High Surf Advisory 4, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
930 PM PST Sun Dec 6 2020

.SYNOPSIS...
Santa Ana winds will impact the area Monday and Tuesday, strongest
Monday night through Tuesday in the mountains, foothills and
valleys. Tuesday and Wednesday will be warm with above average
temperatures. Thursday and Friday will be a little cooler with the
return of a shallow marine layer, but still warmer than normal.

&&

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

Just a few high clouds over SoCal this evening with an isolated spot
or two of fog or low clouds over the coastal waters. Dewpoints have
jumped along the coast and some light fog has been reported near
Oceanside. Winds were mostly light as the offshore sfc pressure
gradient slowly strengthened. In terms of forecast changes...A small
adjustment was made for winds with new model data, and some patchy
fog has been added to the coastal areas of San Diego County
overnight. If it does materialize, it should not last long as the
offshore gradient strengthens tomorrow morning.

From previous discussion...

The 500 mb pattern with this Santa Ana is unusual. It is not common
to have a trough diving south across SoCal and stalling out as a
deep closed low over the ocean. This adds a little more uncertainty
to the timing, the wind speeds and just where the winds will
surface.

The models this morning are projecting a stronger wind event than
previous runs, especially in San Diego County. There is strong upper
support with this cold system, with 50 kt 850 mb winds in SoCal
associated with the circulation around a cold 561 dm closed upper
low that stalls out west of northern Baja Tuesday morning. These
east winds will crest the mountain ridges and flow down the western
slopes into the foothills and eastern valleys. Based on the recent
data, wind gusts to 70 mph are possible in the back county,
especially in the east-west transverse ranges of San Diego and
Riverside Counties. In collaboration with Oxnard and others, a High
Wind Watch was issued for the mountains due to the elevated threat
for high winds. The watch runs from Noon Monday to Noon Tuesday in
the SBD and ORA County mtns, and from 6 PM Monday to 6 PM Tuesday
for RIV and SAN County mtns.

This colder system might have a greater potential for the winds to
lower farther into the San Diego Valley and parts of the Inland
Empire, and especially in the eastern valleys. Sustained winds 20 to
30 mph with gusts to 55 mph are possible from Monday night through
late Tuesday afternoon. It'll be a windy day in these valley
areas.

In addition to the winds, the RH will be very low and this Santa Ana
necessitated yet another Red Flag Warning.

Mid and high level moisture will increase tonight and Monday as an
upper trough over Oregon digs south across California before
stalling off the coast of northern Baja Monday night. This moisture
will result in mid and high level clouds and a small increase in
PWATs Monday, but surface RH increases will be modest and not too
significant.

Temperatures: Monday will be cooler day in the deserts, mountains
and inland valleys as the surge of cold air moves in from the north.
Tuesday and Wednesday will be warmer, especially west of the
mountains with downslope warming raising high temps into the 80s in
some spots.

Thursday and Friday: A weak trough passes by to the north bringing
weaker offshore gradients and perhaps some weak onshore flow and
some cooling. Later in the week the ensembles show a another
offshore/Santa Ana type pattern, but the model spread is too large
to project its strength.

&&

.AVIATION...
0670500Z...BKN high clouds at/above 20000 ft MSL with unrestricted
vis will continue through Monday.

East to northeast winds will strengthen along mountain slopes and
below passes after 12Z Monday, with local gusts 35-40 kts and strong
up/downdrafts west of the mtns. The winds increase further Monday
evening, esp aft 02z, with gusts 45-55 kts at times, continuing into
Tuesday. Gusts at the terminal sites will not be as strong, although
gusts to 40 kts will be slightly possible at KONT, but likely to see
gusts to 30 kts. There also could be patchy blowing dust west of the
mountains, with visibility restrictions as low as 1 SM at times in
isolated spots.

&&

.MARINE...
No marine concerns through Monday. A long-period WNW swell will
likely bring combined seas around 10 feet in the outer waters Tue
and Wed, mainly south of San Clemente Island.

&&

.BEACHES...
A long-period WNW swell of 8-10 ft from 290 degrees in the outer
waters will likely bring high surf Tue/Wed. Surf is expected to be 4-
6 ft at Orange County beaches and 5-8 ft at San Diego County
beaches. Isolated sets up to 10-12 ft are possible at select spots
from La Jolla southward. There will be strong rip currents and
dangerous swimming conditions during that time. Swell and surf will
gradually subside Wednesday eve through Thursday. A Beach Hazards
Statement has been issued and a High Surf Advisory may be necessary.

&&

.FIRE WEATHER...
Santa Ana winds start to pick up Monday morning, but Red Flag
conditions will not be too widespread due to the 20-30 percent RH.
The fire weather threat increases and spreads into the southern
areas in the afternoon, and is most widespread Monday night through
Tuesday when the winds will be strongest and the RH falls further.
Wind gusts from 40 to 60 mph are expected in most mountain and
foothill areas, but the strongest winds will likely be in San Diego
County and Riverside County where gusts to 70 mph will increase in
the back country and the RH drops below 15 percent. This will be the
time of the most impactful Red Flag conditions.

Winds begin to diminish Tuesday evening, but moderate offshore winds
will continue into Wednesday morning. Onshore flow will briefly
return Thursday and Friday for cooler days and modest RH recovery.

&&

.SKYWARN...
Skywarn activation will not be needed, but reports of wind damage or
measured winds gusts on Monday and Tuesday are welcome.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Red Flag Warning from 4 AM Monday to 10 PM PST Tuesday for
     Orange County Inland Areas-Riverside County Mountains-
     Including The San Jacinto Ranger District Of The San
     Bernardino National Forest-San Bernardino County Mountains-
     Including The Mountain Top And Front Country Ranger
     Districts Of The San Bernardino National Forest-San
     Bernardino and Riverside County Valleys  -The Inland Empire-
     San Diego County Inland Valleys-San Diego County Mountains-
     Including The Palomar And Descanso Ranger Districts of the
     Cleveland National Forest-San Gorgonio Pass Near Banning-
     Santa Ana Mountains-Including The Trabuco Ranger District
     of the Cleveland National Forest.

     High Wind Watch from Monday evening through Tuesday afternoon
     for Riverside County Mountains-San Diego County Mountains.

     High Wind Watch from Monday afternoon through Tuesday morning
     for San Bernardino County Mountains-Santa Ana Mountains and
     Foothills.

PZ...NONE.

&&

PUBLIC/FIRE WEATHER...10/Moede
AVIATION/MARINE/BEACHES...JMB