Excessive Heat in California, June 14 to 20, 2021

Area Forecast Discussion
National Weather Service Phoenix AZ
505 AM MST Mon Jun 14 2021

.UPDATE...Updated 12Z Aviation Discussion.

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.SYNOPSIS...
Extreme heat will continue through at least Saturday as high
temperatures generally remain in a 114 to 120 degree range for
most lower desert locations. Excessive Heat Warnings are currently
in effect through Saturday across the lower deserts of southeast
California and south-central and southwest Arizona. These warnings
are also in effect through Friday over the higher terrain of
southern Gila County. Very dry conditions will persist through
today before isolated storm chances increase across southern
Arizona and the higher terrain of northern Arizona by Tuesday.

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.DISCUSSION...
Our weather continues to be dominated by a strong ridge of high
pressure that now extends into the Great Basin according to
the latest water vapor imagery. The warming trend of hot-to-hotter
continued yesterday as the ridge center drifted westward to its
current location over the San Francisco Mountain range along the
AZ/NM border. Sunday ended as the hottest day of the year so far,
taking away any surprise that early morning temperatures are once
again running 3-5 degrees warmer compared to this time yesterday.
Temps have cooled below 90 degrees for most locations with another
5-8 degrees of cooling expected through sunrise.

The ridge center will migrate towards the Four Corners today and
tomorrow causing H5 heights of 594-600 dm to expand across all of
the Southwest through Wednesday. These values are climatological
maximums for the northern half of AZ and the 99.5th percentile
everywhere else in the region. Ensemble guidance broadens the
ridge later in the week, suppressing heights to 590-594 dm, but
still climatologically significant near the 90th percentile. This
progression means we should anticipate the heat-up to continue with
widespread readings of 114-120 degrees across the lower desert through
Friday. The aforementioned weakening of the ridge may provide a hardly
noticeable cooldown of a few degrees Saturday as highs remain well
above 110 degrees. Meanwhile, daily heat surpluses and a modest
moisture increase will also allow overnight low temperatures to rise
into the mid 80s/low 90s supplying little reprieve from the heat.

NBM probabilities for attaining certain thresholds highlight the
significance of the heat. For Phoenix, probabilities of reaching
115 degrees each day Tue-Fri vary between 55-75% before dropping
to 45% next Sunday. Chances for reaching 118 degrees vary between
10-35% through Saturday with the highest chances tomorrow. An official
reading of 120 degrees at Sky Harbor cannot be ruled out either through
Saturday with daily odds between 2-10%. Similar chances exist farther
west in places like Yuma, El Centro and Parker, but with chances
slightly tilted towards more extreme temperatures. The minor
oscillations in temperatures from day-to-day do not undercut the
danger and potentially deadly effects of this heat, even by local
standards. As such, an Excessive Heat Warning remains in effect
everywhere through Friday, and then for all lower desert elevations
west of Gila County.

Aside the danger of heat exposure, the intense heat is drawing moisture
into the region. The latest GOES-TPW satellite channel shows
precipitable water values of 0.50-0.60" now extending through the
southern half of AZ with locally higher values closer to the Gulf of
California. Deeper moisture with PWs in excess of 1.00" remain farther
south in Sonora but we should see moisture levels increase, as
illustrated by both the ECMWF Ens and GEFS mean PWATs, settling near
1.00" by midweek. Moisture increases and daytime heating should be
enough to destabilize the atmosphere while subtle upper level waves
embedded within the ridge's circulation could trigger afternoon and
evening convection for parts of the region nearly every day this week.
The most suspect areas to see convection include the Mogollon Rim,
White Mountains, and the higher terrain of southern AZ and northern
Mexico. Total atmospheric moisture will still be on the lower side
of the spectrum thus limiting the coverage of thunderstorms and
rainfall amounts. The chances for showers or thunderstorms
surviving/developing off the higher terrain over the lower desert,
including the Phoenix area, is fairly low most days around 15% or less.
The PoPs in the forecast could be an indication of debris clouds from
dissipated convection that occurred elsewhere. Hints of this
solution can be derived from the slight disparity between the
ECMWF Ens and GEFs QPF guidance, in which the former's ensemble
members hardly contain any indication of rain at KPHX while the
latter has about 50% of its members with just a trace of rain at
KPHX on any given day through the weekend.

Even without rain, conditions for impactful weather exists. High
dew point depressions of 50-80 degrees means that any storms that
do develop, regardless of location, will likely produce strong
downburst winds. Details aren't quite clear yet but gusts of 30-45
mph could easily be expected with potential for even stronger
gusts. The relatively dry atmospheric column also means that distant
thunderstorms also have potential to produce long-traveling outflow
winds, and depending on trajectory, could produce blowing dust.
Climatological chances for dust storms, like haboobs, are increasing
and the conditions for such an event impacting the Valley or locations
between Phoenix and Tucson sometime this week are in play.

