Excessive Heat in California, September 7 to 9, 2021

Area Forecast Discussion
National Weather Service Phoenix AZ
505 AM MST Tue Sep 7 2021

.UPDATE...
Updated 12Z aviation discussion.

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.SYNOPSIS...
Strong high pressure aloft will be the predominant weather feature
across the Southwestern U.S. this week providing above normal
temperatures for much of the region. Generally dry conditions
will also prevail despite somewhat higher humidity levels, though
slight chances for storms may become possible later this week
across southeast California and the Arizona higher terrain. Daily
high temperatures will mostly range from 105 to 108 degrees across
the Arizona lower deserts to a couple of degrees warmer across
southeast California and southwest Arizona.

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.DISCUSSION...
The unseasonably strong high pressure ridge across most of the
Western U.S. will continue to bring above normal temperatures and
generally dry conditions to our region through the rest of the
week. The ridge center is currently analyzed across central Nevada
with 500mb heights around 598dm, likely around record climo for
the date in that area. The expansiveness of the ridge also
engulfs the entire Desert Southwest creating strong subsidence
aloft which will continue to suppress most if not all of
convection over the next couple days and likely longer.

The main forecast concern through the rest of the week will be the
warming temperatures and the potential for localized High
HeatRisk. The GoC moisture surge yesterday increased boundary
layer moisture and kept daytime highs near normal readings, but
as boundary layer moisture slowly gets mixed out over the next
couple days temperatures will bounce back. Forecast highs through
the rest of the week generally top out between 105-108 degrees
across the Arizona deserts to 106-110 degrees across southeast
California. These forecast readings do result in very localized
areas of High HeatRisk, particularly within eastern Riverside
County (where an Excessive Heat Warning is in effect through
Wednesday). If the NBM trends warmer over the next few model runs,
additional localized areas may need an Excessive Heat Warning.

Models mostly agree the upper level ridge will shift eastward
through Friday before stalling out somewhere across our region.
The ridge should also weaken slightly, but heights and thicknesses
aloft generally remain stable across our region through the
weekend meaning very little change in the air mass. NBM
probabilities of reaching 110 degrees increases over the weekend
for Phoenix and Yuma (topping out at 40% and 50% respectively on
Sunday) likely due to even further loss of boundary layer moisture
and lowering surface dew points. Any highs reaching or exceeding
110 degrees in Phoenix this time of year would be near record
territory. Model ensembles eventually point toward the ridge
weakening further and becoming more elongated at some point
during the first half of next week. This would yield some gradual
cooling across the region, but likely keep dry conditions in
place.

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.AVIATION...Updated at 1205Z

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT;
Southeast California/Southwest Arizona including KIPL and KBLH:

Minimal aviation impacts are expected through Wednesday afternoon
as light diurnal or variable winds prevail under mostly clear
skies. Winds will generally be southerly or S-SW at KBLH and
southeasterly at KIPL at 10-15 knots. Winds will generally be
light and variable this morning at the Phoenix terminals, with a
preference for westerlies at KPHX and West Valley sites and
easterlies at KIWA. All Phoenix sites will become westerly at 5-10
knots by 17-18Z before switching back to easterly overnight.
Isolated thunderstorms over southeast Arizona may push weak
southerly outflows into Phoenix after 06Z, but confidence is too
low to include them in any TAF at this time.

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.FIRE WEATHER...
Thursday through Monday:
Strong high pressure will dominate over the Southwestern U.S. late
this week into the weekend providing generally dry conditions and
above normal temperatures. Chances for showers and thunderstorms
will be very limited through the period, especially over the
Arizona lower deserts where chances will be less than 5%. High
temperatures are likely to run a few degrees above normal
throughout the period with some lower desert areas potentially
approaching 110 degrees. Min RH levels will range from 15-25%
through Saturday with readings dropping to 10-20% starting Sunday,
while max RHs generally range from 35-55% (locally higher). Winds
will overall remain light through the period while following
typical diurnal patterns.

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

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...Excessive Heat Warning from 10 AM this morning to 8 PM PDT
     Wednesday for CAZ561-570.

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DISCUSSION...Kuhlman
AVIATION...Hopper
FIRE WEATHER...Kuhlman