640
FXUS65 KPSR 280530
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1030 PM MST Thu Aug 27 2026

.UPDATE...06Z Aviation Discussion

&&

.KEY MESSAGES...

- Extreme Heat Warnings remain in effect through Saturday with
  areas of Major to Extreme Heat Risk, despite temperatures slowly
  decreasing.

- Today and Friday will bring low end chances for showers and a
  few thunderstorms, focused more across southern and central
  Arizona.

- A pattern change this weekend will bring much better chances
  for rain, including including a chance of strong to severe
  storms across Southern Arizona mainly on Sunday or Monday.

&&

.SHORT TERM /TODAY THROUGH FRIDAY/...
A broad area of anticyclonic flow aloft remains entrenched over
the Desert Southwest, with current RAP analyzed H5 heights near
596-597 dam (values around the 99th percentile of climatology for
this time of year) centered over eastern AZ/western NM. A gulf
surge combined with upper level moisture advection from the
remnants of a TC well offshore Northern Baja/SoCal has increased
atmospheric moisture over the forecast area, but only modestly.
The 12Z PSR sounding this morning indicated PWAT around 1.26", up
from 1.07" at that same time yesterday. However, boundary layer-
averaged mixing ratios remain unfavorable for any deep moist
convection over the lower deserts today, with guidance keeping
values around 8 g/kg at most through the remainder of the day and
into this evening. Strong anticyclonic subsidence will also help
to keep thunderstorms anchored mostly to terrain this afternoon
into the early evening. Clusters of strong storms along the
international border/far Southern AZ will be capable of sending a
long lived outflow towards South-Central AZ lower deserts this
evening, potentially strong enough to kick up dust in some of the
more prone areas of Pinal County or Southern Maricopa County.
This outflow, coupled with perhaps gravity waves from earlier,
more intense convection over Sonora, may be able to trigger
elevated showers late this evening into the overnight hours.

As we head into Friday, ensembles agree that the subtropical high
and associated anticyclonic flow will begin to be displaced
eastward as upper level troughing draws near/over the West Coast.
During this time, the forecast area will be split between
southeasterly flow aloft to its east and drier southwesterly flow
to the west/northwest. If it weren`t for this split flow with
troughing impinging to the northwest, better quality moisture may
be able to surge into the area for Friday. However, current models
show only marginally better moisture in the low levels Friday
(8-10 g/kg mixing ratios at most), resulting in continued low end
storm chances focused mostly over higher terrain areas. A very
low probability of strong to severe winds gusts from isolated
convection over prominent terrain of Central to Northeastern La
Paz County and from decaying storms moving off the Baja Spine into
Southern Imperial County exists late Friday afternoon.

Aside from the low end storm chances through the end of the work
week, the main story will be continued extreme heat conditions.
NAEFS/ENS mean 850 mb temperatures above the 98th-99th percentile
of climatology, peaking between 29C-33C, will support afternoon
highs above 110F across the typically hotter lower desert locales.
This translates to values between 5F-10F above daily normals.
Increased cloud cover the next couple days will also prevent more
efficient radiative cooling overnight, resulting in much above
normal morning lows. In fact, the temperature at Phoenix Sky
Harbor Airport never cooled below 95F this morning, a new record
warm low if it holds through midnight and 4F above the previous
record warm low.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
A significant pattern change is set to begin this weekend with a
large Pacific trough slowly moving southeastward across the
Western U.S., eventually pushing the subtropical high into the
Southern Plains by Sunday. Our main forecast concern with this
change will be the increasingly favorable conditions for
thunderstorms, especially by Sunday and Monday. The influence of
the trough may occur as early as late Friday night and early
Saturday as a relatively strong jet streak moves through
California into the Great Basin. This should result in increasing
upper level support for showers and thunderstorms, but we will
also have to have enough moisture. For now, guidance is unclear on
the rain chances for early Saturday, but the NBM is trying to put
a 30-40% chance in after midnight. The daytime hours Saturday are
just as unclear as moisture is likely to continue to improve, but
will it be enough to overcome a modest capping inversion.
Guidance does show PoPs of 30-40% during the evening hours
Saturday from Phoenix and areas to the north and east of Phoenix,
but little chance to the west. Temperatures Saturday will cool a
bit from Friday, but there should still be enough residual heat to
push daytime highs to around 110 degrees across the western lower
deserts to 107-109 degrees in the Phoenix area.

