Excessive Heat in California, August 30 to 31, 2022

Area Forecast Discussion
National Weather Service Phoenix AZ
400 AM MST Tue Aug 30 2022

.UPDATE...Updated Aviation discussion.

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.SYNOPSIS...
Excessive heat warnings are in effect for many areas, including
Phoenix, beginning Today. Prolonged hot temperatures are forecast
across the Desert Southwest. Many lower desert areas will eclipse
the 110 degree mark this week, with the California deserts likely
experiencing multiple afternoons of 110 plus degrees. With high
pressure in place, thunderstorm chances will lessen considerably
on most days and affect primarily higher elevations.

&&

.DISCUSSION...

***EXCESSIVE HEAT WARNING BEGINS TODAY FOR METRO PHOENIX,
 SOUTHWEST ARIZONA, AND SOUTHEAST CALIFORNIA***

High temperatures at or a few degrees above normal were observed
yesterday and the abnormal heat is here to stay for at least the
next 7 days. Today and Wednesday are expected to be the hottest
days of the week with the greatest atmospheric warming as high
pressure strengthens overhead. Temperatures at 850mb the next
couple of days will reach 28-31C over the area, which will
translate to lower desert surface temperatures up to 110-115F
(hottest in Southeast CA). An Excessive Heat Warning is in effect
now for Today and Wednesday for most lower deserts with widespread
Moderate HeatRisk and localized High HeatRisk expected.

There will be a slight reprieve in high temperatures after
Wednesday with high pressure becoming centered near NV/UT, but
only by a few degrees and highs will still remain above seasonal
norms across the region. However, the Excessive Heat Warning will
continue through Labor Day Weekend for Southeast CA, as there
will be less of a reprieve than other areas. Even on Thursday and
Friday, when NBM forecast highs do not reach 110F in most
locations, high humidity could make the heat just as, if not more,
intense. Overnight moisture fluxes from the Gulf of California
will push morning dew points into the 70s through the end of the
week which will lead to morning and early afternoon heat indices
up to 10-15 degrees above ambient temperatures. There will also be
little relief during the overnight hours across all lower deserts
with lows only dipping into the mid-80s. NBM even supports a
10-30% chance for a 90+ degree low in Phoenix each morning through
the end of the week. All global model clusters hold onto this
pattern, with strong high pressure rooted over the Great Basin,
through this weekend. As far as when this might break down it is
difficult to say, but it may not be until at least early next week
and it might be a Pacific trough and/or some tropical influence
that helps bring a more noticeable cooling.

Rain chances through the next 7 days remain minimal, with NBM PoPs
<10% for the lower deserts and 10-20% in Southern Gila County,
despite the synoptic setup being favorable for Rim-to-Valley.
In the short-term, drier air will greatly limit rain chances.
Objective analysis suggests PWATs have fallen several tenths of an
inch overnight, down to around 0.9-1.2 inches across the area,
slightly below normal. Still there may be days with slightly
better odds than others. A cutoff low seen on satellite this
morning in Eastern NM is expected to push south and west through
Wednesday, with the main vorticity mostly pushing through northern
Mexico and clipping Southeast AZ. While this will increase the
east to northeast flow and shear across Southern AZ, the drier air
and warm air aloft is expected to be too much to overcome. Some
mid-level clouds off the Rim in the evening and overnight and
possibly some embedded ACCAS showers cannot be ruled, but most
precipitation will likely fall as virga. Hi-res models do show a
backdoor front pushing east to west across Southern AZ tonight,
brining a slight bump in PBL moisture as well as some breezy
conditions. Moisture is expected to continue increasing, with
PWATs climbing back to around 1.5-1.8" by the end of the week with
additional northeasterly waves possible. So, there may be one day
with better odds of storms surviving off the high terrain Friday
into this weekend.

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.AVIATION...Updated 1100Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Diurnal winds are anticipated today, though with high pressure to
the north, the onset of westerly flow may occur a few hours later
than normal. Meanwhile, monsoon storms will generally remain
isolated today and relegated to eastern and southeastern Arizona.
However, an early shift back to easterly flow is likely across the
Phoenix area as the pressure gradient tightens between the
aforementioned high to the north and a low pressure system across
northern Mexico. Gusts may reach as high as 20 kt across the
Valley late tonight and Wednesday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation weather concerns anticipated. Light, diurnal winds
and mostly clear skies will prevail through Wednesday.

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.FIRE WEATHER...
Substantial warming and drying early this week will keep
afternoon MinRH values around 15-25% through most of the week,
while overnight MaxRH recovers to around 40-60% most places.
Afternoon temperatures will be several degrees above normal for
most of the next 7 days. Shower and thunderstorm chances will be
very limited through the week, focused mainly in the AZ high
terrain areas. Apart from thunderstorms, winds will tend to follow
familiar diurnal patterns with a slight increase in southerly to
southeasterly breezes after today.

&&

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

     Excessive Heat Warning from 10 AM this morning to 8 PM MST
     Wednesday for AZZ531>533-535>537-540>544-546-548>551.

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

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DISCUSSION...Benedict
AVIATION...Hirsch
FIRE WEATHER...Benedict