256
FXUS65 KPSR 071132
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
432 AM MST Mon Sep 7 2026

.UPDATE...Updated 12Z Aviation Discussion.

&&

.KEY MESSAGES...

- Above normal temperatures will prevail starting Tuesday,
  resulting in widespread Moderate Heat Risk and areas of Major
  Heat Risk.

- Extreme Heat Warnings have been issued beginning Tuesday for
  portions of the lower deserts.

- Mostly quiet weather conditions are expected for the first half
  of the week, then chances for thunderstorms increase across
  Central and Eastern Arizona during the latter half of the week.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...
Early morning midlevel water vapor imagery shows a trough pushing
towards the Intermountain West from the Pacific Northwest, with
an attendant SSW to NNE oriented jet streak originating over
SoCal. This feature is helping to squeeze out some lingering,
elevated light showers/virga across portions of the region which
should mostly conclude by around sunrise. The Desert Southwest is
already falling more so under the influence of a subtropical high
centered approximately over the TX panhandle at this hour, which
will become the predominant influence over local conditions as it
shifts westward through the middle of the week.

Ensembles have come into much better agreement over the last
24-36 hours or so, with the consensus now advertising increasing
midlevel heights as subtropical high pressure passes overhead and
strengthens midweek. This will be accompanied by a return of
easterly/southeasterly flow aloft, which will gradually allow
better low-level moisture to push into the area from Sonora and
support increasing thunderstorm chances during the latter half of
the work week across at least South-Central AZ. In the short term,
however, the main concern will be building extreme heat
conditions. Ensemble mean H5 heights will increase upwards of
594-597 dam by Wednesday, near climatological maximum values for
the time of year. Additionally, the increasing low level moisture
through the middle of the week will bring dewpoint temperatures
well into the 60s across much of the forecast area and locally
into the lower to middle 70s for areas such as the Lower CO River
Valley and Imperial Valley. This fairly rapid increase in surface
dewpoints also likely reflects the influence of gulf surges
returning as early as Tuesday night. Higher humidity and near
climatological maximum H5 heights will support above normal
temperatures, however, overnight lows appear to be the main driver
for the areas of Major HeatRisk developing across the region.
Afternoon highs around 5F above normal will become common across
the lower deserts, but less effective overnight radiative cooling
from the higher humidity and likely some mid-high level cloud
cover overnight (whether it be debris clouds from Sonora
convection or otherwise) will keep lows closer to 7F-10F above
daily normals, providing little relief from the daytime heat.
Apparent temperatures also near or exceed 110F across some of the
typically hotter spots along the desert valleys of the western
CWA. As a result, Extreme Heat Warnings have been issued for
portions of the lower deserts staring on Tuesday (starting
Wednesday for Lower CO River Valley, Desert Center, and La Paz
County).

Rain and thunderstorm chances remain limited through Wednesday
thanks at first to unsupportive low level moisture (7-9 g/kg mean
mixing ratios between 1000-700 mb), then primarily due to
anticyclonic subsidence centered directly overhead Wednesday into
Thursday. Low level moisture and instability would otherwise likely
be sufficient with a strong enough lifting mechanism to result in
thunderstorm chances across South-Central AZ lower elevations as
early as Wednesday. Expect storms to mostly be confined to the
Southeastern/Eastern AZ high terrain through midweek. However,
the change in the pattern during the middle of the week also
brings the possibility of easterly waves providing a focus for
thunderstorm development or convective complexes upstream/over
Sonora to influence our area.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
Latest NBM/WPC guidance depicts the greatest coverage of Major
HeatRisk across the forecast area on Thursday, with morning lows in
the middle to upper 80s across the lower deserts and forecast highs
106F-110F. These temperatures during the hottest time of the year
may not prompt extreme heat headlines, but as we head into
meteorological fall, daily normals drop off noticeably, resulting in
more sensitive Major HeatRisk thresholds than what folks may have
gotten accustomed to over the last few months. It also makes sense
that Thursday would be the warmest of the next several days, as
thermal profiles would have had more time to respond to the near
climatological maximum strength ridge overhead. Heat indices will
likely also be at their highest values, peaking between 108F-117F
across the lower deserts. Temperatures then begin a gradual
downtrend heading into the weekend as the subtropical high weakens
and is displaced from overhead.

