Frost Advisory 9, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
820 PM PDT Sun Apr 2 2023

.SYNOPSIS...
A strong trough moving down the California interior will bring cold,
windy weather Monday with isolated showers west of the mountains,
and a period of snow showers on the southwestern slopes of the San
Bernardino Mountains above 5,000 feet. Tuesday will be another cold
day, but warmer weather is forecast Wednesday through Sunday.

&&

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

EVENING UPDATE:

-Highlights-
--Wind Advisories and High Wind Warnings Monday and Monday night.
--Blowing dust and sand in the deserts with significant reductions
  in visibility at times Monday.
--Cold Monday night and Tuesday night with frost in the valleys.
--Warming trend Wednesday through Saturday.

The 00Z sounding this evening showed the marine layer depth at
~2,200 ft. The marine layer will steadily deepen tonight, and will
be followed by more rapid deepening Monday morning, reaching a depth
of 6,500 feet by midday. Isolated showers are possible in the
coastal and valley area Monday morning from this modest influx of
low level moisture, but the best chance for rain and snow showers
will be on the west and southwest facing mountains slopes due the
additional lift from orographic forcing.

Strong cold air advection Monday morning will lower snow levels in
the SBD Mountains from 6,500 feet at sunrise, then to 5,000 feet by
noon, and then to 4,000 feet through the afternoon. Strong jet
dynamics coupled with strong vertical motions with the cold front
and orographics may very well support a burst of snow showers in the
SBD mountains tomorrow morning into early afternoon.

Winds will be VERY strong in the mountains and deserts Monday
afternoon and evening, potentially dangerous. 850mb winds accelerate
to 40-70 kts and some of this wind energy will crest the mountain
ridges and surface down the eastern slopes into the lower deserts as
a mountain wave. Wind guidance is projecting sustained of 30-40 mph
gusting 60-70 mph at times in some desert areas. If that unfolds
there will be widespread blowing dust/sand and significant
visibility restrictions, a particularly dangerous situation for
motorists traveling through the deserts.

Winds in the coastal and valley areas will also be strong, but
nothing like the mountains and deserts. The strongest wind gusts of
40-45 mph are most likely near the coast as the wind blows in
unabated from over the ocean. Farther inland the gusts will range
from 30 to 35 mph.

*From Previous Discussion*

These winds will propel another cold airmass across the region, and
with clear skies and winds dropping off, especially in the valleys,
temperatures could crater Tuesday morning, easily dropping through
the 30s. This will likely require a Frost Advisory. Atmospheric
thickness values bottom at this time, with modest recovery expected
through Wednesday morning, but some of those cold spots in the
valleys are likely to be quite cold again on Wednesday morning.

After that, it looks like our temperature trajectory will be slowly
upward as heights/thickness slowly rise. By the end of the week, we
are still forecasting above normal temperatures, although it still
isn't completely clear how we get from here to there. The latest
ECMWF ensembles have backed off on the warming and now show
considerable uncertainty as we move into next weekend. The GFS
ensembles remain bullish overall on the warming. Regardless of the
near term trends, some warming is still likely as a broad ridge
builds over the West. Dry weather is still likely through at least
the first half of next week as well.

&&

.AVIATION...
030300Z...BKN-OVC low clouds with bases 1500-2000 feet MSL will
continue spreading inland to the coastal mountain slopes overnight.
Ceilings likely at KONT and KSBD by 05Z. Slight chances for light
and scattered SHRA increase after 06Z. SHRA will be brief and
intermittent. Cloud bases will rise to 2000-2500 feet MSL after 12Z,
with ceilings persisting into Monday afternoon at all terminals west
of the mountains.

West winds will become gusty over and east of the mountains starting
tonight, with gusts 40-50 kts. Strongest winds will be over
ridgelines, below passes, and through wind-prone canyons. Winds will
be even stronger Monday with gusts of 60-80 kts in similar wind-
prone regions, with gusts of 30-40 kts occurring across much of the
coastal, valley, and lower desert areas. Strong up/downdrafts and
LLWS expected over/east of the mountains.

&&

.MARINE...
Winds strengthen from the northwest tonight, with gusts around 20 kt
near San Clemente Island. Gale force west-northwest winds and large,
rough seas are likely Monday and Tuesday. Gale Warnings are in
effect starting at 5 AM Monday for the outer waters, and at 11 AM
for the inner waters, with both warnings in effect through 8 AM
Tuesday. Gusts of 35-40 kt are possible Monday morning through early
Tuesday morning, lowering to 20-25 kt for the remainder of Tuesday
into Wednesday morning.

&&

.BEACHES...
Large, short-period west northwest swells will bring elevated to
high surf conditions for Monday into early Tuesday. Surf of 5-7 feet
expected, with higher surf of 8-10 feet possible on exposed west-
facing beaches in San Diego County. High Surf Advisories have been
issued for the Orange and San Diego County Beaches from 11 AM Monday
to 11 AM Tuesday.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory from noon Monday to 5 AM PDT Tuesday for Orange
     County Coastal Areas-Orange County Inland Areas-San
     Bernardino and Riverside County Valleys-The Inland Empire-
     San Diego County Coastal Areas-San Diego County Valleys-
     Santa Ana Mountains and Foothills.

     High Wind Warning from 5 AM Monday to 5 AM PDT Tuesday 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.

PZ...Gale Warning from 11 AM Monday to 8 AM PDT Tuesday for Coastal
     Waters from San Mateo Point to the Mexican Border and out
     to 30 nm.

     Gale Warning from 5 AM Monday to 8 AM PDT Tuesday for Waters
     from San Mateo Point to the Mexican Border Extending 30 to
     60 nm out including San Clemente Island.

&&

EVENING UPDATE...Moede
PUBLIC...10
AVIATION/MARINE/BEACHES...Connolly