054
FXUS65 KPSR 180502
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1000 PM MST Mon Aug 17 2026

.UPDATE...
Updated Aviation

&&

.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 reveals a
broad, 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 east and southeast AZ this afternoon and early evening.
The highest PoPs today will only be around 15-20% focused in S Gila
County where a few isolated storms will be possible. High-res models
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 heights
remaining around 594-596 dam. Hghts of this magnitude are exceeding
the 90th percentile of climatology for mid August. These positive
height 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 0500Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major weather concerns will exist through Tuesday night under a
few mid/high cloud decks. Confidence in timing of wind shifts
remains low as easterly outflow winds have nearly stalled east of
KPHX, but still should incur a wind shift during the overnight and
earlier than the past several days. Models also continue to exhibit
directional variability Tuesday afternoon as winds revert back to a
westerly direction, followed by the potential for another easterly
outflow wind shift Tuesday evening.


Southeast California/Southwest Arizona including KIPL and KBLH:
No weather concerns will exist through Tuesday night under clear
skies. Trends in wind speeds and timing of directional shifts will
be similar to the past 24 hours featuring little to no gustiness,
and extended periods of variability 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...18
FIRE WEATHER...Salerno/Kuhlman