As anticyclonic subsidence weakens, especially by late Thursday and
into Friday, chances for lower desert thunderstorms become
introduced across South-Central AZ (20-30%). A familiar monsoon
thunderstorm evolution may begin to take place beginning Thursday
and could last into the weekend, with afternoon thunderstorms
focused over the eastern AZ high terrain, which will push outflows
towards South-Central AZ in the evening that may be capable of
sparking additional thunderstorms. Current thermodynamic profiles
suggest all the typical monsoon thunderstorm hazards may come into
play, including strong, gusty winds, blowing dust, localized heavy
rain resulting in flash flooding, and frequent lightning. However,
as we head into the weekend, the forecast confidence decreases
substantially. WPC cluster analysis reveals large discrepancies,
with close to 40% of ensemble members comprising a solution which
shows negative midlevel height anomalies diving into the
northwestern and north-central CONUS, possibly impacting the flow
aloft as far south as the Desert Southwest and turning winds out of
the west, beginning a drying trend. Other solutions comprise
clusters which show subtropical ridging remaining in place somewhere
near/over the Southwest US and keeping the influence of northern
stream energy far enough north so as not to influence flow aloft
significantly. Either way, drying will likely not be pronounced
enough to preclude rain/thunderstorm chances once a sufficient
forcing mechanism becomes introduced. Ensemble mean PWATs even
remain near or above seasonal levels this weekend. Current NBM/WPC
PoPs and QPF suggest higher/more widespread chances heading into
Sunday, perhaps picking up on the influence of a distant/elongated
shortwave trough axis rotating through from the Northwest US
troughing that a good chunk of solutions show.

&&

.AVIATION...Updated at 1130Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major weather issues will exist through the TAF period under
occasionally thick mid/high cigs. Winds early this morning are
light and variable at KSDL and KDVT. Winds should remain westerly
at KPHX, but may briefly go light and variable at times this
morning. At KIWA, winds may briefly go SE`rly between 14-16Z this
morning. Winds will quickly reestablish a more defined W/SW
direction heading by early afternoon. Some occasional gusts into
the upper teens to around 20kt are possible, mainly at KPHX and
KDVT. Winds should return to their typical diurnal tendencies
tonight with a SE`rly component at KPHX and KIWA, a NE`rly
component at KDVT, and light and variable to calm winds at KSDL.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major weather issues will exist through the TAF period under
periodically thick mid/high cigs. Winds will generally favor a
W/NW trajectory through the period, though periods of light and
variable winds will be common.

&&

.FIRE WEATHER...
Temperatures will warm to above normal levels through the middle
of the week. Afternoon minimum humidities will generally range
between 20-30% areawide through much of the week following fair to
good overnight recoveries. Fairly typical diurnal winds with
occasional afternoon upslope breeziness can be expected through
much of the week. Rain and thunderstorm chances will remain low
and mostly limited to the eastern Arizona high terrain through
midweek before slight chances become introduced to the lower
deserts of at least the eastern districts during the latter half
of the week.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...     Extreme Heat Warning from 10 AM Tuesday to 8 PM MST Thursday for
     AZZ531-532-535>544-546-548>551-559.

     Extreme Heat Warning from 10 AM Wednesday to 8 PM MST Friday for
     AZZ530-533.

CA...Extreme Heat Warning from 10 AM Tuesday to 8 PM PDT Friday for
     CAZ566-567.

     Extreme Heat Warning from 10 AM Wednesday to 8 PM PDT Friday for
     CAZ569-570.

&&

$$

SHORT TERM...Whittock
LONG TERM...Whittock
AVIATION...Berislavich/18
FIRE WEATHER...Whittock