Excessive Heat in Arizona, August 4, 2023

Area Forecast Discussion
National Weather Service Phoenix AZ
429 AM MST Fri Aug 4 2023

.UPDATE...Updated Aviation Discussion...

&&

.SYNOPSIS...
Dry and very hot conditions are expected through this weekend with
afternoon high temperatures reaching 110 to 115 degrees across the
lower deserts. An Excessive Heat Warning begins today, Friday, for
South-Central Arizona and expands to Southwest Arizona and
Southeast California beginning Saturday. An increase in humidity
is anticipated by early next week. However, storm chances will be
slight and remain confined to primarily over the higher terrain of
southern and eastern Arizona.

&&

.DISCUSSION...
The regional environment continues to rapidly dry out and
strengthening high pressure is leading to hotter temperatures.
Latest mesoanalysis and satellite imagery places the H5 high over
far Southeast AZ/Northern Mexico. Deep southwesterly flow remains
in place across much of AZ and SoCal, which is the leading cause
of the drying trend. Surface dew points are already down into the
30s-40s for most areas and analysis shows PWATs are under 0.8" for
nearly all of AZ to under 0.5" in SoCal. Suffice to say, regional
monsoon thunderstorms will be taking a small break under this dry
pattern. The focus once again returns to the heat in what has
already been a record-breaking summer and more records are
forecast to be broken.

An Excessive Heat Warning goes into effect beginning today,
Friday, for South-Central AZ lower deserts, including Phoenix.
Aircraft soundings out of Phoenix show a slight lower tropospheric
warming over the past 24 hours. So, afternoon temperatures today
are expected to be another degree or two warmer than Thursday.
This will be enough of a warm up to push HeatRisk into the high-
end Moderate level with areas of Major. Through this weekend the
H5 high is expected to remain just to the southeast, but continue
to strengthen. The most recent 00Z sounding out of Tucson measured
a 595dm 500mb height. Global ensembles are in great agreement that
H5 heights will climb up to 598-600dm this weekend. With the
continued warming into the weekend, areas west of Maricopa County,
AZ get added to the Excessive Heat Warning beginning Saturday.

The Excessive Heat Warning currently extends through Monday for
all areas. H5 heights are forecast to lower a few decameters
during early next week and a considerable moisture return is
expected Sunday into Monday, but latest forecast temperatures
from the NBM remain above normal to the point where an extension
of the Excessive Heat Warning could be considered. Forecast highs
are around 110-114F (lower deserts) for much of next week, with
morning lows in the mid-80s to 90F. In the short-term, through
this weekend, the drier conditions will make the mornings more
pleasant than they have been in quite some time. As the heat
continues it is important to continue to practice heat safety and
take actions to protect yourself and others from the heat and the
sun.

As previously mentioned, the H5 high is expected to remain to the
southeast of the local area through the weekend. Global ensemble
clusters in the 8-10 day window even show little variance and
have the high sticking around in a similar location. This high
placement is not an ideal location for shower and thunderstorm
chances for the lower deserts of South-Central AZ and points west,
with the persistent west to southwest flow aloft. There are
certainly some mesoscale changes that can happen to influence the
environment going forward, but it is difficult for models to
resolve any features in the extended forecast. Global models do
keep the high pressure relatively flat through the next several
days, to the point where easterly winds across northern Mexico
will support potential easterly perturbations, which could fuel
Sonoran thunderstorm complexes. As mentioned, a moisture return to
the deserts is forecast Sunday into Monday, likely associated with
a gulf surge from the south and possibly enhanced by Sonoran
activity. This moisture return at a minimum will bring PoPs back
up to slight chances (15% or higher) in typical terrain favored
areas of southern, eastern, and northern areas of AZ, mainly Southern
Gila County in our CWA, beginning Monday afternoon.

It may still be quite a while yet before the next good chance at
rain in the Phoenix area and the official Phoenix climate site
(Sky Harbor Airport) will be making a push for the second longest
streak without measurable rain. The current streak is at 134 days,
which is already the 4th longest streak behind 160 days (1972) and
143 days (2006 & 1960). Will note that it has rained in the
greater Phoenix area on a few occasions this summer so far. There
are a few spots that have measured over 1 inch. The airport
unfortunately has barely missed out on a few occasions with a
couple reports of a trace.

&&

.AVIATION...Updated at 1130Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major weather issues will exist through Saturday morning under
clear skies. Light east winds this morning are likely to become
variable for a brief period around noon before making the typical
shift back to westerly. Little if any gusts are anticipated this
afternoon/early evening.

Southeast California/Southwest Arizona including KIPL and KBLH:
No significant weather issues will exist through tonight under
clear skies. Northerly drainage winds this morning at KBLH will
eventually revert back to a southerly wind by early this
afternoon. At KIPL, west winds are likely to weaken later this
morning becoming variable or light easterly this afternoon. While
the evening shift to westerly can be expected at KIPL, gusts
should be notably absent.

&&

.FIRE WEATHER...
Abnormally hot temperatures are expected through the next several
days under strong high pressure. Lower desert temperatures will
push 110-115F in the afternoons. Very dry conditions are also
expected today through this weekend, with afternoon minimum
humidity (RH) values at or below 15%. A moisture increase heading
into Monday will push min RH values up to 15-20% across South-
Central AZ. Much of the remainder of the week min RH values will
range from 10-20%. Overnight recoveries will be highest Sunday
night with values around 35-45%. Otherwise, recoveries will be
around 20-35%. Winds are forecast to remain below critical
thresholds, with 15-25 mph afternoon westerly gusts, most days.
Strongest winds are expected Sunday afternoon, with westerly gusts
potentially reaching up to 30-35 mph in parts of Arizona. Winds
during overnight hours will become light and follow typical
diurnal and terrain trends. With the moisture return Monday,
thunderstorm activity will return, primarily in high terrain areas
of southern and eastern AZ, but chances are only slight (15-25%)
at this time. Chances are mostly under 10% across lower desert
locations. Still, erratic winds from distant thunderstorms will
become possible again.

&&

.CLIMATE...

Phoenix Daily Record High Temperatures

8/4:  116 (1975)
8/5:  115 (2019)
8/6:  114 (2018 & 1995)
8/7:  112 (2012 & 1905)
8/8:  116 (2012)
8/9:  114 (2012)

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM Saturday to 8 PM MST Monday
     for AZZ530>533-535-536.

     Excessive Heat Warning from 10 AM this morning to 8 PM MST
     Monday for AZZ534-537>555-559>561.

CA...Excessive Heat Warning from 10 AM Saturday to 8 PM PDT Monday
     for CAZ560>570.

&&

DISCUSSION...Benedict
AVIATION...18/Kuhlman
FIRE WEATHER...Benedict
CLIMATE...Benedict