Sunday into possibly Monday very well could be quite active as
guidance is pointing at very good moisture advection into Arizona,
somewhat directed by a passing tropical system to the southwest of
southern Baja. Atmospheric parameters improve on Sunday with
MUCAPEs anywhere from 1000-2000 J/kg and little if any CIN during
the afternoon hours. The presence of the trough over or just to
the northwest of our area should also provide some modest shear
and upper level support for storm potential on Sunday and/or
Monday. Depending on the evolution of the trough, heating, and
how much moisture gets pulled into our area, we may need to worry
about a strong to severe storm potential as well as a heavy
rainfall threat as PWATs may reach near 1.75". Drier air will also
be trying to work into western portions of our area by Sunday,
leaving little chance for rain over southeast California and
potentially southwest Arizona. NBM PoPs reflect this with at most
10-20% chances over the western deserts, but PoPs rise to 40-60%
from Phoenix and areas to the north and east of Phoenix on Sunday.
There is much higher model uncertainty going into the mid to
latter half of next week as GEFS favors moisture staying mostly in
place while the EPS favors some more significant drying. Part of
this uncertainty is due to a highly active tropic region to our
southwest which very well could influence our region at times.

Temperatures are at least slated to drop a good amount over the
weekend into early next week with forecast highs falling back to
near normal on Sunday and then at or just below the normal range
early next week. We can`t even rule out having highs in the 90s
for a day across south-central Arizona if the rain activity is
widespread enough on Sunday and/or Monday.

&&

.AVIATION...Updated at 0530Z.

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

No major aviation concerns are expected through Friday afternoon.
Some VCSH, and potentially a very isolated VCTS, may pop up during
the morning hour, primarily between 08-12Z, but any impacts should
be minimal. Winds will favor light and diurnal trends with periods
of VRB conditions during directional shifts. There are signs of an
outflow moving through the region Friday evening, but exact timing
and strength is uncertain at this time. TAFs reflect this lower
confidence with through a TEMPO group, but adjustments to these
lines should be anticipated.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation concerns are expected through Friday afternoon.
Winds will follow familiar diurnal trends with periods of VRB
conditions, especially overnight into the mid-morning hours. VCTS
may be present around KIPL by the afternoon, but forecast
uncertainty remains regarding timing, therefore, a PROB30 has
been added to the TAF. If convection does come to fruition, gusty
outflow winds, and reduced VIS due to HZ/BLDU will be the main
impacts. KBLH may see an outflow from distant TS activity move
through, but that is not currently included in the TAF due to low
forecast confidence.

&&

.FIRE WEATHER...
Hot and relatively dry conditions will continue into this weekend,
with low end chances for isolated to widely scattered showers and
thunderstorms. Chances then increase Friday night into Saturday,
especially across Southern and Central Arizona. Afternoon minimum
humidities will improve slightly to 15-20% Friday and then 17-25%
Saturday further over the weekend as rain chances increase over
Arizona. Outside the influence of distant thunderstorm outflows,
winds will be relatively light and follow diurnal tendencies
through Friday, with modest upslope/upvalley gusts in the
afternoon. Southerly to southwesterly winds then increase
Saturday, particularly across the western deserts, where
afternoon gusts increase up to 20-30 mph.

&&

.CLIMATE...
Record highs:

Date       Phoenix         Yuma         El Centro
----       -------         ----         ---------
Aug 27   114 in 2023    120 in 1981    117 in 1981
Aug 28   117 in 2023    118 in 2023    117 in 2009
Aug 29   116 in 2023    117 in 1954    117 in 2005

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Extreme Heat Warning until 8 PM MST Saturday for AZZ530>555-559-
     560.

     Extreme Heat Warning until 8 PM MST Friday for AZZ556-561-562.

CA...Extreme Heat Warning until 8 PM PDT Saturday for CAZ562-565>567-
     569-570.

     Extreme Heat Warning until 8 PM PDT Friday for CAZ561-563-564-
     568.

&&

$$

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