611
FXUS65 KPSR 181058
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
Issued by National Weather Service Tucson AZ
358 AM MST Tue Aug 18 2026

.KEY MESSAGES....

- Temperatures will remain several degrees above daily normals
  this week with lower desert highs 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 through mid-week
  with chances returning to the south-central Arizona lower
  deserts as early as Wednesday.

&&

.SHORT TERM /TODAY THROUGH THURSDAY/....
Upper level ridging is becoming more established across the
Southwestern U.S., but it remains elongated from west to east
with a weak disturbance moving westward near the Mexican border of
New Mexico and Arizona. H5 heights have climbed to around 596dm
helping to push daytime highs to above 110 degrees across the
majority of the lower deserts. Widespread Major HeatRisk is
expected to persist with highs mostly between 109-113 degrees
through at least Wednesday with a bit more uncertainty starting
Thursday. Models continue to pivot more toward increasing moisture
starting today and more widespread storm chances by Wednesday.

The elongated ridge is expected to form an established center
somewhere across western Arizona tonight before shifting more over
northern Arizona on Wednesday. This shift in the ridge is expected
to allow for better moisture advection into southern and central
Arizona from the southeast and off the Gulf of California.
Moisture is still expected to be quite limited today with only
high terrain scattered showers and isolated storms, but that
should change starting Wednesday. Forecast low level mixing ratios
are now expected to climb to a respectable 10-12 g/kg by Wednesday
afternoon. The exiting disturbance across northern Mexico is also
likely to help provide some upper level support for storms on
Wednesday with southeast Arizona positioned near the right
entrance region of a jet streak just to our south. The 06Z HRRR is
starting to pick up on a better signal for storm activity late
Wednesday afternoon across the AZ high terrain and into the south-
central AZ lower deserts during the evening. Any storms will
still have to fight against some weak to modest CIN over the lower
deserts, but if that can be overcome there should be plenty of
instability for further storm development. We have added 10-20%
PoPs for Wednesday evening through the Phoenix area to account for
this possibility. The environment will favor strong gusty winds
on Wednesday with DCAPEs over 1500 J/kg likely.

A similar monsoon storm set-up is expected for Thursday, but this
time the NBM recognizes the potential with more reasonable 15-30%
PoPs across the area. However, if the convection is more
widespread on Wednesday, storm chances Thursday may be too
optimistic. Additionally, if we do have more storm coverage than
anticipated, it is plausible (but not very likely) for
temperatures to meaningfully cool off by Thursday or Friday.

&&

.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 0900Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

No major weather concerns will exist through the TAF period under
a FEW/SCT mid/high cloud decks. Current easterly winds will make
a slow turn to westerly this afternoon with a brief period of
southerly cross winds expected prior to the westerly shift. Models
continue to show potential for another easterly outflow wind
shift this evening (around 03-04Z). However, this looks to be
brief and winds may go back westerly (except at KIWA) as models
also indicate a Gulf surge tomorrow evening making the winds go
back southerly/southwesterly.

Southeast California/Southwest Arizona including KIPL and KBLH:

No weather concerns will exist through the TAF period under clear
FEW/SCT high cloud decks. Trends in wind speeds and timing of
directional shifts will be similar to the past 24 hours featuring
little to no gustiness.

&&

.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 temperatures topping out around 110-113
degrees. Widely scattered showers and thunderstorms are forecast
for the high terrain today with an increase in coverage expected
Wednesday into Thursday. Relative humidity values of 15-25% today
will increase to 20-30% starting Wednesday. 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 this morning to 8 PM MST Friday
     for AZZ560.

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

&&

$$

SHORT TERM...Kuhlman
LONG TERM...Kuhlman
AVIATION...18/Berislawich
FIRE WEATHER...Kuhlman