719
FXUS65 KPSR 172108
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
208 PM MST Mon Aug 17 2026

.KEY MESSAGES....

- Temperatures will remain several degrees above daily normals
  this week, with lower desert highs climbing to 108-113 degrees
  and overnight lows in the mid 80s to low 90s.

- Extreme Heat Warnings are in effect through Friday evening due to
  widespread Major HeatRisk.

- Thunderstorm activity will be mostly confined to the Arizona
  high terrain over the next few days, but chances of affecting
  the lower deserts should increase by Wednesday or Thursday.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...
Latest mid-lvl wv imagery and 500 mb streamline analysis reveal a
broad an expansive ridge of high pressure over the entire western
half of the CONUS. Under this ridge, a weakness in the height field
is apparent across NM and far W TX where an easterly wave/inverted
trough is present. Although dry air is in place over south-central
AZ, instability and lift associated with the approaching shortwave
will provide a focus for shower and thunderstorm activity mainly
across far E And southeast AZ this afternoon and early evening. The
highest PoPs today will only be up around 15-20% focused in S Gila
County where a few isolated storms will be possible. High-res models
do suggest more widely scattered activity in the TWC forecast area,
but the chances for any storms reaching the lower deserts is
currently <10%. As storms begin to weaken early this evening, a
residual outflow boundary could make it into far E Maricopa and
Pinal Counties, briefly causing winds to gusts around 20-25 mph and
kicking up a bit of dust. Otherwise, the outflow will likely
dissipate before making it into the Phoenix Metro. A gulf surge
early Tuesday morning will foster low-level moisture advection from
Mexico into southeast California and southwest Arizona. This will
promote a slight increase of precipitable water values up to 1.2"-
1.3" across the lower deserts which is still not optimal for
convective activity. Isolated storms will again be confined to the
higher terrain with a bit higher coverage along the Mogollon Rim and
possibly S Gila County Tuesday afternoon, but chances for the lower
deserts will remain <10%.

The unseasonably strong subtropical high will continue to dominate
the weather pattern through much of this week with H5 hghts
remaining around 594-596 dam. Hghts of this magnitude are exceeding
the 90th percentile of climatology for mid August. These positive
hght anomalies will result in high temperatures reaching 108-113
degrees across the lower deserts, or around 5-7 degrees above
normal. An Extreme Heat Warning is now in effect for much of the
forecast area and will continue through the rest of this week.
Make sure to take all necessary heat precautions this week,
especially if spending prolonged periods of time outdoors.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Forecast uncertainty has actually increased for the latter half
of the week as guidance is now favoring slightly better moisture
than previously shown while also repositioning the expected
subtropical ridge center closer to the Four Corners area by
Thursday. The forecast strength of the ridge has increased too,
but if the ridge center is more removed from southern and central
Arizona it will increase the convective potential. Although there
is a good amount of ensemble spread for Wednesday and Thursday,
the amount of members indicating QPF spreading into the lower
deserts has more than doubled. Forecast mean PWATs are now shown
to reach 1.3-1.5" Wednesday and 1.4-1.6" Thursday, or right around
normal for the latter half of August. If the ridge does in fact
end up moving far enough to the north, it should help to decrease
the CIN over the area by Thursday. As of now, Wednesday should
still involve mostly high terrain convection, but we can`t rule
out decent outflows pushing into the south-central Arizona lower
deserts. Despite the uncertainty, storm chances for Thursday have
definitely increased enough to go higher than NBM guidance. There
are also indications of shear increasing by Thursday which could
result in some stronger storms. However, we can`t put a lot of
stock in this so far as models have a poor track record with more
mesoscale features as of late. Beyond Thursday, guidance mostly
keeps with the near normal moisture levels across the area which
means day-to-day somewhat low-end monsoon chances are likely to
persist into at least next weekend.

The temperature forecast for the latter half of the week is also
more uncertain, at least for eastern and central Arizona where
higher chances for convection may eventually affect temperatures.
NBM/WPC forecast temperatures have only slightly started to
account for the potential increase in convection with lower desert
highs roughly a degree or two cooler for Wednesday-Friday, but
still easily high enough to keep the Extreme Heat Warning in
place. However, it definitely is plausible the warning could end
up being cancelled early across portions of Arizona if convection
increases enough later this week.

&&

.AVIATION...Updated at 1735Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No aviation weather concerns expected under few mid/high cloud
decks can be expected throughout the TAF period. The overall wind
pattern will exhibit the typical diurnal tendencies with speeds
aob 10 kts. Thunderstorms over the higher terrain areas well east
of Phoenix will likely send an outflow into the Phoenix area late
this evening, most likely moving in after 03-04Z. The outflow will
mostly serve as an easterly wind shift. No significant gusts are
anticipated and no slantwise visibility reductions from dust
expected with the outflow passage at this time.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major aviation weather concerns under some passing mid/high
cloud decks can be expected throughout the TAF period. At KIPL,
very light westerly/variable winds this morning will shift out of
the southeast early this afternoon before shifting back out of
the west early this evening. At KBLH, winds will fluctuate out of
the south to southwest. Overall wind speeds will remain aob 10
kts, although some modest afternoon/early evening gusts will be
possible. A period of light variability will be common during the
early morning hours.

&&

.FIRE WEATHER...
Above normal temperatures with a steady increase in low-level
moisture is anticipated over the next several days. Most lower
desert communities will see temperatures topping out around
110-113 degrees, or around 5-7 degrees above daily normals.
Widely scattered showers and thunderstorms are forecast for the
high terrain through Tuesday with an increase in coverage expected
on Wednesday. Better rain chances for the lower deserts will
arrive by the latter half of this week. Relative humidity values
will bottom out between 15-25% through Tuesday before increasing
to 20-30% starting Wednesday. Overnight recoveries will also
increase from fair to good starting Wednesday night. Outside of
gusty and erratic winds near any thunderstorm, overall winds will
follow the typical diurnal tendencies with some modest afternoon
upslope gustiness.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Extreme Heat Warning until 8 PM MST Friday for AZZ530>544-546-
     548>551-553>555-559.

     Extreme Heat Warning from 10 AM Tuesday to 8 PM MST Friday for
     AZZ560.

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

&&

$$

SHORT TERM...Salerno
LONG TERM...Kuhlman
AVIATION...Lojero/Benedict
FIRE WEATHER...Salerno/Kuhlman