Excessive Heat in California, August 3 to 5, 2021

Area Forecast Discussion
National Weather Service Phoenix AZ
430 AM MST Tue Aug 3 2021

.UPDATE...Updated aviation discussion.

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.SYNOPSIS...
Excessive heat will prevail on the lower elevations for at least
the next couple of days...especially over southeast Califonria
and southwest Arizona where temperatures of 115 to 120 degrees are
anticipated. It is not expected to be quite as hot over south-
central Arizona including the Phoenix area but highs of 110 to 113
are possible. A cooling trend begins Thursday as humidities
increase but southeast California and southwest Arizona will still
have excessive heat. Expect storm activity to be confined to
places like the White Mountains, eastern Mogollon Rim, and
portions of southeast Arizona. Storm chances trend upward into
early next week. At the same time temperatures trend down. In
fact, highs over south-central Arizona are anticipated to dip
below normal beginning Saturday with southwest Arizona and
southeast California lowering to early August normals.

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.DISCUSSION...
Storm activity has wrapped up for the night. The downward trend in
moisture continues today and Wednesday. Storms will struggle to
form over our CWA with PoPs peaking at around 10% and that is only
over easternmost portions of Gila County. PoPs peak in the single
digits Wednesday. But, outflows from isolated storms over the
White Mountains and eastern Mogollon Rim could still affect
portions of southern Gila County although gusts exceeding 35 mph
look to have less than 10% likelihood per HREF.

The main weather impact for today through Thursday will be the
heat. With the drier air, lack of clouds, and a ridge centered
over the region (500mb heights 594-596dm), high to very high Heat
Risk can be expected over the lower elevations today and
Wednesday. The Excessive Heat Warning was expanded to include the
rest of western Maricopa County. High temperatures over southeast
CA and southwest AZ will reach and exceed 115 degrees in many
locations. In fact, some places will make a run at 120 degrees. On
Thursday, the beginning of a cooling trend is anticipated due to an
increase in moisture/humidity. It is expected to be just enough
to keep south-central AZ from reaching high Heat Risk levels.
However, much of southeast CA and southwest AZ are now looking
like they will still reach criteria as the ridge is a bit slower
to weaken. Thus, the Excessive Heat Warning there has been
extended through Thursday. While it's possible that portions of
Yuma County and eastern Imperial County may have highs dip below
criteria, it will be due to very high dew points - with full sun.
It is also uncertain how much cooling there will be so those areas
were included in the extension.

As touched on previously, the ensemble models continue to show
that the ridge weakens (most noticeably Friday and Saturday) due
to a trough passing to the north which gets reinforced by a
stronger system further north. Also they depict a sizable
inverted trough tracking across Mexico. In the process,
significant moisture advection would take place (starting as early
as Wednesday night). However, both the EPS and GEFS mean appear to
have trended toward the inverted trough not extending quite as
far north. Separately, there is a smaller disturbance (currently
centered over Colorado) which is expected to track southward.
Previously, the ensemble means hinted at it (or some of it)
breaking off and turning west across southeast AZ and far NW
Mexico. But this looks to be less distinct now. In turn, some of
the ensemble means are also a little less optimistic on how much
the PWAT increases. But, overall, PoPs trend upward Friday through
Sunday with a subtle drop Monday. Though for southeast CA, storm
chances remain in the single digits. At this point, we are not
anticipating a multi-day widespread precip event like we had last
month. But, it will be a significant uptrend in storm activity
compared to the lull of the next few days (including today).

Temperatures trend down more noticeably on Friday and slowly
continue to decrease through Sunday with little change Monday. In
all, anticipate a 10-15 degree drop in high temperatures by Sunday
compared to the peak on Wednesday (dipping below normal over
south-central AZ over the weekend)

&&

.AVIATION...Updated at 1140Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Expecting a quiet day for thunderstorms across most of south-
central Arizona, with any development likely to remain well east
of the Phoenix area. As a result, all terminals should experience
typical diurnal wind patterns with speeds remaining less than 10
kt. Clouds bases should remain mainly aoa 12 kft with FEW to SCT
coverage.

