Excessive Heat in California, August 25 to 29, 2021

Area Forecast Discussion
National Weather Service Phoenix AZ
350 AM MST Wed Aug 25 2021

.UPDATE...Updated Aviation Discussion.

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.SYNOPSIS...
Increasing high pressure over the region will bring hot
temperatures over the next few days with lower desert highs near
110 degrees today and then 110-113 degrees for Thursday and
Friday across south central Arizona and approaching 115 degrees
across portions of southeast California and southwest Arizona. An
Excessive Heat Warning is in effect for much of the area. Storm
chances will gradually return to the higher terrain of eastern
Arizona starting Friday and potentially into the south central
Arizona lower deserts as early as the weekend. Cooler temperatures
and better rain chances look likely for next week.

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.DISCUSSION...
Hot temperatures and excessive heat will be the main concern
starting today and lasting through at least Friday. A strong
subtropical ridge remains centered over the Southern Plains, but
continues to expand westward and will provide our region with
modest warming over the next couple days. 500mb heights have
already risen to around 592-594dm (near 90% of climo for date) and
are forecast to stay there through Thursday before falling
slightly into the weekend as the ridge eventually weakens.
Boundary layer moisture did tick upward a bit yesterday, raising
surface dew points roughly 10 degrees, but this will not be enough
to stop the rising temperatures. High temperatures around 110
degrees are expected across a good portion of the lower deserts
for today before topping out between 110-113 degrees in the
Phoenix area to as high as 115 degrees over the western deserts
for Thursday and Friday.

As the ridge weakens late Thursday into Friday, and new high
center is expected to form somewhere over Arizona before drifting
more toward the Four Corners area over the weekend. This shift in
the high center should eventually provide a more favorable
southeasterly moist flow aloft, but model timing for this remains
somewhat uncertain. Initially on Friday, a slight improvement in
moisture and better jet positioning should allow for more
scattered storm activity mainly east of Globe. As moisture levels
continue to improve into the weekend, daily storm chances should
also increase over the high terrain with chances eventually
spreading westward into the lower deserts by Sunday. Model
ensembles continue to support some sort of a tropical system
developing well south of Baja on Friday before likely approaching
the Baja peninsula maybe by Sunday. Uncertainty increases going
into early next week, but both the EPS and GEFS support the system
tracking a bit farther northward along Baja, and this is likely
to increase a moist southerly flow across northern Mexico into our
region. Confidence is increasing for a good portion of the region
seeing decent storm chances during the first half of next week,
but timing is much more uncertain.

Gradual cooling is expected starting this weekend, but
temperatures are likely to remain a few degrees above normal
through Sunday. Highs across the western deserts may still hit
110 degrees even through Sunday, but as moisture levels increase
early next week, temperatures are expected to eventually fall to
slightly below normal readings.

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.AVIATION...Updated at 1050Z

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major aviation weather concerns expected through the TAF
period. Light easterlies should prevail until an early afternoon
switch to westerlies. Occasional periods of light and variable
wind conditions will also continue. Afternoon wind speeds will be
mostly light at around 4-8 kt. Storm chances will remain very
unlikely with any activity far removed from the terminals. Skies
will be clear to mostly clear with just FEW mid- high clouds
across the region.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation weather concerns are in the forecast through the
rest of the TAF package. At KIPL, light easterly winds will
transition to west or northwest winds for the rest of the TAF
forecast. Wind speeds will generally be light at 4-10 kt, though a
few gusts may exceed 10 kts this evening. For KBLH winds will
also be mostly light and variable until the late afternoon and
early evening when they will become southwest at 5-8 kts. Winds
will stay light, but gradually shift towards the northwest. Skies
should remain mostly clear.

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.FIRE WEATHER...
Friday through Tuesday:
Dry conditions will prevail across the majority of area on Friday
with above normal but cooling temperatures likely continuing into
the weekend. Moisture levels are forecast to gradually increase
throughout the period with shower and thunderstorm chances first
over the eastern Arizona high terrain through Saturday and then
spreading into the lower deserts starting Sunday or at the latest
Monday. After minimum afternoon humidity levels mostly in the
teens on Friday, improving moisture should push minimum levels
closer to 25% by Sunday or Monday. Overnight recovery will be
mostly good in a 35-55% range through Sunday, improving to 45-70%
late in the period. Winds should be relatively light with an
afternoon upslope component and overnight drainage common. Some
enhanced afternoon and evening gusts will be possible at times in
western districts.

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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 AZZ530>544-546-548>556-559>562.

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

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DISCUSSION...Kuhlman
AVIATION...Hodges/Sawtelle
FIRE WEATHER...Kuhlman