374
FXUS65 KPSR 210550
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1050 PM MST Wed Aug 20 2025

.UPDATE...Updated Aviation Discussion.

&&

.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 through Friday.

- Monsoon thunderstorm activity will remain primarily limited to
  the higher terrain today with better chances of thunderstorms
  extending into the lower deserts starting Thursday.

- 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/...
The overall upper level pattern remains relatively unchanged from
yesterday as the subtropical ridge of high pressure remains centered
over the Four Corners. Objective analysis shows 500 mb heights
around 595-596 dm over the area with similar heights expected
through the end of the week. As the ridge maintains its strength
over the region through the next few days, temperatures will run
well above normal through at least the end of the workweek.
Afternoon highs will come within 1-3 degrees of daily record
highs, while record warm lows will be in jeopardy through the rest
of this week. NBM highs across the lower deserts climb to upwards
of 110-115 degrees over the next couple of days with tomorrow
expected to be the hottest day. The well above normal, and
potentially record-breaking, temperatures in the forecast will
promote widespread Major HeatRisk with localized areas of Extreme
HeatRisk through at least Friday. Thus, an Extreme Heat Warning
remains in effect through Friday evening. Everyone should exercise
the proper heat safety precautions, including staying hydrated
and limiting time outdoors, to avoid heat-related health issues.

Meanwhile, the position of the high remains in a favorable position
to promote increasing monsoonal activity for the latter half of the
week. Moisture continues to increase across the CWA with the latest
SPC mesoanalysis showing PWAT values upwards of 1.3-1.5". The
increase in moisture and instability (MUCAPE ~1000 J/kg) is leading
to more thunderstorm development across southern Gila County this
afternoon. Thunderstorm activity will continue to develop across the
higher terrain east of Phoenix through the afternoon. With inverted-
V profiles and DCAPEs in excess of 1000 J/kg, any stronger
thunderstorms that develop today will be capable of producing strong
outflow winds as indicated by the latest HREF, which shows
probabilities for outflow gusts in excess of 35 mph climbing upwards
of 50-70%. Shower and thunderstorm activity is likely to diminish
as it progresses westward towards the lower deserts into a more
unfavorable environment. However, strong gusty outflow winds will
be more likely to impact the Valley this evening and could promote
some localized areas of blowing dust.

Rain chances will continue to increase over a broader area,
including portions of the lower deserts, over the next couple of
days as moisture and instability continues to increase. The latest
guidance suggests thunderstorms will fire up along the Rim to the
White Mountains Thursday afternoon before the favorable easterly
steering flow helps to advance the thunderstorms westward towards
the lower deserts. Multiple HREF members show thunderstorms
surviving into the lower elevations of south-central Arizona,
particularly into Pinal County, late Thursday afternoon into the
evening. Strong outflow winds will be the primary concern with this
activity with the HREF showing a broad area of higher probabilities
across south-central Arizona around 50-70% for outflow gusts
exceeding 35 mph as well as a 10% chance of exceeding 58 mph. Given
the potential for very strong outflow winds, dense blowing dust will
be an increasing concern tomorrow, especially south of the 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 0550Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Main aviation concern throughout the TAF period will be potential
for another outflow tomorrow evening. Hi-Res models are showing a
stronger signal than the previous night (tonight`s), with gusts
expected near 25kts from the NE. During which SHRA can be possible
but currently uncertain of any TS activity. Otherwise, winds may
continue to be erratic for the next few hours as the current outflow
from the south passes, but should settle out of the east for the
remainder of the overnight/morning period, wind aob 10kts once
gusts subside. Typical westerly shift is expect by the afternoon.
SCT-BKN high cloud deck will continue to be common.

Southeast California/Southwest Arizona including KIPL and KBLH:
No weather concerns will exist through the forecast period 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. Possible periods of variability or nearly calm
conditions can be common.

&&

.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 until 8 PM MST Friday for AZZ530>556-559-
     560-562.

CA...Extreme Heat Warning until 8 PM PDT Friday for CAZ560>570.

&&

$$

SHORT TERM...Smith
LONG TERM...Berislavich
AVIATION...Ryan
FIRE WEATHER...Berislavich CLIMATE...18