691
FXUS65 KPSR 201110
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
410 AM MST Wed Aug 20 2025

.UPDATE...
Updated Aviation

&&

.KEY MESSAGES...

- High pressure will strengthen over the region today and tomorrow
causing temperatures to climb to near record levels to end the
workweek, which will result in widespread major HeatRisk.

- Extreme Heat Warnings are in effect across the majority of the
region today through Friday.

- Slight chances for monsoon thunderstorm activity moves into the
lower deserts today with better chances confined to the higher
terrain in E and SE Arizona.

- Better rain chances move into the lower deserts to end the
workweek and even expand into SW Arizona and SE California by this
weekend and continue into the beginning of next week.

&&

.SHORT TERM /Today through Friday/...
RAP analysis of the subtropical ridge early this morning shows it
centered over the Four Corners region at a strength of 596 dm. This
high is expected to further strengthen today and tomorrow, with 500
mb heights rising to around 597-598 dm. This will lead to
temperatures rising even further. Afternoon high temperatures are
forecasted to rise into the 109-116 degree range across the lower
deserts and in the 100-108 degree range across the higher terrain
areas today through Friday, with Thursday currently forecasted to be
the hottest day. These forecasted high temperatures are within a few
degrees of the daily record high temperatures for all three of our
climate sites (Phoenix, Yuma, and El Centro; see the Climate section
below for the daily records). There will also be little to no
overnight relief as morning low temperatures are forecasted to be in
the upper 80s to low 90s. Lows of this magnitude are also near
record warm readings for our three climate sites. The combination of
these high and low temperatures will result in widespread Major to
locally Extreme HeatRisk across much of the area. The Extreme Heat
Warning remains in effect for much of the region for today through
Friday. Everyone should practice smart heat safety, such as staying
hydrated and limiting their time outdoors, in order to avoid heat-
related heath issues.

With the subtropical high remaining virtually stationary over the
Four Corners region to end the workweek, this will provide the
region with a much better set up for an increase in monsoonal
activity. The flow overhead will become deep and persistent
E`rly/SE`rly, which will lead to increasing moisture across the
region, which in turn will lead to increasing shower and
thunderstorm chances, including over the lower deserts. Models
continue to show that mixing ratios will increase to around 10 g/kg
with PWATs rising into the 1.4-1.7" range (currently they are in the
1.1-1.4" range early this morning). This will support daily
thunderstorm chances (30-50%) across the higher terrain, with rain
chances increasing to around 15-40% across the lower deserts. Gusty
erratic winds, lightning, and locally heavy downpours will be the
biggest threats with these storms.

For today, the best shower and thunderstorm chances remain over the
higher terrain, with the biggest threat being strong gusty winds.
The HREF shows a 70% chance of wind gusts in excess of 35 mph
tonight across the higher terrain in southern Gila County. For the
Phoenix Metro area, the main threat will be these strong gusty winds
from outflow boundaries surviving into the Valley. The HREF shows a
50- 70% chance of wind gusts in excess of 35 mph across the Phoenix
Metro this evening. There is less than a 20% chance of showers this
evening in the Phoenix Metro area. However, if there are multiple
outflow boundaries and those boundaries collide, then there will be
better chances for showers and storms to develop. Shower and
thunderstorm chances increase to around 20-30% tomorrow across the
south-central lower deserts, chances remain around 30-50% across the
higher terrain. Still, the main threat will be strong gusty winds
from the thunderstorms themselves and any outflows they produce. The
HREF shows a 70% chance of wind gusts in excess of 35 mph and a 10%
chance of wind gusts in excess of 58 mph across the higher terrain
of southern Gila County and eastern Maricopa County, including the
Phoenix Metro.

&&

.LONG TERM /Saturday through Tuesday/...
Ensemble guidance continues to show the ridge weakening slightly
(around 594 dm) this weekend. This weakening will result in
temperatures slightly cooling off. Afternoon high temperatures are
forecasted to be in the 106-112 degree range across the lower
deserts this weekend. Morning lows are also forecasted to remain on
the warmer side, with morning lows currently forecasted to range
from the mid 80s to near 90 degrees this weekend. These temperatures
don`t cool off enough to get rid of all of the Major HeatRisk, as a
good chunk of the lower deserts remain in the major category this
weekend. So, the Extreme Heat Warning may need to be extended
through this weekend. However, with some uncertainty still
surrounding the increase in moisture expected this weekend and
thunderstorm activity, the decision to extend the Extreme Heat
Warning will need to be made later this week when there is more
confidence in this weekend`s forecast.

