538
FXUS65 KPSR 090521
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1020 PM MST Tue Sep 8 2026

.UPDATE...
Updated Aviation

&&

.KEY MESSAGES...

- Above normal temperatures will prevail through the rest of the
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 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 0520Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major weather concerns will exist through Wednesday night under
occasional mid/high cigs, patchy virga, and isold SHRA. Confidence
in wind forecast are somewhat lowered tonight as a SW heading may
redevelop due to a Gulf surge before weakening again and becoming
east/variable closer to sunrise. Directions should switch back to
W/SW by noon Wednesday with limited afternoon gusts.


Southeast California/Southwest Arizona including KIPL and KBLH:
No major weather concerns will exist through Wednesday night under
occasional mid/high cigs and patchy virga/SHRA. Confidence in wind
trends is only moderate with a tendency for a NW direction to become
more southerly during the day Wednesday. Extended periods of nearly
calm conditions may be common during this transition.

&&

.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...18
FIRE WEATHER...Whittock

NWS Phoenix (PSR) Office