174
FXUS65 KPSR 282121
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
221 PM MST Tue Jul 28 2026

.KEY MESSAGES...

- High pressure will center itself overhead and strengthen late
  this week, resulting in hotter temperatures and widespread Major
  to Extreme Heat Risk across the forecast area.

- Extreme Heat Warnings remain in effect through the weekend and
  expand to cover the entire forecast area starting Thursday as
  the extreme heat conditions build late week.

- Daily chances for showers and thunderstorms will continue over
  Eastern Arizona high terrain and portions of Southeast Arizona
  but will decrease considerably and become almost entirely
  confined to the most prominent terrain features by the latter
  half of the work week.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...
A couple MCVs tracked northward through Gila/Pinal/Maricopa
Counties this morning providing forcing for showers and releasing
elevated instability of up to 1000-1500 J/kg (backed up by earlier
RAP analysis and the 12Z PSR sounding) to produce isolated
thunderstorms and locally heavy rainfall for some. While not all
saw rainfall, portions of the East Valley received anywhere from a
few hundredths of an inch up to around 0.3" from this morning`s
activity. Further east in Southern Gila County, automated gauges
that were lucky enough to have convective cells track overhead
recorded upwards of 0.25" to as much as 1.0" in some spots. Rain-
cooled, humid air overspread much of South-Central AZ this
morning, dropping morning lows to a few degrees below the daily
normals, ranging from the upper 70s to middle 80s and increasing
dewpoint temperatures to between the middle 60s and lower 70s.
With cooler temperatures to start the day and delayed surface
heating, expect daytime highs several degrees cooler than
originally forecast, likely reaching near normal values (105F-107F
in the Phoenix Metro) across South-Central AZ and a portion of
Southwest AZ, whereas temperatures will remain several degrees
above normal for the western deserts.

The forecast area remains under easterly to southeasterly flow aloft
on the western periphery of a now weaker subtropical high centered
approximately over Northern NM at this hour. Current 500 mb RAP
analysis and midlevel water vapor imagery suggests a subtle easterly
wave and yet another vort max or MCV feature just east of the
northern tip of the Gulf of California. These will pass into Western
AZ and be pulled northward today thanks to the anticyclonic flow
about the subtropical high. As we are now on the backside of a
few MCVs and the aforementioned easterly wave, subsidence seems
to be favored over the region and at the very least over South-
Central AZ. Latest GFS bufr soundings for KPHX display a midlevel
inversion this afternoon, indicative of this subsidence, and poor
850-700 mb lapse rates can be seen in early afternoon PHX ACARS
soundings, putting a considerable cap on the local environment.
However, this cap will erode through the remainder of the
afternoon, and CAMs do indicate the potential for thunderstorms
to form over some of the typically favored terrain such as the
Tabletop Mountain Wilderness and the Rim/far eastern Gila County.
These could produce locally gusty winds and heavy rainfall. Wind
shear is also still present, with bulk effective shear upwards of
25-35 kts over the southeastern third of the state. While storm
coverage is very uncertain and likely to be quite low based on
latest CAMs, any that do form could develop rotating updrafts and
be capable of severe wind gusts. HREF consensus is that the threat
is greater towards the AZ/NM border in the southeastern corner of
the state, where the cap will erode quicker and allow surface
based convection to develop late this afternoon. A few severe-
warned cells have already developed over the White Mountains.

