Flash Flood Watch 3, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
838 AM PST Fri Nov 6 2020

.SYNOPSIS...
An early winter storm will impact Southern California this weekend
with periods of showers, mountain snow and strong winds. Fair, dry
and cool weather is expected for much of next week.

&&

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

Cloud cover was rather chaotic this morning over much of SoCal with
patchy low clouds over coastal areas and swaths of higher clouds
moving from SW to NE. The 12Z sounding had a sfc-based inversion,
with cooling throughout the column, which was most notable between
2K and 14K FT. Winds had flipped from weak easterly to westerly and
increased with height. The sfc pressure gradients were weak and
trending onshore now. Still, we had several sites over the high
deserts and mts with SW winds 20-25 MPH and gusts 30-35 MPH. Plan on
those winds increasing, especially this afternoon. No updates
planned.

From previous discussion...

A vigorous low pressure trough currently impacting the Pacific
Northwest will drive down the coast today and tonight. A coastal
eddy and falling heights will allow more coastal clouds to form
today and spread inland this afternoon and evening. Some light
showers are possible west of the mountains after dark and
overnight. This enormous trough over the West will contain two
waves rotating like a dumbbell through our region. Wave one arrives
Saturday morning bringing widespread showers pretty much all day.
There's even a slight chance of thunderstorms during the
afternoon mainly near the coast. Showers will relent somewhat
Saturday evening and night, but isolated showers will continue.
Wave two arrives on Sunday with similar effect and timing as
Saturday's wave one. The positive tilt trough and westerly flow
favors San Diego County to harvest the most rainfall with this
legit winter storm. Two-day precipitation will range from about
one- third inch at coastal Orange County to about 2.5 inches
around Cuyamaca Peak. The snow level will fall rapidly to around
5,000-5,500 feet elevation. Accumulations up to about 8 inches at
resort levels, but much more is expected on the higher slopes and
peaks. Winds will pick up and increase today, becoming quite
strong this afternoon and evening. Wind gusts in mountain ridges
and adjacent desert slopes will top 50 mph, with isolated gusts
reaching 60 mph. The double barrel energy from this storm will
maintain these strong winds until Sunday night. Winds west of the
mountains will have some gusts topping 30 mph, especially at the
coast. Temperatures will nosedive starting today and bottom out
over the weekend. Thursday's highs were 10-20 degrees above
average for early November. Today we'll drop temps into the
average range, then plummet to 20-30 degrees below average both
days this weekend. Yeah, it's time to dust off the jackets,
umbrellas and furnaces... and safe driving skills. The storm moves
out Sunday night with rapidly clearing skies, which could lead to
some frost in the high desert. Fair, dry and cool weather will
follow much of next week. A touch of offshore flow should remind
the marine layer it's November, so coastal clouds and fog should
be mostly absent. There are a couple troughs that move through the
West, but too far north and too weak to bring us a credible
chance of rain. The overall weather pattern, the long wave pattern
over the northeast Pacific, appears to be changing. We've endured
a powerful ridge of high pressure for months, but the jet stream
flow appears zonal or westerly and gaining strength, essentially
opening the door for waves of energy in the jet stream to bring
storms to California, particularly Northern California. It will be
interesting to see if this materializes.

&&

.AVIATION...
061630Z...Patchy low clouds and fog across coastal areas this
morning with bases 400-800 FT MSL and reduced VIS of 1-3 SM. Low
clouds will progress inland through this morning and afternoon with
bases rising to 1500-2000 FT MSL after 18z, and 2500-3000 FT MSL
after 02z Sat. West to southwest winds will increase after 20z today
across the mountains and valleys, with winds of 10-20 kts and
isolated gusts to 45 kts. This will lead to up/downdrafts and LLWS.
Isolated showers will be possible early Sat morning, but the best
chance of rain will be after 12z Sat.

Otherwise, BKN clouds AOA 15,000 ft through tonight.

&&

.MARINE...
Strong northwest winds of 20-30 kts will arrive this afternoon along
with combined seas of 8-12 ft and steep waves, creating conditions
hazardous to small craft. The winds and swells will peak Sat and Sun
before subsiding Monday. A Small Craft Advisory is in effect from
this afternoon through early Mon morning for gusty winds and high
combined seas. Periods of showers are likely tonight through Sun
night. No hazardous marine conditions are expected after Mon morning.

&&

.BEACHES...
Surf will increase through today, with the highest surf on Sat and
Sun. Surf of 4 to 6 feet with local sets to 7 feet is expected,
highest in southern San Diego County, producing a high rip current
risk and dangerous swimming conditions. A Beach Hazards Statement is
in effect from this afternoon through Sun night for elevated surf
and strong rip currents.

&&

.FIRE WEATHER...
Increasing onshore flow will drive higher RH inland today. The
developing stronger southwest to west winds will still be of concern
though, especially on the desert slopes of the mts and across high
desert areas where minimum RH values of around 15% are expected.
Southwest to west winds of 20 to 35 MPH with gusts 50 MPH or more
are possible through tonight over the San Bernardino County
mountains and deserts. This will produce elevated fire weather
conditions in these areas through this evening.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Weather Advisory from 9 PM this evening to 10 PM PST
     Sunday for Riverside County Mountains-San Bernardino County
     Mountains.

     Wind Advisory from noon today to 10 PM PST Sunday 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 from 8 AM PST this morning through late
     Sunday night for Orange County Coastal Areas-San Diego
     County Coastal Areas.

PZ...Small Craft Advisory from 1 PM this afternoon to 4 AM PST Monday
     for Coastal Waters from San Mateo Point to the Mexican
     Border and out to 30 nm-Waters from San Mateo point to the
     Mexican Border Extending 30 to 60 nm out including San
     Clemente Island.

&&

PUBLIC...10/MM
FIRE WEATHER...10
AVIATION/MARINE/BEACHES...Schenk