122
FXUS65 KPSR 190517
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1017 PM MST Tue Aug 18 2026

.UPDATE...Updated Aviation Discussion.

&&

.KEY MESSAGES....

- Temperatures will remain several degrees above daily normals
  through this weekend with lower desert highs ranging between
  107-113 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 is expected to increase in coverage
  through mid-week with chances returning to the south-central
  Arizona lower deserts as early as Wednesday.

&&

.SHORT TERM /TODAY THROUGH THURSDAY/....
Latest mid-lvl wv imagery and 500 mb streamline analysis reveals an
elongated area of high pressure stretching across the western CONUS.
Currently the ridge axis is situated over western AZ allowing 500 mb
hghts to reach 596-597 dam across our forecast area. Hghts of this
magnitude are above the 90th percentile of climatology for this
time of year. As such, high temperatures will once again top out
well above normal this afternoon, ranging from 108-113 degrees
across the lower deserts. The ridge axis will begin to shift NE
over the AZ/UT border on Wednesday which will promote a slight
decrease in overall heights/thickness. Although highs will cool by
a degree or two Wednesday and Thursday, apparent temperatures
(heat indices) are actually expected to increase to around 109-117
degrees as dewpoints rise into the mid 60s both afternoons. Thus,
widespread Major HeatRisk is expected to persist and the Extreme
Heat Warning will continue.

Moisture is still expected to be very limited over south-central AZ
this afternoon with PWATs hovering around 1.2-1.3". Therefore,
only isolated high-terrain convection is anticipated across S Gila
County. The northward migration of the subtropical ridge will
allow for much better moisture advection into the region starting
on Wednesday with PWAT values rising to around 1.5-1.7". This
noticeable increase in moisture along with greater instability
(ML CAPE 800-1000 J/kg) and a less robust capping inversion (ML
CIN -80 to -100 J/kg) will allow for better odds of storms
reaching the lower deserts by Wednesday evening with NBM PoPs now
around 20-30%. The overall environment will also be favorable for
longer- lived outflow boundaries and strong winds which could
result in blowing dust across the favorable dust prone regions of
N Pinal and SE Maricopa Counties. Almost identical environmental
conditions are anticipated on Thursday, although convective
potential across the lower deserts will be highly dependent on how
much storm activity we see Wednesday evening. For now, NBM PoPs
are currently 20-30% across southcentral and southwest AZ Thursday
afternoon/evening which seems reasonable.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/....
Ensemble guidance generally agree the subtropical high will
recenter itself near the Four Corners region on Friday and then
gradually shift eastward through New Mexico this weekend. The
position of the high should allow for fairly persistent south
southeasterly flow into Arizona, likely maintaining moisture
levels for much of the period. As the ridge shifts more to the
east, an upper level trough may also attempt to nudge into western
portions of our region by early next week. We expect at least
day-to-day low-end monsoon storm chances each day, but there are
no clear indications of any decent disturbances that may enhance
chances. Above normal temperatures are forecast through at least
the upcoming weekend, but as the ridge shifts more to the east
temperatures are expected to drop off a bit with highs falling
below 110 degrees by next Sunday or Monday.

&&

.AVIATION...Updated at 0520Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
The only aviation weather issue through Wednesday afternoon will
be elevated westerly winds overnight and the potential for modest
LLWS conditions from approximately 07-11Z (below TAF criteria).
Confidence is low on exact wind directions Wednesday morning,
however, speeds should relax to AOB 6 kts, followed by a typical
increase in westerly winds late morning into the afternoon.
Occasional late afternoon gusts between 15-20 kts are anticipated.
Cloud bases will remain generally above 8 kft AGL through the TAF
period, with FEW-SCT and occasionally BKN coverage of mid and
high cloud decks.

By Wednesday evening, conditions will be more favorable for
strong, gusty outflows to survive into the lower deserts. These could
spark additional SHRA/TSRA in the Phoenix Airspace. Chances for
SHRA/TSRA remain around 20%. Additionally, strong gusty outflow
winds from the south or southeast could bring blowing/lofted dust
and result in temporary surface VIS or slantwise VIS
restrictions.

Southeast California/Southwest Arizona including KIPL and KBLH:
No weather concerns will exist through the next 24 hours under
FEW-SCT high cloud decks. Winds at KIPL will favor southeasterly
with speeds AOB 10 kts sustained. Winds at KBLH will continue to
oscillate between SSE and SW with occasional gusts to around 20
kts, especially by Wednesday morning.

&&

.FIRE WEATHER...
Above normal temperatures with a steady increase in low-level
moisture is anticipated over the next few days. Most lower desert
communities will see afternoon high temperatures topping out around
108-113 degrees through the rest of this week. Widely scattered
showers and storms are forecast for the AZ high terrain today with
an increase in coverage, including the lower deserts expected
Wednesday into Thursday. Relative humidity values of 15-25% today
will increase to 20-30% starting Wednesday with continued fair to
good overnight recoveries. 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>555-559-560.

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

&&

$$

SHORT TERM...Salerno
LONG TERM...Kuhlman
AVIATION...Whittock
FIRE WEATHER...Kuhlman/Salerno