This evening/overnight tonight, less evidence exists that an
upstream MCV will form and track northward over the forecast area,
however, the possibility cannot be entirely ruled out. It is highly
dependent on thunderstorm coverage and intensity upstream over
Sonora or the southeastern corner of the state. CAMs do seem to be
picking up on the influence of another axis of vorticity being
pulled northward around the anticyclone and providing enough lift
ahead of it to produce showers or even an isolated storm over
Southwest AZ tonight. On Wednesday, ensembles remain in good
agreement that flow aloft will shift southerly to even a slightly
southwesterly orientation, starting a W-E drying trend. By late
Wednesday, lower desert near-surface mixing ratios drop below 10
g/kg, essentially ending lower desert storm chances. Daily
thunderstorm activity across the region will diminish and remain
diminished heading through the rest of the work week. Ensemble
mean H5 heights will continue in a 594-596 dam range, between the
90th and 95th percentile of CFSR climatology, supporting above
normal highs around 109F-113F for the typically hotter spots
Wednesday.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Ensembles remain in good agreement that the subtropical high over NM
will retrograde into AZ and strengthen, especially by Friday-
Saturday, with mean H5 heights building to a 599-601 dam range, near
or just above CFSR climatological maximum values. NAEFS/ENS mean 850
mb temperatures will also approach climatological maximum values
around 32C-35C. Under drier conditions, much clearer skies thanks to
reduced thunderstorm activity, and strong anticyclonic subsidence,
heights/temperatures aloft in this range will support afternoon
surface temperatures peaking between 115F-120F, with values on the
higher end of that range favored in Southeast CA. Saturday is
shaping up to be the hottest day, with NBM advertising 25-40%
probabilities of reaching 120+F in some of the hottest spots across
Southeast CA and along the lower Colorado River Valley. Overnight
lows are also forecast to warm into the upper 80s to lower 90s,
providing very little relief from the heat. Coverage of Major
HeatRisk builds starting Thursday and is widespread Friday-Sunday,
with areas of Extreme HeatRisk indicated in those areas where
overnight lows do not cool below 90F. As such, widespread Extreme
Heat Warnings go into effect for the entire forecast area Thursday
and will continue through the weekend.

Long range global guidance continues to indicate moisture increasing
across the area as the subtropical high weakens late this weekend
into the following week. However, details are very uncertain this
far out. Current consensus of the GEFS and ENS is for the northern
stream to impinge further south, flattening the subtropical ridge.
meanwhile, flow aloft on the southern periphery of the subtropical
anticyclone strengthens and turns easterly to northeasterly,
exposing the region to the possibility of easterly waves and better
shear/steering flow to carry storms off Eastern/Southeastern AZ
terrain features. Regardless of the eventual increase in moisture
and thunderstorm activity, this will also help end the extended
period of extreme heat conditions by early next week.

&&

.AVIATION...Updated at 1745Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major aviation concerns are expected through Wednesday morning
under SCT-BKN cloud cover. Winds will follow familiar diurnal
trends with some afternoon gusts in the mid to upper teens. KPHX
will deviate slightly from typical directional patterns at W`rly
winds look to prevail overnight.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major aviation concerns are expected during the next 24 hours.
SE`rly winds will be common at KIPL, with perhaps a brief switch
to the SW/W this evening. S`rly flow, with periods of gusts around
20-25 kt, will persist at KBLH. VCSH may pop near KBLH overnight,
but chances are too low to warrant any inclusion in the TAF at
this time. SCT-BKN cloud decks will be present across the region
through Wednesday morning.

&&

.FIRE WEATHER...
Fairly typical monsoon conditions remain in place through tonight
before more notable drying occurs from west to east on Wednesday.
Low chances (10-20% at most) for scattered showers and even some
isolated storms remain possible this evening and overnight for
portions of Southwest and South-Central AZ, with gusty, erratic
winds and locally heavy rainfall the main concerns. Near to
slightly above normal temperatures will continue through mid week
before even hotter temperatures arrive late week into the weekend.
Humidities will decrease through the rest of the week, with
afternoon minimums dropping into a 7-15% range across the western
districts and 12-20% range across the eastern districts, following
poor to fair overnight recoveries starting Wednesday night. Winds
outside of thunderstorm outflows will follow fairly typical trends
with upslope/upvalley afternoon and early evening gusts into a
20-25 mph range common. As the drying occurs during the latter
half of the week, locally elevated fire weather conditions are
likely to develop, with low afternoon humidities and the typical
upslope/upvalley breeziness.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Extreme Heat Warning until 8 PM MST Sunday for AZZ530-532-537-
     540-542>544-546-548-550-551.

     Extreme Heat Warning from 10 AM Thursday to 8 PM MST Sunday for
     AZZ531-533>536-538-539-541-545-547-549-552>563.

CA...Extreme Heat Warning until 8 PM PDT Sunday for CAZ567-569-570.

     Extreme Heat Warning from 10 AM Thursday to 8 PM PDT Sunday for
     CAZ560>566-568.

&&

$$

SHORT TERM...Whittock
LONG TERM...Whittock
AVIATION...RW
FIRE WEATHER...Whittock