Southeast California/Southwest Arizona including KIPL and KBLH: No
significant aviation weather concerns anticipated through
Wednesday morning. Winds will generally remain light and diurnal
under mostly clear skies.

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.FIRE WEATHER...
Thursday through Monday:
A cooling trend takes place starting Thursday and continuing
through Sunday. This is due to weakening high pressure, higher
atmospheric moisture, and an uptrend in storm activity. Humidities
will be at their lowest on Thursday with Min RH on the most of the
lower elevations of 10-20% (15-25% higher terrain of south-central
AZ). Of note, a Gulf surge is anticipated to be felt on Thursday
so those numbers are 5-10% higher than Wednesday. By Sunday,
anticipate lower elevation Min RH values of 20-35% (lowest over
eastern Riverside County) and 30-45% higher terrain. Little change
Monday. Max RH will increase 20-30% by Sunday morning compared to
Thursday morning. High temperatures will go from roughly 5-10
degrees above normal on Thursday to 0-5 degrees BELOW normal by
Sunday (little change Monday). For winds, anticipate southerly
breeziness over much of southwest AZ and southeast CA and lighter
winds elsewhere favoring southwest directions. But, the uptrend in
storm activity will lead to the likelihood of areas of strong
erratic winds at times (mainly for south-central AZ).

&&

.CLIMATE...

Record highs and year of occurrence for 8/3 - 8/5...

                   8/3        8/4        8/5
                   ---        ---        ---
Phoenix (PHX)    114/2020   116/1975   115/2019

Yuma (NYL)       116/1998   116/1995   116/2019

El Centro (IPL)  115/2011   118/1969   116/2019

&&

.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
     Wednesday for AZZ534-537>544-546-548>551-553>555-559.

     Excessive Heat Warning from 10 AM this morning to 8 PM MST
     Thursday for AZZ530>533-535-536.

CA...Excessive Heat Warning from 10 AM this morning to 8 PM PDT
     Thursday for CAZ561>570.

&&

DISCUSSION...AJ
AVIATION...Rogers
FIRE WEATHER...AJ
CLIMATE...AJ

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
855 AM MST TUE AUG 3 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1205 PM     FLASH FLOOD      8 WSW FLORENCE JUNCTION 33.19N 111.46W
07/25/2021                   PINAL              AZ   PUBLIC

            *** DELAYED REPORT *** SOCIAL MEDIA POST
            SHOWS WATER GREATER THAN 6 INCHES (REPORTED
            AS KNEE DEEP) COVERING ROADWAY IMPEDING
            TRAVEL ALONG SKYLINE DR. NEAR FELIX RD.

&&

GL

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
942 AM MST TUE AUG 3 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0540 PM     TSTM WND DMG     GADSDEN                 32.55N 114.78W
07/30/2021                   YUMA               AZ   TRAINED SPOTTER

            *** DELAYED REPORT *** POWER POLES REPORTED
            TO BE DOWN WITH NUMBER UNKNOWN. DOWNED POWER
            POLES WERE SCATTERED NOT IN GROUPS. LOCATION
            AND TIME ESTIMATED BASED OFF RADAR AND APS
            POWER TRACKER. RADAR ESTIMATED WINDS BETWEEN
            45-60 MPH.

0540 PM     TSTM WND DMG     1 W SOMERTON            32.60N 114.72W
07/30/2021                   YUMA               AZ   TRAINED SPOTTER

            *** DELAYED REPORT *** REPORTED POWER POLES
            DOWN AND APS REPORTED POWER OUTAGES. UNKNOWN
            HOW MANY POLES BUT SEEMED TO BE SCATTERED,
            NOT IN GROUPS. TIME ESTIMATED FROM RADAR AND
            APS OUTAGE TRACKER. RADAR ESTIMATED WINDS
            BETWEEN 45-60 MPH.

&&