization later in the day more
uncertain. However based on a most likely outcome, would expect much
of SE California to maintain cloud cover and limited instability
Tuesday afternoon with convective initiation focused further to the
east. In fact, most high resolution modeling erupts deep convection
along the international border in central/western Pima County where
MLCape in excess of 1000 J/kg could be tapped by another vorticity
center emanating from an inverted trough spinning over central
Mexico. With the synoptic pattern remaining favorable, there`s
better than a 50% chance of strong, gusty winds in excess of 30 mph
thrusting north and west from the initiation point creating areas of
dense blowing dust. Thermodynamics should be supportive for
additional development behind the outflow boundary as activity again
sweeps through the western CWA becoming an evening/nocturnal event.
Aside from strong winds and dust, heavy rainfall could yield
localized flash flooding with the forward motion of the boundary and
individual storms the only apparent limiting factor towards a
greater hydrologic threat.

&&

.LONG TERM /Wednesday Through Sunday/...
With more favorable S/SE flow in place, moisture advection will be
sustained and allow for daily chances for showers and thunderstorms
remainder of the week and into the weekend for parts of the region.
Most of the activity at first will be focused around enhanced
terrain features such as the Kofa and Chocolate Mountains, areas
around Joshua Tree NP, and the typical spots in southern Gila
County. Areas along the desert floor will also have chances during
the middle to latter portions of the week to see rainfall activity,
but that will be determined by any triggering mechanisms (such as
outflow boundaries) that move off those high terrain areas.

For the Phoenix metro, outside of any isolated activity that pops
due to any aforementioned outflow boundaries during the first part
of the week, the best timeframe to rainfall looks to be the Thursday-
Saturday window. Models points toward a secondary surge of moisture
that will push PWATs closer to 150% of normal, which should be
enough to help initiate some more widespread showers and storms
across South-Central Arizona. Now, can rain be expected all three of
these day? That seems unlikely as what occurs on one day will
influence what occurs on the next, so pinpointing the exact timing
of rainfall for the metro is still a bit uncertain at this time.
Looks like by Sunday, flow aloft will switch back to the southwest,
drying the region out and suppressing rain chances for next week.

In terms of temperatures, high pressure overhead is expected to
weaken slightly thanks to a disturbance off the Pacific Coast,
indicating the presence of a cooler atmospheric profile. This,
combined with increasing moisture along with rainfall activity and
associated cloud cover, will send temperatures on a downward
trajectory over the next several days with some lower desert areas
potentially struggling to reach into the triple digits by as early
as Thursday.

&&

.AVIATION...Updated at 1750Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Easterly winds will continue through the morning and into the
afternoon, where the typical afternoon west shift is not
anticipated. Instead, a period of variability or light southerly
component looks to develop around mid to late afternoon. Good
confidence for outflow wind impacts his evening with winds most
likely from the east to northeast. Timing as early as 00Z is
possible, but 01-03Z is most favorable at this time. Latest odds of
outflow gusts >30kt are around 40-50%. Confidence is too low (15% or
less) for VCSH/VCTS mention in the TAFs as activity looks to
dissipate before reaching the metro, but we cannot rule out a stray
shower or storm surviving near one or multiple terminals late this
evening. VFR conditions with cloud bases staying mostly at above 10K
ft AGL will prevail.


Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation weather concerns are expected through the TAF period
under mostly BKN high clouds. Winds will favor the SE at KIPL
throughout the period, and winds at KBLH will favor SSE with periods
of SSW. Expect wind speeds to remain mostly aob 7 kts at both
terminals. Towards the end of the period, shower/storm activity will
encroach from the east, thus the addition of a VCSH group as early
as 09-10z for primarily KIPL but KBLH may see activity as well,
however confidence is lower and not mentioned in he TAF at this time.

&&

.FIRE WEATHER...
Increasing moisture levels across the Desert Southwest will result
in a gradual enhancement of daily shower and thunderstorm chances
over the next several days. Due to this increased moisture flux,
MinRH values will remain around 15-25% through most of the week,
potentially rising even higher later in the week due an additional
moisture push and cooling temps. MaxRHs will follow a similar trend,
with modest overnight recoveries Monday into Tuesday becoming good,
to even excellent, by late week. Above-normal temperatures will last
one more day before temperatures gradually cool toward below
seasonal levels by mid to late week.

&&

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

&&

$$

SHORT TERM...18
LONG TERM...RW
AVIATION...Young/Ryan
FIRE WEATHER...RW