797
FXUS65 KPSR 190525
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1025 PM MST Mon Aug 18 2025

.UPDATE...Updated Aviation Discussion.

&&

.KEY MESSAGES...

- High pressure will strengthen over the region this week, causing
  temperatures to climb to near record levels and result in the
  return of widespread major HeatRisk.

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

- Monsoon thunderstorm activity will remain primarily confined to
  the high terrain of E and SE Arizona through the first half of
  this week before rain chances expand to the lower deserts by the
  end of the week and into this weekend.

&&

.SHORT TERM /Today through Tuesday/...
Dry conditions are in place as we start off the new workweek,
with early afternoon radar remaining quiet across much of Arizona,
while visible satellite imagery shows mostly clear skies
prevailing across much of our CWA. PWATs today are around 50-80%
of normal with this morning`s PHX 12Z sounding recording a value
of 0.99". Given the dry conditions in place, thunderstorms are
expected to remain confined to far eastern and southeastern
portions of the state through this afternoon and evening with
rain chances less than 15% across southern Gila County.

The latest objective analysis shows the subtropical ridge of high
pressure retrograding westward today and is currently centered near
NM/CO. The subtropical ridge will continue to migrate westward
before settling near the Four Corners tomorrow. As this occurs, the
ridge will also strengthen over the area, leading to 500 mb heights
climbing to around 594-596 dm tomorrow before further strengthening
into the middle part of the week. As a result, temperatures will
trend upwards over the next several days with highs Tuesday
afternoon forecast to top out around 109-113 degrees across the
lower deserts. These temperatures will result in widespread
Moderate HeatRisk Tuesday, so folks, particularly those sensitive
to the heat, should take the necessary heat precautions to avoid
any heat-related health impacts. Even hotter temperatures in
excess of 110 degrees are in the forecast for the mid to latter
part of this week.

The positioning of the ridge will also be favorable for advecting
better monsoonal moisture into the region this week. Starting
tomorrow, we`ll begin to see better moisture and instability creep
into southern Gila County, leading to increasing chances for
monsoonal activity. PoPs across southern Gila County will climb to
around 20-30% tomorrow afternoon. The primary concerns with
thunderstorms will be lightning and gusty, erratic outflow winds.
The potential for gusty outflows tomorrow is supported by the
latest HREF, which indicates upwards of a 30-50% chance of outflow
gusts in excess of 35 mph across the higher terrain east of
Phoenix.

&&

.LONG TERM /Wednesday through Sunday/...
We will then turn our attention to the latter half of this week
which will feature another bout of extreme heat. The subtropical
ridge is expected to strengthen further on Wednesday, with 500 mb
hghts peaking around 596-598 dam over the Four Corners region.
Positive hght anomalies will overspread the Desert Southwest,
boosting highs above 110 degrees across much of the lower deserts
starting on Wednesday. Highs will max out near record levels by
Thursday and Friday, topping out around 112-115 degrees many
desert communities including as Phoenix, Yuma, Blythe, and El
Centro. There will also be little to no overnight relief with lows
expected to remain very warm in the upper 80s to lower 90s.
Temperatures of this magnitude will result in widespread Major to
locally Extreme Heat Risk. The Extreme Heat Watch has been
upgraded to an Extreme Heat Warning and is in effect for much of
the region from Wednesday through Friday. Latest ensemble
guidance does show an eventual weakening of the upper lvl ridge by
this weekend along with improving moisture and thunderstorm
activity which will bring HeatRisk back down to the Moderate
category.

The subtropical high remaining stationary over the Four Corners
region will actually become beneficial later in the week as we
receive a more persistent and deep E-SE flow over our region.
This will provide a better opportunity for scattered thunderstorm
activity, including the lower deserts beginning Wednesday and
persisting into this weekend. GEFS and EPS member are still in
good agreement that mixing ratios will increase to around 10 g/kg
and PWATs rise to around 1.5-1.6" from Wednesday through the end
of this week which will promote daily thunderstorm activity over
the high terrain of southcentral AZ where NBM PoPs remain around
30-50%. Rain chances increase to around 20-40% for the lower
deserts on Thursday and Friday. By this weekend, a slight shift in
the upper lvl high to the S-SW will shift the focus for rain
chances to the western deserts of southwest AZ and southeast CA.
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.

&&

.AVIATION...Updated at 0525Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major weather concerns will exist through Tuesday evening under
FEW-SCT and at times BKN mid to high cloud decks. Winds will
follow typical diurnal patterns, with speeds generally AOB 10 kts
and occasional late afternoon/early evening gusts into the mid
teens. A period of spotty/widely scattered virga showers is
possible between 08-14Z across the Greater Phoenix Area. Late
Tuesday evening, confidence is low to moderate that a distant
thunderstorm outflow will reach the Phoenix area, switching winds
out of the SE earlier than usual but otherwise causing no real
aviation weather concern.

Southeast California/Southwest Arizona including KIPL and KBLH:
No weather issues will exist through the next 24 hours under
periods of FEW-SCT mid to high cloud decks. Winds will follow
typical trends for the time of year, varying between SE and SW,
with speeds generally AOB 10 kts and an extended period of
variability anticipated Tuesday morning.

&&

.FIRE WEATHER...
Above normal temperatures and mostly dry conditions will persist
through the first half of this week. Much hotter temperatures are
anticipated mid to late week as highs approach record 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 10-20% this afternoon before improving to
15-25% beginning Tuesday. Overnight recovery will range from poor
across the western districts to fair in the eastern districts.
Thunderstorm activity will likely remain confined to the higher
terrain of E and SE AZ through the first half of this week before
expanding to the lower deserts later in the week. The chances for
wetting rainfall will also increase across the foothills and
higher terrain of southcentral AZ later this week.

&&

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

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

&&

$$

SHORT TERM...Smith
LONG TERM...Salerno
AVIATION...Whittock
FIRE WEATHER...Salerno
CLIMATE...18