Excessive Heat in California, June 2 to 4, 2021

Area Forecast Discussion
National Weather Service Phoenix AZ
449 AM MST Wed Jun 2 2021

.UPDATE...Updated Aviation Discussion.

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.SYNOPSIS...
The warmest temperatures so far this year are expected through the
rest of the week, with Friday looking like the hottest day.
Afternoon highs will reach 104 to 107 degrees across south-
central Arizona and up to 109 to 113 degrees in southeast
California. Widespread moderate heat risk, and localized high heat
risk, is expected with these temperatures. Showers and isolated
thunderstorms are likely in the Arizona high terrain each
afternoon through the end of the weak with only a low, 5 to 15
percent, chance of light showers in south-central Arizona during
the overnight and morning hours. A cooling trend is possible late
in the weekend and early next week as low pressure increases from
the west.

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.DISCUSSION...
A rex block which set up a couple days ago across the western U.S.
is still in place this morning, with a weak closed low to the
south of our area over Northern Baja and an anomalously strong
ridge through California and the Great Basin. This setup has put
our CWA under weak east to northeast steering flow. While moisture
levels (PWAT ~0.6-0.7" across southern AZ) are near normal for
this time and has been sufficient for afternoon convective
development over the AZ high terrain, the thermodynamic profile
has generally been just warm enough, resulting in a stable
environment over the lower deserts. So activity has not been able
to survive into the lower elevations. Instead we have mostly seen
the occasional outflow boundary try to push toward lower
elevations and debris clouds, with embedded virga showers,
drifting over the area during the overnight and morning hours.

Through the rest of the week, global and hi-res ensembles favor a
gradual weakening of the low to the south, allowing the strong
ridge to slowly become the more dominant feature across the
region. The ridge axis is expected to remain to the northwest,
which would still keep our area under a northeasterly flow. With
little change in moisture levels anticipated, afternoon convection
is likely to continue developing each day, primarily along the
Rim, in the White Mtns, and over the higher elevation of southeast
AZ. While the lower elevations of our CWA continue to see mid and
upper level clouds debris during the overnight and morning hours.
Most nights the PoP for the lower deserts is barely seen reaching
10%, but the NBM, along with a few individual ensemble members
and deterministic runs, are optimistic that rain showers could
spread into the Valley Thursday night-Friday morning with PoPs
closer to 20-30%. However, it may still be hard pressed to get
rain down to the Valley floor, let alone measurable rain
(>0.01"), as forecast soundings still show a very dry sfc-600mb
layer. Suggesting mostly virga.

The more sensible impact from the expected pattern evolution will
be the much warmer temperatures. Beginning today, afternoon
temperatures are expected to reach 5-7 degrees above normal in
south-central AZ to up to 8-12 degrees above normal across
southeastern CA. The hottest day currently looks to be Friday,
with afternoon highs reaching upwards of 105-107 in the Phoenix
area to 109-113 in southeast CA. Saturday could be just as hot as
Friday, depending on the temporal evolution of the flow pattern.
So the rest of the week will feature widespread moderate HeatRisk
for nearly all populated lower elevation communities, with a few
areas of high HeatRisk in southeast CA today through Friday. An
Excessive Heat Warning has been issued for portions of eastern
Riverside and Imperial counties beginning late this morning and
extending through Friday evening. If the forecast trends warmer,
additional communities zones or communities may have to be added.
Regardless of a warning or not, these will be the hottest
temperatures so far this year. So adequate hydration, having
access to effective cooling, and taking breaks from strenuous
outdoor activities is encouraged.

This weekend into early next week, cluster analysis supports a
flattening and weakening of the ridge, allowing temperatures to
cool down slightly. Weak high pressure will still keep
temperatures slightly above normal and a shifting flow pattern
back to more of a westerly component, as a trough digs into the
PAC NW, will support a drying trend as well. By the middle of next
week, cluster analysis shows a considerable amount of variance
among global ensemble members. Whether temperatures continue to
gradually cool as the trough digs closer, flat-line near normal,
or start warming again is uncertain at this time.

&&

.AVIATION...Updated at 1150Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
There are no major aviation weather concerns. However, the
westerly switch is anticipated to occur earlier than usual,
between 17-19Z, with a few hours of a light and variable wind
prior. Occasional westerly gusts of 18-22 kts are possible this
afternoon. A very late evening gulf surge could also produce
temporary gustiness up to 20 kts between 02Z-05Z but confidence in
this occurring at any TAF site is too low for further mention at
this time. Otherwise, chances for showers/thunderstorms and
related impacts should stay well east of the terminal area today
with only periods of thick mid to upper level clouds are
anticipated.

Southeast California/Southwest Arizona including KIPL and KBLH:
There are no major aviation weather concerns. At KIPL, a
southeasterly wind direction will predominate for much of the next
24 hours with speeds with a period of 15-20 kt gusts this
afternoon. At KBLH, a southerly to southwesterly wind will persist
with speeds reaching 10-15 kts and gusts nearing 25-30 kts this
afternoon. Otherwise, a few periods of high clouds will pass
overhead.

&&

.FIRE WEATHER...
Friday through Tuesday:
Isolated thunderstorms, mainly over the AZ higher terrain, could
drift into portions of Maricopa and Gila Counties, including the
Tonto NF Thursday-Friday. The main threats with any storms, even
distant ones, will be lightning and gusty outflow winds of 30-40
mph. Otherwise, the subtle moisture increase will not do much to
improve RH with afternoon values dropping into the 7-15% range and
overnight recoveries mostly in the 20-35% range. Besides outflows,
winds will be breezy each afternoon with gusts up to 25-35 mph out
of the west to southwest most areas. Which may result in elevated
fire weather conditions. Outside the afternoons, winds should
follow typical diurnal/terrain influences.

&&

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

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...Excessive Heat Warning from 11 AM this morning to 9 PM PDT
     Friday for CAZ561-562-566-570.

&&

DISCUSSION...Benedict
AVIATION...AD
FIRE WEATHER...Benedict/Sawtelle/AD