739
FXUS65 KPSR 081128
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
428 AM MST Tue Sep 8 2026

.KEY MESSAGES...

- Above normal temperatures will prevail through the rest of the
 work week, resulting in widespread Moderate Heat Risk and areas
 of Major Heat Risk.

- Extreme Heat Warnings will be in effect for portions of the
  lower deserts through the end of the work week.

- Thunderstorm coverage is expected to increase during the latter
  half of the week across Eastern and Central Arizona, with
  slight chances becoming introduced for the South Central Arizona
  lower deserts by Thursday.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...
Southwesterly flow aloft is still apparent in midlevel water
vapor imagery, as the region is sandwiched between troughing over
a good chunk of the Western US and expansive ridging centered over
the Southern Plains and Mississippi River Valley region. Mid and
high level clouds continue to stream northeastward over the area,
with some areas of embedded virga showers/sprinkles registering
on early morning MRMS composite reflectivity. These will likely
continue through the early morning hours until the influence of
the southwesterly flow aloft diminishes. The forecast remains
largely on track, with a pattern shift occurring by the middle of
the week. Troughing over the Western US is shown retreating to the
north, allowing the subtropical high to retrograde westward and
settle almost directly overhead Wednesday. During this time, the
subtropical high will quickly strengthen with H5 heights aloft
briefly peaking near climatological maximum values for this time
of year, around 596-597 dam. NAEFS/ENS mean 850 mb temperatures
between the 80th-95th percentiles of climatology, around 26C-30C
over the forecast area, will support afternoon highs around 5F
above daily normals. Periods of mid to high clouds have been
maintained overnight due to the continued southwesterly flow and
are expected the next few nights due to decaying or distant
upstream convective activity over Sonora/Southeastern AZ, which
will prevent more efficient radiative cooling. Thus, morning lows
are forecast to attain a 7F-10F above normal range, offering
little relief from the daytime heat. These temperatures will
translate to widespread Moderate HeatRisk with areas of Major
HeatRisk across the typically hotter lower desert locales. As a
result, Extreme Heat Warnings go into effect later this morning
for the Phoenix Metro, portions of South-Central and Southwest AZ
including the Yuma area, and portions of Imperial County including
the Imperial Valley. Extreme Heat Warnings expand slightly
further north Wednesday along the Lower CO River Valley and
surrounding communities.

With the pattern change occurring by the middle of the week, flow
aloft will shift southeasterly, and daily gulf surges are
expected to reestablish. These factors will combine with vigorous
upstream convection over Sonora to help push abundant low level
moisture into the area, especially by Wednesday night. Mixing
ratios will surge back above 10-12 g/kg across South-Central and
even Southwest AZ, supportive of lower desert storms if there is a
sufficient lifting mechanism. However, anticyclonic subsidence
centered nearly directly overhead will act to suppress deep moist
convection except over prominent terrain features of Southeastern
and Eastern AZ through Wednesday. Latest HREF does indicate the
potential for spotty elevated showers to develop tonight across
portions of the forecast area, but this is likely conditional on
sufficiently strong gravity waves propagating north and west from
earlier strong storm clusters over Sonora.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Late this week is shaping up to be a more active period for
monsoon thunderstorms across at least South-Central AZ. By
Thursday, anticyclonic subsidence will begin to diminish as the
subtropical high weakens overhead, and abundant low level moisture
will remain in place through the rest of the week and linger into
the weekend. Latest NBM guidance seems to favor Friday with the
best chances for lower desert thunderstorms, which makes sense
given the weaker subsidence aloft and plenty of moisture and
instability to support numerous thunderstorms over the high
terrain by the afternoon, sending outflows into the lower deserts
and triggering additional thunderstorms by the evening. GFS bufr
soundings indicate a good balance between CAPE and DCAPE Thursday
evening, with values between 1000-1500 J/kg, which is typically a
good indicator for severe weather potential over the Valley.
However, CIN may be a bit stronger than preferred, and the
development of thunderstorms that can utilize this environment
will be entirely dependent on the presence and timing of a
sufficient lifting mechanism (i.e., thunderstorm outflow
boundary). This conditional threat continues Friday, but with
perhaps lower DCAPE and shallower inverted-V soundings. Strong
thunderstorm winds, dense blowing dust, locally heavy rainfall
resulting in instances of flash flooding, and frequent lightning
will all be on the table for thunderstorm hazards later this week.

The forecast for this weekend into early next week is much less
certain, however, a few more details are coming into focus tonight
compared to the guidance available in previous forecast updates.
Ensembles are in better agreement that mean troughing will
establish over the Northwest US, pushing the northern stream and
likely a series of upper lows and their attendant jet streaks as
far south as the Great Basin. WPC cluster analysis reveals
discrepancies on how far south the mean troughing will establish
and how strongly negative the midlevel height anomalies will
become over the Northwest US. The current consensus is for flow
aloft to shift south to southwesterly as a result, meaning the
more typical monsoonal pattern which briefly sets up this week
will be short-lived, becoming something more akin to a transition
season setup. This does not, however, mean that thunderstorm
chances leave the forecast. Drying will begin to occur from west
to east, but low level moisture will not be scoured out from
Southern AZ. Thunderstorms will likely continue to favor
development over prominent terrain features, and lower desert
storm chances will be dependent on outflows or a nearly uncapped
environment. As for now, NBM/WPC PoPs have come in lower this
weekend compared to Thu/Fri, and timing of the better chances
seems a bit unrealistic, reflecting the high uncertainty and
nebulous forcing mechanisms. Regardless, the main message for this
weekend into early next week will be continued thunderstorm
chances over at least South-Central AZ higher terrain areas, but
likely a decrease in activity from Thu/Fri due to the pattern
shift and until southeasterly flow returns.

With the subtropical high diminished and then displaced by
Northwest US troughing this weekend, temperatures will follow a
downward trend, attaining near normal values likely by Saturday or
Sunday, ending the extreme heat conditions.

&&

.AVIATION...Updated at 1125Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No significant weather issues will exist through tonight under
periodic mid/high cigs. Light east winds for the rest of the
morning will turn back to the west around noon with limited
afternoon/early evening gusts.

Southeast California/Southwest Arizona including KIPL and KBLH:
No weather issues will exist through tonight under occasional
mid/high cigs. Winds will generally favor a W/NW heading through
the afternoon before switching to a southerly direction. Periods
of nearly calm conditions will also be common.

&&

.FIRE WEATHER...
Above normal temperatures will prevail through the end of the work
week, then temperatures are likely to decrease closer to normal
over the weekend. Afternoon minimum humidities will generally
range between 18-30% areawide through much of the week following
fair to good overnight recoveries between 35-70%. Fairly typical
diurnal winds with occasional afternoon upslope breeziness can be
expected through Thursday, followed by an increase in southerly to
southwesterly winds Friday into this weekend. 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 this morning 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 this morning to 8 PM PDT Friday
     for CAZ562-566-567.

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

&&

$$

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