Excessive Heat in California, August 5 to 7, 2023

Area Forecast Discussion
National Weather Service Phoenix AZ
442 AM MST Sat Aug 5 2023

.UPDATE...Updated 12Z Aviation Discussion.

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.SYNOPSIS...
Strong high pressure over the region will continue to bring very
hot temperatures region-wide this weekend with afternoon high
temperatures reaching 110 to 116 degrees across the lower deserts.
An Excessive Heat Warning remains in effect for South-Central
Arizona, expanding to include Southwest Arizona and Southeast
California today, lasting through Monday. An increase in moisture
is anticipated Sunday into Monday resulting in a return of overall
low chances for monsoonal thunderstorms starting Monday and
eventually leading to a slight cooling trend early next week.

&&

.DISCUSSION...
Unseasonably dry air continues to impact the region under a
strengthening sub-tropical high pressure ridge which is currently
centered near the Arizona/New Mexico/Mexico border. H5 heights of
593-595dm seen yesterday are forecast to rise to around 597dm by
this evening and persist through Sunday night providing additional
warming to the airmass over the region. With dew points remaining
in the 30s to low 40s today, afternoon high temperatures are
expected to top out in a 111 to 116 degree range across the lower
deserts, threatening the record high in Phoenix which is 115
degrees. The Excessive Heat Warning also goes into effect today
areawide and currently goes out through Monday.

Starting tonight, low level moisture is expected to advect
northward off the Gulf of California into southern Arizona and
southeast California, raising dew points into a 50-60 degree
range, a significant change from the past several days. This
moisture will be fairly shallow initially on Sunday with low level
mixing ratios up to around 8 g/kg, while subsidence from the high
pressure ridge overhead should keep any convection from forming.
However, the moisture will start to impact temperatures with
daytime highs dropping a couple degrees from today's readings and
overnight lows Sunday night/Monday morning warming 2-4 degrees
from tonight's readings.

Additional moisture advection is expected Sunday night into
Monday morning with guidance now suggesting better quality
moisture for Monday, likely enough for convection to develop
Monday afternoon as mixing ratios rise to a more respectable to
10 g/kg. Forecast CAPE also increases to 500-1000 J/kg on Monday
centered over Maricopa, Pinal, and Pima counties. Unfortunately,
despite the improved moisture and adequate instability on Monday,
the flow aloft will remain west southwesterly which is definitely
far from optimal. Even with this moisture and instability
increase, convection will still likely favor high terrain areas
south and east of the Phoenix area which will limit chances to the
south-central Arizona deserts to around 10-20%. Forecast
soundings for Monday currently support at least a minor gusty wind
threat with any storms or storm clusters potentially creating one
or more northward or eastward progressing outflows. NBM forecast
temperatures Monday continue to trend downward reflecting the
potentially higher moisture levels with highs mostly in a 108-112
degree range across the lower deserts.

Starting Tuesday, ensemble guidance heavily favors a weakening of
the sub-tropical high with the center of the high sinking
southeastward into Mexico. At the same time, the flow of moisture
off the Gulf of California is likely to stop or at least slow down
with some drying seen across the area, especially from Phoenix
westward. NBM PoPs for Tuesday and Wednesday still show a slight
chance for showers and maybe a few storms over the high terrain
east of Phoenix, but chances over the south-central Arizona lower
deserts drop to around 10%. With the weakening of the high
pressure ridge, forecast temperatures beyond Monday now look to
just fall below Major HeatRisk, so an extension of the current
Excessive Heat Warning may not be needed. Little change is seen
during the latter half of next week with southwesterly flow aloft
likely dominating across the region and moisture levels possibly
staying just high enough to support daily slight chances over
higher terrain areas.

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.AVIATION...Updated at 1141Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Light NE/E winds will prevail through the morning and shift W
around noon local time (19Z). Afternoon gusts to around 20 kts are
expected and should abate by 3-4Z this evening. A late east shift
is expected again Sunday morning, as late as 11-12Z. SKC will
prevail.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds at KIPL may prevail out of the SE through the TAF period. A
W shift is shown with the latest TAF, but confidence in this
occurring is relatively low. Speeds through the period should
remain aob 10 kts. Winds will favor the S at KBLH with speeds up
to 10-12 kts and gusts to 20 kts this afternoon. Both terminals
may also experience periods of light variability during the
morning. SKC will prevail.

&&

.FIRE WEATHER...
Abnormally hot temperatures are expected this weekend, with
afternoon highs mainly in a 110-115F range across the lower
deserts. It will also be very dry today with minimum afternoon
humidity values falling to 5-15% before moisture begins increasing
tonight into Sunday. This will help to improve MinRHs into a
15-20% range Sunday, while overnight readings improve to 40-60%
Sunday night. Winds today will generally be, but with some
periodic afternoon gusts to around 20 mph. Winds will then
increase Sunday into Monday with afternoon gusts likely closer to
25 mph Sunday and 30 mph Monday. Precipitation chances will
increase to around 20-40% on Monday across south-central Arizona,
but chances for wetting rains (10-20%) will be limited to higher
terrain areas. Lingering chances for high terrain storms are
likely through the middle of next week, but there should be an
overall slow drying trend once again.

&&

.CLIMATE...

Phoenix Daily Record High Temperatures

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

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

     Excessive Heat Warning until 8 PM MST Monday for AZZ534-537>555-
     559>561.

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

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