While the convective forecast this week remains mostly murky, the
picture for today and tomorrow is becoming more clear now that both
days are within the HREF's timeframe. For today, an embedded shortwave
will likely enhance convection this afternoon over the northern
Sierra Madres and northward into Nogales, and then to a lesser extent
over eastern AZ. A few cells may redevelop over southern Gila County
but chances for storms and/or outflows in our forecast area is 10%
with hints of either of these mostly supported by the most aggressive
guidance at this time. However, concerns for Tuesday are growing as
the HREF and other deterministic guidance (U of A WRF models) show
more robust convection occurring over the Rim and Pima County by
early afternoon. Conditions for strong downbursts will remain, and
based on the HREF mean wind gusts, outflow winds (could be
multiple boundaries from the NE to SE) will move through portions
of the eastern half of the forecast area sometime after about 2
PM. As of now, HREF mean wind gust speeds associated with outflows
are mostly at 15-25 mph with localized areas of 30+ mph. The
potential for stronger winds would be enhanced if new storms
develop along any boundaries as they move through the lower desert
and reinforce with additional downbursts. By 5 PM, all areas from
Buckeye eastward have a 30-50% chance for seeing 35+ mph storm
related wind gusts and a 10% chance for 58+ mph (severe criteria)
wind gusts. The next HREF run later this morning should provide
additional clarity and/or confidence in the forecast but as of
now, this has the makings of becoming the area's first
convectively driven dust storm of the summer.

After Tuesday, confidence in any particular outcome related to
convection remains low and will have to play the "wait and see how
things develop" card while we all endure the extreme heat.

&&

.AVIATION...Updated at 1203Z.

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

Light diurnal winds are anticipated through the morning and
overnight hours, while speeds in the afternoon increase to around
8-12 kts with gusts up to 20 kts. The diurnal west shift is
expected early today, around 16-17Z. Scattered debris clouds with
bases as low as 15-16 kft are expected this evening. Isolated
virga showers within these clouds is possible. Otherwise,
anticipate FEW to SCT high clouds.

Minor density altitude impacts are possible this week during the
afternoons as surface temperatures reach up to the 115-118 degree
range, with brief 119-121 temperatures also possible.

Southeast California/Southwest Arizona including KIPL and KBLH:

Light winds are expected at both terminals through most of the
period. At KBLH, afternoon gusts could reach up to 20 kts.
Directions at KBLH will favor the south through the period, while
KIPL favors the southeast during the day/overnight and briefly
southwest this evening. FEW to SCT (BKN at times) clouds above 15
kft will move in this afternoon and persist through the rest of
the period.

&&

.FIRE WEATHER...
Wednesday through Sunday:
Very hot temperatures will be the primary concern, especially for
any ongoing fire operations, as lower elevations will see highs in
the 114-120 degree range through at least Friday before cooling
slightly next weekend. The higher elevations will not be spared
from the heat either with highs reaching 96-106 degrees. Afternoon
minimum RH values will struggle to stay above 10% through Friday
before dropping into a 10-15% range for the weekend. Overnight RH
recoveries will generally reach the 20-40% range. Aside from
thunderstorm outflows, winds will follow typical diurnal/terrain
influences with afternoon breeziness. Chances for storms over the
Arizona higher terrain along the Rim and White Mountains is 15-25%
most afternoons, with lower 5-15% chances in south central
Arizona. Strong winds are the primary concern for any storms that
do develop and even very distant storms may produce long traveling
outflow winds. The chance of wetting rains is less than 5%.

&&

.CLIMATE...

Daily Record Highs

  Date               Phoenix       Yuma       El Centro
------------         -------       ----       ---------
Monday    6/14      115 (1987)   119 (1961)   117 (1940)
Tuesday   6/15      115 (1974)   118 (1917)   115 (1940)
Wednesday 6/16      115 (1974)   119 (1917)   114 (1974)
Thursday  6/17      114 (2015)   115 (1981)   115 (1971)
Friday    6/18      115 (2015)   116 (2015)   117 (2015)
Saturday  6/19      118 (2017)   120 (2016)   119 (2016)

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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 until 8 PM MST Saturday for AZZ534-
     537>544-546-548>551-553>555-559.

     Excessive Heat Warning until 8 PM MST Friday for AZZ545-547-552-
     556>558-560>563.

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

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

&&

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