Excessive Heat and Wildfire in California, June 13 to 21, 2021

Area Forecast Discussion
National Weather Service San Diego CA
907 AM PDT Sun Jun 13 2021

.SYNOPSIS...
High pressure will strengthen and expand over the Desert Southwest
through the middle of next week. This will bring a long duration
heat wave to much of the region, with temperatures peaking Tuesday
through Friday. A sea breeze and shallow marine layer will keep
temperatures moderated in the coastal areas, along with some patchy
night and morning low clouds and fog at times. The ridge over the
Southwest will keep it hot well inland into next weekend, with
cooling expected across the entire region early next week.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

.Morning Update...

Low clouds and fog have cleared much of the coastal areas this
morning. Today's high temperatures are shaping up to be five to
ten degrees above average, but this will be the coolest day for
much of the week for many as a strong high pressure system comes
into the region. Patchy low clouds will continue nights and
mornings along the coastal areas through much of the week along
with patchy fog as well. The forecast is still on track for this
large high pressure system to move slowly into the region over
this week, bringing in a prolonged heat wave.

.Previous Discussion as of 3:00 AM Sunday...

After some morning clouds/fog over coastal and western valley areas,
fair skies are expected to prevail away from the immediate coast
each afternoon for the next several days. The marine layer will
shrink into Tuesday, providing a better chance for fog along coastal
areas. An influx of higher clouds is likely by Tuesday as moisture
aloft is drawn north and westward.

Low pressure centered off the PacNW Coast will release it's hold
over CA over the next few days as a strong high builds over the
Southwest. Little has changed with this advertised scenario,
building confidence that this feature will peak over the Four
corners area at 600 DM midweek. A SW-NE oriented ridge axis around
this high will pass SoCal from south to north late Tuesday,
opening the door to easterly flow aloft Wed-Fri. This will spread
mid-high level moisture over the region with more high clouds and
higher humidity at higher elevations, but current indications are
that there instability will be insufficient for any monsoon-type
convection.

The main concern will be the heat, which will reach excessive levels
inland beyond the reach of a shallow marine layer along the coast.
Temperatures will climb above average today, and continue to climb
through midweek when temperatures will be some 10-20F above average,
except along the immediate coast and beach areas. With the northward
passage of the ridge aloft, increasing onshore flow, and a possible
coastal eddy may push cooling inland along the coastal plain as
early as Wednesday, when forecast temperatures begin to decrease
there. Not so much farther inland though. Excessive Heat Warnings
are in effect for the deserts, and Heat Watches farther west across
the mountains and into the valleys much of the week. A secondary
threat is wildfire (See FIRE WEATHER discussion below).

For what it's worth, there is better agreement between the 00Z runs
of the GFS/ECMWF models with respect to the deamplification of the
strong high over the Southwest later in the week. The center does
drift over SoCal as it weakens over the coming weekend, maintaining
heat over the deserts as strong, moderating onshore flow prevails
west of the mountains. This is echoed by the 00Z ensembles of both
models, which show clear cooling trends after Saturday, peaking
toward the middle of next week. These ensembles similarly show
pronounced cooling at Ontario and John Wayne on Wednesday. The trick
will be how far inland this will get. It isn't as evident at San
Bernardino. For now, expect the hottest day to be Tuesday, closer to
the coast, with a possible increase in temperatures again later in
the week if the eddy fades.

&&

131600Z...Coast...Low cloudiness has cleared out quickly this
morning with only patchy clouds near 1000 ft MSL remaining. A mostly
clear sky will prevail this afternoon. For tonight, overall low
cloud coverage should be considerably less, but there could be re-
development near the coast, especially along the San Diego coast.
Bases will be between 600-1,000 ft MSL. Any low clouds will likely
clear by 16Z Monday.

Valleys/Mountains/Deserts...A clear sky with unrestricted visibility
will prevail through Monday.

&&

.MARINE...
Northwest winds gusting 15-20 knots will develop over the outer
coastal waters each afternoon and evening through Tuesday. No
hazardous marine conditions are expected later in the week.

&&

.FIRE WEATHER...
Increasing temperatures and minimum daytime RH values of 10-20% (far
inland valleys/mountains/deserts) will result in elevated fire
weather conditions in the afternoons and evenings. The main concerns
are with the inland push of the sea breeze in the afternoons and
evenings, especially through and below the desert passes...and the
coming extreme heat (temperatures 15-20 degrees above average),
resulting in very high mixing heights, creating the potential for
large, plume-dominated fires over the far inland valleys and
mountains.

These elevated fire weather threats while not critical, will be
greatest Tuesday through at least Friday. Gradual cooling and
increasing humidity will help to ease conditions over the weekend
and into next week.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Warning from 10 AM Monday to 9 PM PDT Friday for
     Apple and Lucerne Valleys-Coachella Valley-San Diego County
     Deserts-San Gorgonio Pass Near Banning.

     Excessive Heat Watch from Tuesday morning through Friday evening
     for Riverside County Mountains-San Bernardino County
     Mountains-San Diego County Mountains-San Diego County
     Valleys.

     Excessive Heat Watch from Monday morning through Friday evening
     for San Bernardino and Riverside County Valleys-The Inland
     Empire.

     Excessive Heat Watch from Tuesday morning through Thursday
     evening for Orange County Inland Areas-Santa Ana Mountains
     and Foothills.

PZ...NONE.
&&

UPDATE...APR
PUBLIC/FIRE WEATHER...10
AVIATION/MARINE...Gregoria