Wildfire in California, June 28 to 29, 2021

Area Forecast Discussion
National Weather Service San Diego CA
238 PM PDT Mon Jun 28 2021

.SYNOPSIS...
High pressure will continue hot and dry weather for inland areas
this afternoon with the marine layer keeping coastal areas cooler
with areas of night and morning low clouds and fog. An influx of
monsoonal moisture for Tuesday into Wednesday will bring an
increase in humidity with more clouds, high temperatures that are
not as hot for inland areas, and chances for mainly afternoon and
early evening showers and thunderstorms, mostly for the mountains
and deserts. Decreasing thunderstorm chances on Thursday with
drying going into next weekend.

&&

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

-Forecast Highlights-

* Scattered thunderstorms Tue-Thu afternoon mountains/deserts
* Main potential impacts: Lightning strikes, downburst winds
* Less impact: Localized heavier rainfall/low flash flood risk

The heat continues inland today with Palm Springs up to 117 so
far and San Bernardino at 106. The sky is mostly clear except for
a few cumulus developing over the mountains. One small cell tried
to develop just south of Big Bear. Could see the growing cumulus
from the office, but that has since dissipated. Have placed a
slight chance of showers/thunderstorms in for the eastern San
Bernardinos for the remainder of this afternoon. The main
potential impact, should a storm manage to develop, would be
strong, downburst winds. Little rainfall is expected.

Mid-high level moisture will be increasing from the northeast
through Tuesday, underneath the anomalous ridge over the Pacific
Northwest, which is resulting in record-shattering and very high
impact heat there. The moisture on Tuesday will be highest aloft
over San Bernardino County. Hi-res models of the HRRR and local
WRF are continuing to show the only cells developing on Tuesday
afternoon being over the San Bernardino County mountains and the
Mojave desert. Most cells only dropping about 0.10-0.25", though
the local WRF does show heftier cells over the Mojave Desert to
about 0.50"/hr, but given the dry low levels this may be overdone.
HRRR shows less activity and lower rainfall rates. The higher
potential impact looks to be potential downburst winds versus
flash flooding.

There could be a few showers and thunderstorms moving southwest
from Nevada Tuesday night, but model consensus keeps the bulk of
this convection over SE California and just outside of our
service area, but will have to monitor this as some of this
could skim the Coachella Valley Tue night-Wed morning.

The coverage of tstorms looks to increase across the mountains on
Wednesday afternoon as better lower level moisture moves in from
the southeast, but not looking too deep/significant as it had
looked a few days ago. The prevailing flow will be westerly, so
the coast and valleys likely will stay dry, except perhaps
portions of the Inland Empire. The tstorms on Wednesday afternoon
will favor the desert mountain slopes with the heaviest cells
occurring over the Riverside (Santa Rosa Mountains) and San Diego
County mountains where cells could drop about 0.50"/hr. Very
localized flash flooding is possible, particularly along the
desert mountain slopes and over the deserts. Again, the
microburst/damaging wind potential looks to be a greater risk,
though that would be localized as well.

With the moisture aloft and fairly dry air continuing in the
lower levels, thunderstorms will be capable of producing strong
and potentially damaging downburst winds, which could lead to
blowing dust over the deserts.

The moisture will be advecting east on Thursday. There could still
be isolated thunderstorms in the afternoon over the mountains and
deserts, but less activity compared to Wednesday, and it is
possible it could be dry if not much moisture remains.

Friday and through the holiday weekend will see a slow warming
trend along with expected dry conditions. Monsoonal moisture will
be just off to our east over SE California and Arizona. The model
consensus keeps moisture at bay here until perhaps later next week
when a moisture return is possible. Until then, the upcoming
weekend and into the following week looks tranquil with above
normal temperatures likely.

&&

.AVIATION...
282030Z...Coast/Valleys...BKN-OVC stratus will continue over the
coastal waters and up to a few miles inland, with bases mostly 700-
1000 ft MSL and tops to 1300 ft MSL. Local vis 4-6 miles in haze.
Clear remaining areas through about 29/03z this evening, then the
low clouds will push inland, reaching 20 miles inland by about
29/12z Tuesday morning. Local vis 2-4 miles will occur in haze over
higher coastal terrain and in the coastal valleys. Moderate to high
confidence for continued BKN-OVC conditions at KSAN through Tuesday
morning so confidence is moderate to high for continued BKN-OVC low
clouds at KSAN as seen in the TAF.

Mountains/Deserts...SCT-local BKN cumulus with bases around 9000
feet MSL will occur into this evening, with isolated TCU. Mostly
clear skies will prevail overnight into Tuesday, then SCT-local BKN
cumulus with bases around 9000 feet MSL will occur into early
evening, with isolated TCU. Visibility will be unrestricted through
Tuesday morning.

&&

.MARINE...
No hazardous marine conditions are expected through the week.

&&

.FIRE WEATHER...
High pressure across the western states will continue hot dry
weather for inland areas today. Lowest humidity for the deserts
will fall to around 10 to 15 percent this afternoon. This low
humidity combined with gusty southwest to west winds will
continue elevated fire weather conditions for the deserts this
afternoon.

An influx of monsoonal moisture will bring a recovery in humidity
for Tuesday into Wednesday with a chance for mainly afternoon and
early evening thunderstorms, mainly in the mountains and deserts.
There is greater potential for dry lightning strikes on Tuesday
afternoon as the moisture begins to increase, then again on
Thursday afternoon as the moisture is drying.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Warning until 9 PM PDT this evening for Apple and
     Lucerne Valleys-Coachella Valley-Riverside County Mountains-
     San Bernardino County Mountains-San Diego County Deserts-
     San Diego County Mountains-San Gorgonio Pass Near Banning.

     Heat Advisory until 9 PM PDT this evening for San Bernardino and
     Riverside County Valleys-The Inland Empire.

PZ...NONE.
&&

PUBLIC...Gregoria
AVIATION/MARINE...Small