Excessive Heat in California, August 24 to 28, 2020

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
445 AM MST Mon Aug 24 2020

.UPDATE...Updated aviation discussion.

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.SYNOPSIS...
Temperatures are expected to warm to excessive heat levels
today and Tuesday across the lower deserts with the excessive
heat likely persisting through Friday across the Phoenix area.
Storm chances also increase across eastern and south-central
Arizona through the first part of the week, while mostly favoring
higher terrain areas. Strong gusty winds, blowing dust,
lightning, and brief heavy rain capable of producing flash
flooding will be possible with or near storms that develop.

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.DISCUSSION...
Significant drying in the mid levels yesterday along with weak
subsidence that formed across southeast Arizona led to a quieter
than expected storm day. However, temperatures continued to climb
as most lower deserts areas warmed close to 110 degrees Sunday.
The rain forecast for the next couple days remains a challenge due
to multiple factors. The drier air mass that has moved into much
of the area will limit instability today, but a positive will be
the stronger heating and high temperatures easily topping 110
degrees over the lower deserts. As the Excessive Heat Warning goes
into effect today, temperature guidance continues to show most
lower desert areas topping out between 112-114 degrees today.

The subsidence problem we had on Sunday should not be as big of
an issue today over the high terrain allowing more widespread
coverage of storms across the high terrain of eastern Arizona and
even across southern Maricopa and western Pima Counties. There
doesn't seem to be much agreement within the HREF members as far
as storm coverage this afternoon and evening, but it would take
some significant outflow boundary collisions for storms to develop
over the south-central lower deserts. Very strong DCAPE today
should yield decent outflow boundaries with the best signal in the
00Z HREF across Pima County. Any strong outflow coming northward
into Maricopa County will be capable of bringing some blowing
dust and potentially kicking off a few storms near Gila Bend, but
storm redevelopment becomes less likely closer to Phoenix.

For Tuesday, the uncertainty as far as storm coverage is even
more than today. A slight moisture increase mainly across eastern
Arizona and a potential weak inverted trough moving westward out
of New Mexico may be enough to bring more widespread showers and
storms across the eastern Arizona high terrain. Along with the
potential shortwave trough, some model solutions indicate mid
level northeasterly flow to increase across eastern Arizona. If
this does come to fruition, developing storms across the high
terrain will have better chances of surviving into the lower
deserts. Some of the more aggressive CAMs do show storms making
it into the Phoenix area Tuesday evening while others have storms
dying out. As with many monsoon days, storm chances over the lower
deserts will be conditional and will depend on how well the high
terrain storms organize. This particular northeasterly mid level
flow synoptic set up may persist into Wednesday, but the bulk of
the guidance currently shows decreasing storm chances due to
decreasing instability.

Temperatures this week will continue to be quite hot with daily
highs mostly running 5-10 degrees above seasonal normals. After
the hottest temperatures today, we should lose a degree or two
each day through Wednesday before leveling off through Friday or
Saturday. Daily highs Wednesday through Friday should mainly top
out between 108-112 degrees over the lower deserts. Rain chances
during the latter half of the week do look to decrease overall
with storms likely confined to high terrain areas starting
Thursday. Increased model spread into the weekend is still a
factor as models have had a difficult time resolving an incoming
trough likely to affect northern California and a trough coming
out of Canada. A few days ago, models mostly agreed the stronger
Canadian trough would dip southward into the Northwestern U.S.
late in the weekend turning our flow out of the west and drying
out much of the region. This trough now looks to be delayed and
more shallow, likely keeping the westerly flow to our north. It is
too early to know how much moisture will be around after Friday,
but at least a slow decline in temperatures seems to be likely by
the weekend into early next week.

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.AVIATION...Updated at 1145Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

For the rest of the morning, just FEW to SCT mid and high level
debris clouds can be expected in the Phoenix area, along with light
east wind. By afternoon winds should return to light west with
speeds mostly 10kt or less. Skies to stay mostly clear through the
afternoon. We could see isolated storms along the rim and across
southeast Arizona late this afternoon and evening that could push
outflows into the Phoenix metro and affect the terminals, although
it looks to be a bit less active today than the past couple of days
so confidence is a bit low with respect to the outflows or any
storms/showers making it into the lower desert. Will make a nod to
the potential for modest outflows, initially from the south to
southwest, then switching to northeast or east. Most speeds should
be less than 15kt. After 06z winds should tend to favor an easterly
component. We will not mention VCTS or VCSH in the TAFs at this time
but could always add this in with later issuances if needed.
Otherwise expect SCT-BKN mid/high decks this evening with most bases
aoa 14k feet.

Southeast California/Southwest Arizona including KIPL and KBLH:

Very minimal chances for any convection across the western deserts
over the next 24 hours; skies to be relatively clear with mainly FEW
decks of cirrus or high mid cloud. Winds a bit variable but mostly
below 15kt. Winds to favor the south at KBLH, and at KIPL they will
favor west through mid morning then turn more to the southeast
before returning to the west by late afternoon or early evening. For
the most part there are no aviation concerns at the TAF sites over
the next 24 hours.

&&

.FIRE WEATHER...
Wednesday through Sunday:
High temperatures will be near or above 110 degrees across the
lower deserts through much of the period with slight cooling into
the weekend. This will mean above normal high temperatures and Min
RH values generally in the 15-20% range (20-25% higher terrain).
Max humidities will generally be 35-45% lower elevations and
45-70% higher terrain. Storm chances lower during the middle part
of the week with likely only isolated high terrain storms through
Friday. An slight uptick in moisture during the weekend should
lead to an increase in storm coverage, potentially returning
chances back into the Arizona lower deserts. Apart from
thunderstorm activity, winds will follow warm season patterns.

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.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM this morning to 8 PM MST
     Friday for AZZ534-537-540>544-546-548-550-551-553.

     Excessive Heat Warning from 10 AM this morning to 8 PM MST
     Tuesday for AZZ530>533-535-536-538-539-545-547-549-552-
     554>556-559>562.

CA...Excessive Heat Warning from 10 AM this morning to 8 PM PDT
     Tuesday for CAZ565-567-569-570.

&&

DISCUSSION...Kuhlman
AVIATION...CB
FIRE WEATHER...Feldkircher/Kuhlman