Ensembles continue to show a slight shift W-SW of the subtropical
high heading into the weekend. This slight shift will concentrate
PoPs (20-50%) over SW AZ and SE CA, leaving a hole of lower PoPs (10-
30%) over the greater Phoenix Metro area. Due to the hit-or-miss
nature of shower and thunderstorm activity, it is difficult to
discern where the highest rainfall totals will occur and this will
hopefully become more clear over the next few days. However, locally
heavy downpours are expected with any thunderstorms, which could
lead to localized flooding problems.

By the end of the weekend and heading into early next week, the
subtropical high will weaken even more and shift S/SE. This will
allow for temperatures to cool off even more, with forecasted highs
returning to near to slightly above normal by Monday and falling
below normal on Tuesday. The shift in the subtropical high will
cause the flow overhead to become more S/SE. And will shift better
PoPs into the south-central AZ lower deserts, including the Phoenix
Metro. Current NBM PoPs show 20-40% across the south-central AZ
lower deserts for the beginning of next week. However, with this 4-6
days out there remains an ample amount of uncertainty and each day`s
thunderstorm chances will heavily rely on how thunderstorms the
previous day shakes out. The uncertainty can also be seen in the
maxT IQR scores for next week, with Monday having an IQR of 7
degrees and Tuesday having and IQR of 9 degrees. All ensembles show
a trough digging off the Pacific coast, by the beginning of the
workweek next week. The discrepancies come from if the trough pushes
inland and if so how far. The ECMWF Ensemble and Canadian Ensemble
models show the trough moving further inland causing heights aloft
to lower further leading to a cooler solution. Whereas, the GEFS
shows the trough staying further west, leaving the center of the
subtropical ridge closer to us and therefore, higher heights aloft
and warmer temperatures than the Canadian and ECMWF solutions.

&&

.AVIATION...Updated at 1110Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Uncertainty regarding timing of wind shifts and potential for gusty,
outflow winds this evening will be the primary weather issues under
occasional periods of mid/high cigs. Confidence is good that E/SE
winds will prevail well into the afternoon with lower confidence
directions fully switch to the traditional westerly, instead
possibly just becoming light and variable. TSRA/SHRA should remain
well east of terminals over mountains, however some form of an
easterly outflow should approach the Phoenix metro (better than a
50% chance). East winds may tap wildfire smoke and haze capable of
reducing slantwise visiblities. Modeling has backed off the
intensity of the outflow, as well as delaying the passage into mid
evening, so confidence in major impacts to operations is waning.
Otherwise, TSRA/SHRA should dissipate well before entering the local
airspace with less than a 20% chance of any SHRA surviving.


Southeast California/Southwest Arizona including KIPL and KBLH:
No weather concerns will exist through Thursday morning under
occasional passing mid/high cloud decks. Wind trends will be similar
to the past 24 hours with directions generally varying between SE
and SW. Extended periods of variability or nearly calm conditions
will be common during the morning hours.

&&

.FIRE WEATHER...
Temperatures warm well above normal today through Friday as highs
approach record levels and lows approach record warm levels. Winds
will continue to follow diurnal upslope/downvalley patterns with
afternoon gusts in the 15-20 mph range. Relative humidity will
bottom out around 15-25% through the weekend, increasing to 20-30%
for the beginning of next week. Overnight recovery will be poor
across the western districts and fair across the eastern districts
through the end of the workweek, increasing to fair area wide this
weekend. Thunderstorm activity will likely remain confined to the
higher terrain of E and SE AZ today before expanding to the lower
deserts to end the week. The chances for wetting rainfall will also
increase across the foothills and higher terrain of southcentral AZ
to end the week. Rain chances move westward into SW AZ and SE CA by
Friday and continue through the weekend.

&&

.CLIMATE...

Record highs later this week:

Date       Phoenix          Yuma          El Centro
----       -------          ----          ---------
8/20     113 in 2019     114 in 1982     114 in 1969
8/21     114 in 2019     115 in 1969     117 in 1969
8/22     113 in 2011     115 in 1969     117 in 1969

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Extreme Heat Warning from 10 AM this morning to 8 PM MST Friday
     for AZZ530>556-559-560-562.

CA...Extreme Heat Warning from 10 AM this morning to 8 PM PDT Friday
     for CAZ560>570.

&&

$$

SHORT TERM...Berislavich
LONG TERM...Berislavich
AVIATION...18
IRE WEATHER...Berislavich
CLIMATE...18