414
FXUS65 KPSR 020520
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1020 PM MST Tue Sep 1 2026

.UPDATE..06Z Aviation Discussion

&&

.KEY MESSAGES...

- Temperatures should remain in a near to below normal category
  for the first week of September, resulting in widespread Minor
  Heat Risk and localized areas of Moderate Heat Risk.

- Drier conditions will prevail through the middle of the work
  week before potentially another period of wet weather
  materializes late week into the weekend.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...
Another strong W-E gradient in surface dewpoints was noted this
morning in the vicinity of the CO River Valley, with dewpoint
temperatures in the 40s across much of Southeast CA and 60s to lower
70s across Southwest AZ. The higher dewpoints across our eastern CWA
are indicative of the better low level moisture that remains in
place, however, the 12Z PSR sounding reveals a very dry layer
around/just above 500 mb, thanks to the sustained southwesterly to
westerly flow aloft. With daytime heating, anticipate further
drying of the low levels as the drier air aloft mixes down into
the boundary layer. However, there will be enough moisture to
support isolated deep moist convection over prominent terrain
features of Southwest AZ (primarily Central/Northeastern La Paz
County) and perhaps the Tabletop Mountains of Southwestern
Pinal/Southern Maricopa Counties. The intensity of convection will
be reduced compared to yesterday, with a reduction in instability
and the addition of subsidence, which is evidenced by a strong
midlevel inversion present in GFS bufr soundings and the 12Z PSR
sounding. Differential heating across the quasi dry-line in
Southwest AZ may provide a focus for convective initiation as it
progresses eastward this afternoon. This, coupled with lingering
effective bulk shear around 25-30 kts presents the possibility for
some of the La Paz County storms to organize and become strong. A
very isolated risk for a severe thunderstorm or two cannot be
entirely ruled out. Fields of cumulus are already visible on
satellite imagery early this afternoon, stretching from far
Southeast CA through the CO River Valley and into La Paz County.
We will be keeping a close eye on this area for further
development.

Ensembles remain in excellent agreement that southwesterly flow
aloft with deep Pacific troughing along the West Coast will be
maintained through at least the middle of the work week, keeping the
subtropical high suppressed and displaced well east of the region.
In fact, a compact PV anomaly/area of strongly negative midlevel
height anomalies has dived southward and can be seen in midlevel wv
imagery early this afternoon starting to move onshore along the
NorCal Coast, resulting in a restrengthening of southwesterly flow
aloft and further drying through Wednesday. This will lead to
another quiet day across our forecast area with any isolated
convection relegated to the higher terrain of northern and
southeastern AZ. This will also lead to an uptick in daytime
winds, particularly across Southeast CA/Southwest AZ. Ensemble
mean H5 heights will remain in a seasonable range, around 587-590
dam. Mean 850 mb temperatures between 22C-26C (between the
20th-60th percentiles of climatology) will support surface
temperatures within a few degrees of the daily normals through
Thursday, with lower desert highs generally ranging from the upper
90s to around 105F, following morning lows from the upper 70s to
middle 80s.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
The forecast becomes a bit more muddy heading into the end of this
week and through the weekend. The NHC shows Tropical Storm Marie
has formed well of the coast of Southwestern Mexico. Most
forecast models show this storm curving out into open waters by
this weekend, however there is good evidence that at least some
tropical moisture will get pulled up into Mexico and making it as
far north as southern AZ as early as Thursday/Thursday night.
Roughly 90% of the GEFS and EPS members show PWATs increasing from
1.5" on Thursday to around 1.8-2.0" over portions of southcentral
and southwest AZ by Friday or Saturday. Given this likely
increase in moisture, NBM PoPs have increased to around 40-50%
with the initial moisture push late Thursday into Friday.
Thereafter, PoPs remain quite low in a 20-30% range through the
weekend which is most likely due to uncertainty in timing and
coverage of rainfall within the deterministic model output.

Although there will be adequate moisture return, the positioning
of the ridge/trough features over the west will play a key role in
how much rainfall may actually materialize. If the ridge
strengthens too much and retrogrades wwd, then the focus for
heavier rainfall may be much further west into SoCal and north
into UT and CO as indicated by the latest ECMWF and GFS runs. It
will likely be at least 2 or 3 more days until we have a good idea
on what will happen late this week, and any forecast that does
not show a range of possible outcomes should be disregarded at
this time.

Uncertainty in the temperature forecast by this weekend is also
apparent as indicated by large (8-10 degree) spreads within the
NBM inner quartile range. Once our region does begin to draw in
the Pacific moisture, temperatures will likely dip down to around
3-5 degrees below normal by the end of this week. However, as
ridging aloft attempts to build over the area this weekend,
temperatures may warm closer to normal by Sunday or Monday.

&&

.AVIATION...Updated at 0520Z.

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

No major aviation concerns are expected through Wednesday
afternoon. Winds will follow light familiar diurnal trends with
periods of VRB conditions overnight and some occasional afternoon
gusts in the mid teens to around 20 kt. A slower-than-usual
transition from east to west may produce a few hours of southerly
cross winds at KPHX and KDVT, with perhaps a window of directions
160V230 before westerly flow takes over. There are hints of TS
activity moving within the vicinity of the terminals during the
late afternoon to early evening hours, but no mention is included
in the TAF at this time as rain chances are too low ~10%.
Generally clear skies tonight will give way to a FEW low- level
clouds during the back half of the period.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major aviation concerns are expected during the next 24 hours.
Winds will favor a westerly component at KIPL, while south-
southwesterly to southwesterly flow will be common at KBLH. Breezy
conditions with gusts upwards of 20-25 kt will develop at each
terminal during the back half of the forecast window. Skies will
be mostly clear through Wednesday evening.

&&

.FIRE WEATHER...
Drier conditions will prevail through Thursday with only isolated
chances for showers and thunderstorms mostly relegated to
prominent terrain features across AZ. Better chances for wetting
rains area-wide return late this week into this weekend. Afternoon
minimum humidities will range from 7-15% across Southeast CA and
between 15-35% for much of Southern AZ through the next few days.
This will follow poor to fair overnight recoveries across the
western deserts and good to excellent overnight recoveries across
the eastern districts. Daytime winds will remain elevated out of
the south to southwest, with the strongest gusts in Southeast CA
and along the Lower Colorado River Valley peaking between 20-30
mph during the afternoon/early evening.

&&

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

$$

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

NWS Phoenix (PSR) Office