157
FXUS65 KPSR 181121
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
421 AM MST Mon Aug 18 2025

.UPDATE...Updated 12Z 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 Watches remain 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/...
The overall upper air pattern has not changed much over the past 24
hrs as our forecast area remains centered between troughing over the
Pacific NW and ridging over the Southern Plains. The subtropical
ridge is forecast to retrograde back to the west today and set up
over the Four Corners region starting Tuesday, resulting in a
warming trend back to above average temperatures. Today is expected
to remain dry and tranquil across the region as moisture remains
quite limited, even across the higher terrain with PWAT values
hovering around 1.0-1.2". There could be a few isolated storms over
far SE AZ, but the chances for any outflow boundaries reaching the
lower deserts of southcentral AZ will be very low (<10%). Due to a
an uptick in mid-lvl hghts/thickness this afternoon, temperatures
will climb a couple degrees from what we saw yesterday, topping out
around 103-107 degrees across the lower deserts. Low temperatures
will also be slightly warmer by Tuesday morning, mainly in the low
to mid 80s across the lower deserts.

On Tuesday, the aforementioned subtropical high will become
established over the Four Corners region. The positioning of the
high pressure will set the stage for another episode of extreme heat
and bring rain chances back into forecast area later this week. 500
mb hghts on Tuesday afternoon will increase to around 593-595 dam
across southcentral AZ, resulting in highs approaching 110 degrees
in in most lower desert communities including Phoenix. These
temperatures will result in a widespread Moderate HeatRisk. A
gradual rise in moisture across eastern portions of the state will
result in increasing monsoonal thunderstorm activity across the
Mogollon Rim and the higher terrain of southeast AZ Tuesday
afternoon and evening. Rain chances are currently around 20-40% from
the foothills of E Maricopa County through S Gila County. Latest SPC
HREF guidance suggest there is at least a 30% chance of gusty
outflow winds exceeding 35 mph across N Pinal County late Tuesday.

&&

.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. An Extreme Heat Watch remains in
effect or much of the region from Wednesday through Friday and
will likely be upgraded to a warning in subsequent shifts. 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 1120Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
W/NW winds will dominate the majority of the TAF period, with
only the KIWA terminal expected to follow their more typical
diurnal transitions with periods of easterly winds during the
overnight and early morning hours. Periods of light and variable
to calm winds are also expected, mainly during the early morning
hours. Otherwise, wind speeds will generally be aob 10 kt, with
some occasional gusts into the teens possible this afternoon. FEW-
SCT mid and high clouds will be common through the period.

Southeast California/Southwest Arizona including KIPL and KBLH:
Wind behavior will be nearly identical to the past 24 hours as
directions generally vary between SE and SW with limited
gustiness. Skies will remain mostly clear, with FEW mid and high
level clouds moving in during the overnight hours tonight.

&&

.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 Watch from Wednesday morning through Friday evening
     for AZZ530>556-559-560-562.

CA...Extreme Heat Watch from Wednesday morning through Friday evening
     for CAZ560>570.

&&

$$

SHORT TERM...Salerno
LONG TERM...Salerno
AVIATION...Berislavich
FIRE WEATHER...Salerno
CLIMATE...18