615
FXUS65 KPSR 081835
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1135 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/...
Not much has changed in the current upper atmosphere conditions over
the past 24 hours with the region remaining between a large
troughing feature over much of Western CONUS, and an expansive
ridging over the Southern Plains and Mississippi River Valley
region. Mid to high level clouds continue to stream northeastwards
through the late morning hours due to southwesterly flow aloft.
However the flow will begin shifting out of the southeast as the
subtropical high moves westwards towards the Four Corners region
starting this afternoon. The subtropical high is expected to peak
Wednesday, with H5 heights quickly building to 596-597 dam, near to
at NAEFS climatological max for this time of year. This will help
drive afternoon high temperatures across the lower deserts around 5F
above normal for this time of year. Additionally, while
southwesterly flow usually advects drier air into the region,
Pacific tropical moisture was able to be pulled in keeping good
moisture in the mid to upper atmosphere. This has lead to overnight
cloud coverage hindering radiative cooling and PWATs have remained
between 130-150% of normal, with morning dew points consistently
reading in the mid 50s to low 60s. Due to these factors the
overnight low temperatures are in the mid to upper 80s, or near 7F-
10F above normal, and providing very little relief in the heat
throughout this week. Major HeatRisk has resulted across a large
portion of the lower deserts and an Extreme Heat Warning is in
effect 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.

As mentioned earlier the subtropical high will begin shifting
winds aloft out of the southeast starting today and will continue
through the week, with daily gulf surges likely. A notable
increase to 1000- 700 mb mixing ratios will occur Wednesday night
quickly increasing to 10-12 g/kg across SC AZ and even into the
lower western deserts starting wednesday. Unfortunately, with the
subtropical high expected to be directly over head, strong
anticyclonic subsidence storms will struggle to develop in the low
deserts, although not impossible. HREF models continue to show
between a 30-50% chance of storms developing in southeastern AZ,
and chances expand northwards into SC AZ, even into southern Pinal
County by Late Wednesday, but with the ridge overhead it will
remain conditional.

&&

.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 1819Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No significant weather impacts expected through the TAF period
under periodic mid/high cigs. Light E/SE winds will veer to the
W/SW by around 20-21Z at all terminals. A gulf surge tonight
around midnight may briefly increase W/SW winds to 10-14 kts.
Diurnal E winds will redevelop toward 10-12Z Wednesday followed by
an early W/SW shift around 16Z. A few high-based showers cannot be
ruled out around midnight tonight.

Southeast California/Southwest Arizona including KIPL and KBLH:
No significant weather impacts are expected through the TAF period
at both terminals under occasional mid/high clouds this afternoon
and evening and then SCT to BKN mid level clouds tonight after
midnight. A few high-based showers and virga may also pass through
the area after midnight through Wednesday morning, but confidence
is too low to put any mention in the TAFs. Winds will be light
through the TAF period, mostly below 8 kts, and KBLH will favor a
N/NW direction through this afternoon and then slowly shift CCW to
a S direction late tonight. Wind directions at KIPL will favor an
E/SE component through this afternoon, followed by a period of
W/SW this evening - early tonight. There will be storms along the
mountains west of KIPL this afternoon, which may push an outflow
boundary through the terminal, but the storms are expected to stay
in the mountains.

&&

.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 until 8 PM MST Friday for AZZ532.

     Extreme Heat Warning until 8 PM MST Thursday for AZZ531-535>544-
     546-548>551-559.

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

CA...Extreme Heat Warning until 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...Ryan
LONG TERM...Whittock
AVIATION...Benedict
FIRE WEATHER...Whittock