High Wind in California, May 20 to 21, 2021

Area Forecast Discussion
National Weather Service San Diego CA
930 PM PDT Wed May 19 2021

.SYNOPSIS...
A weak high pressure ridge aloft will keep things relatively warm
today. A low pressure system moving into the western states will
bring periods of stronger and gusty west winds in the mountains and
deserts for this afternoon into Friday along with a cooling trend
and deepening of the marine layer, with the coastal low clouds
extending inland onto the coastal mountain slopes by the weekend.
Friday and Saturday will be the coolest days. Sunday and Monday will
be warmer with the low clouds not spreading as far into the valleys
by Monday. Then a little cooler again for the middle of next week.

&&

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

Evening Update: The forecast remains on track this evening and no
updates were planned at this time. The low stratus clouds were
filling in the coastal basin and through the valleys as well with
healthy onshore gradients and marine layer. Because of the strong
onshore gradients, winds will increase overnight in the mountains
and deserts, and the wind advisory will remain in place there
through Friday evening.

Previous forecast discussion (131 pm pdt): Mostly clear this
afternoon, with patches of low clouds over the coastal areas.
Temperatures are mostly a few degrees higher than at this time
yesterday, except in the mtns and the high desert where temps are
mostly a few degrees lower. Sfc pressure gradients are trending
stronger onshore, with 9.4 mb SAN-TPH and 7.7 mb SAN-IPL. Onshore
winds are strengthening and wind gusts of 47 mph have been reported
at the windier locations during the last hour.

Today through Friday...A low pressure system over the coast of
Washington state will move southward and slowly inland through
Friday. Onshore flow will continue to strengthen this afternoon,
continuing through Friday.

A coastal eddy and height falls aloft are expected to bring some
deepening of the marine layer for late tonight into Thursday
morning, with the low clouds spreading farther up the coastal
slopes. The lower level flow goes northwesterly for Thursday night
into Friday. This may limit or at least delay the spread of low
clouds Thursday night into Friday morning across coastal areas of
Los Angeles County and adjacent portions of Orange County and the
Inland Empire.

Lower elevations - mainly the lower deserts - will be a little
warmer today while higher elevations begin to cool with cooling
spreading inland for Thursday and Friday. With the cooling, high
temperatures near the coast will be a few degrees below average to
20 to 25 degrees below average in the mountains on Friday.

The stronger onshore flow will also bring periods of strong and
gusty west winds in the mountains and deserts, with gusts to 45 mph
and isolated gusts to 55 mph in the windier locations. Winds will
generally be stronger during the afternoon and evening each day.

Saturday through Wednesday... Cooler weather and a deep marine layer
will persist into Saturday with high temperatures similar to Friday.
The marine layer could deepen to around 5000 feet with coastal low
clouds and fog extending inland onto the coastal mountain slopes.

For Sunday and Monday, the low pressure system will weaken and move
to the northeast bringing a warming trend for Sunday and Monday.
With the warming, Monday high temperatures would be near average in
the mountains and deserts and a few to around 5 degrees above
average for the coast and valleys. The marine layer will become
shallower and not spread as far into the valleys by Monday.

Another weaker low pressure system moving inland along the West
Coast around Tuesday could bring minor cooling and a little
deepening of the marine layer for the middle of next week.

&&

.AVIATION...

200330Z...Coasts/Valleys...Areas of low clouds with bases 1500-1900
MSL and tops to 2700 ft MSL will fill in west of the mountains
tonight. Local vis 3-5 SM possible in the inland valleys 11-18Z with
the foothills of the mountains obscured in clouds/fog. Clouds and
vis restrictions clearing 17-20Z, but may remain locally BKN along
the immediate coast through the afternoon. Areas of low clouds
redeveloping after 21/00Z and spreading into the valleys Thu night.
Bases will be near 1700-2500 ft MSL with tops to around 3000 ft MSL.

Mountains/Deserts...A few high clouds AOA 20,000 ft MSL through Thu
evening. W-NW winds with sfc gusts 25-35 kt and local gusts to 45 kt
continuing this evening on the desert mountain slopes, desert
foothills, and through the San Gorgonio Pass. Winds will weaken some
overnight, increasing again Thursday afternoon and evening with
isolated gusts to 50 kt. MOD-STG up/downdrafts, LLWS, and rotors
possible over and east of the mountains, including VCTY KPSP and
KTRM. Vis may be locally reduced to 3-5 SM in blowing dust.

&&

.MARINE...
Breezy northwest winds will persist through early Saturday,
strongest over the outer coastal waters. Gusts of 25-30 kts will
occur during the afternoons and evenings. Winds may briefly reach
gale force Thursday evening with gusts to around 35 kt. Combined
seas of 8-10 ft and steep waves are also likely. A Small Craft
Advisory remains in effect through Saturday morning.

&&

.BEACHES...
A south swell of 3 ft/16 sec from 190 degrees will gradually lower
into Thursday. Elevated surf of 3-5 ft is possible on the favored
south facing beaches of Orange County Thu morning, with a high risk
of rip and longshore currents at all beaches. A reinforcing long
period south swell of 3 ft/18-19 sec from 185 degrees arrives later
Friday and continues through the weekend. This may produce surf of 4-
7 ft at select spots in Orange County, highest Saturday morning.
Hazardous swimming conditions are likely through the weekend. A
Beach Hazards Statement is in effect through early Monday morning.

&&

.FIRE WEATHER...
There will be periods of stronger and gusty west winds in the
mountains and deserts into Friday with lowest daytime humidity in
the deserts falling to around 10 percent today, and 10 to 15 percent
Thursday and Friday. This will bring periods of elevated fire
weather conditions in the deserts at times into Friday with
conditions most critical from the combination of stronger winds and
low humidity each afternoon.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory until 11 PM PDT Friday 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.

     Beach Hazards Statement through late Sunday night for Orange
     County Coastal Areas.

PZ...Small Craft Advisory until 5 AM PDT Saturday for Waters from San
     Mateo point to the Mexican Border Extending 30 to 60 nm out
     including San Clemente Island.

&&

PUBLIC/FIRE WEATHER...PG/JMB
AVIATION/MARINE...SS