984
FXUS65 KPSR 072020
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
120 PM MST Mon Sep 7 2026

.KEY MESSAGES...

- Above normal temperatures will prevail starting Tuesday,
  resulting in widespread Moderate Heat Risk and areas of Major
  Heat Risk.

- Extreme Heat Warnings will begin Tuesday for portions of the
  lower deserts.

- Mostly quiet weather conditions are expected for the first half
  of the week, then chances for thunderstorms increase across
  Central and Eastern Arizona during the latter half of the week.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...
The RAP40 500MB analysis and GOES WV imagery continue to show the
subtropical ridge positioned over northwestern TX, a large trough
over NW CONUS. The Desert SW sits between these two features with a
feint stream of upper level moisture being advected into the western
lower deserts resulting in a few light showers/virga early this
morning and lingering upper level clouds.

Ensembles remain in good agreement that the subtropical high will
begin to shift westwards, peaking over the Desert SW by mid week
with H5 heights between 594-597 dam. 500 MB heights at this level
are near climatological maximum values for the time of year. This
will lead to afternoon temperatures 4-5 degrees above normal,
resulting in temperatures between 105F-111F. Overnight lows will
also provide very little relief as the lingering cloud coverage
mentioned earlier will hinder radiative cooling, and dew points
are expected to be well into the 60s across much of the forecast
area and locally into the lower to middle 70s for areas such as
the Lower CO River Valley and Imperial Valley. With all of these
factors taken into consideration Major HeatRisk will make a
return to most of the region tomorrow and even more widespread by
Wednesday, with the overnight lows being the main driver as they
are flirting with the record warm lows through the end of the work
week. As a result, Extreme Heat Warnings have been issued for
portions of the lower deserts staring on Tuesday (starting
Wednesday for Lower CO River Valley, Desert Center, and La Paz
County).

Little to no chance for showers/thunderstorms are expected
through mid week under strong subsidence thanks to the subtropical
high, with the exception being the far eastern high terrain.
However, by Wednesday night, low level moisture ticks up with
1000-700 MB mixing ratios increasing to 10-12 g/kg. Enough
instability, and this moisture in place, SC AZ lower elevations
could develop isolated storms as early as Wednesday night.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
Latest NBM/WPC guidance depicts the greatest coverage of Major
HeatRisk across the forecast area on Thursday, with morning lows in
the middle to upper 80s across the lower deserts and forecast highs
106F-110F. These temperatures during the hottest time of the year
may not prompt extreme heat headlines, but as we head into
meteorological fall, daily normals drop off noticeably, resulting in
more sensitive Major HeatRisk thresholds than what folks may have
gotten accustomed to over the last few months. It also makes sense
that Thursday would be the warmest of the next several days, as
thermal profiles would have had more time to respond to the near
climatological maximum strength ridge overhead. Heat indices will
likely also be at their highest values, peaking between 108F-117F
across the lower deserts. Temperatures then begin a gradual
downtrend heading into the weekend as the subtropical high weakens
and is displaced from overhead.

As anticyclonic subsidence weakens, especially by late Thursday and
into Friday, chances for lower desert thunderstorms become
introduced across South-Central AZ (20-30%). A familiar monsoon
thunderstorm evolution may begin to take place beginning Thursday
and could last into the weekend, with afternoon thunderstorms
focused over the eastern AZ high terrain, which will push outflows
towards South-Central AZ in the evening that may be capable of
sparking additional thunderstorms. Current thermodynamic profiles
suggest all the typical monsoon thunderstorm hazards may come into
play, including strong, gusty winds, blowing dust, localized heavy
rain resulting in flash flooding, and frequent lightning. However,
as we head into the weekend, the forecast confidence decreases
substantially. WPC cluster analysis reveals large discrepancies,
with close to 40% of ensemble members comprising a solution which
shows negative midlevel height anomalies diving into the
northwestern and north-central CONUS, possibly impacting the flow
aloft as far south as the Desert Southwest and turning winds out of
the west, beginning a drying trend. Other solutions comprise
clusters which show subtropical ridging remaining in place somewhere
near/over the Southwest US and keeping the influence of northern
stream energy far enough north so as not to influence flow aloft
significantly. Either way, drying will likely not be pronounced
enough to preclude rain/thunderstorm chances once a sufficient
forcing mechanism becomes introduced. Ensemble mean PWATs even
remain near or above seasonal levels this weekend. Current NBM/WPC
PoPs and QPF suggest higher/more widespread chances heading into
Sunday, perhaps picking up on the influence of a distant/elongated
shortwave trough axis rotating through from the Northwest US
troughing that a good chunk of solutions show.

&&

.AVIATION...Updated at 1715Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major weather issues under occasionally mid/high cloud decks
can be expected throughout the TAF period. The overall wind
pattern will follow the typical diurnal tendencies with some
occasional gusts near 20 kts this afternoon/early evening.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major weather issues under occasionally mid/high cloud decks
can be expected throughout the TAF period. Winds will generally
favor a west to north-northwest direction throughout the period
with speeds generally under 10 kts along with periods of light and
variable conditions.

&&

.FIRE WEATHER...
Temperatures will warm to above normal levels through the middle
of the week. Afternoon minimum humidities will generally range
between 20-30% areawide through much of the week, with Tuesday
expecting a larger area across the lower deserts between 15-20%
before rebounding back near 20% Wednesday. Fair to good overnight
recoveries are expected throughout the week. Fairly typical
diurnal winds with occasional afternoon upslope breeziness can be
expected through much of the week. Rain and thunderstorm chances
will remain low and mostly limited to the eastern Arizona high
terrain through midweek before slight chances become introduced to
the lower deserts of at least the eastern districts during the
latter half of the week.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...     Extreme Heat Warning from 10 AM Tuesday to 8 PM MST Thursday for
     AZZ531-532-535>544-546-548>551-559.

     Extreme Heat Warning from 10 AM Wednesday to 8 PM MST Friday for
     AZZ530-533.

CA...Extreme Heat Warning from 10 AM Tuesday to 8 PM PDT Friday for
     CAZ562-566-567.

     Extreme Heat Warning from 10 AM Wednesday to 8 PM PDT Friday for
     CAZ569-570.

&&

$$

SHORT TERM...Ryan
LONG TERM...Whittock
AVIATION...Lojero
FIRE WEATHER...Whittock/Ryan