Small Craft Advisory 28, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
935 AM PDT Fri Oct 27 2023

.SYNOPSIS...
Onshore winds and cooler temperatures prevail through Saturday
afternoon. Late Saturday into Sunday sees a transition to offshore
flow and warming temperatures with multiple rounds of Santa Ana
winds through the middle of next week. Strongest Santa Ana winds
at this time look to occur Saturday night into Sunday. Gusty winds
and low humidity will result in elevated to critical fire weather
conditions at times Sunday through midweek. Onshore flow returns
from next Thursday onward.

&&

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

Patchy low clouds cover much of the coastal basin this morning,
and the cloud bases are much lower than yesterday morning. The
inversion on the 12Z NKX sounding is fairly weak, so clouds should
scatter out nicely later this morning, with a mostly clear
afternoon in store.

A longwave troughing pattern over the West will maintain another
couple of days of cooler temperatures and onshore flow. Highs
today and Saturday will generally be 5 to 10 degrees below average
west of the mountains, and near average in the lower deserts. Late
tonight into Saturday, a shortwave will progress south across the
interior West, strengthening onshore flow across the mountains
and deserts. Peak wind gusts will generally be 40-50 mph, with the
strongest winds below the passes, over the ridgelines and through
the wind-prone canyons.

As that shortwave moves inland late Saturday, a surface high will
develop over the Great Basin, with resultant Santa Ana winds for
Southern California. The winds will begin to ramp up late Saturday
night, peaking in strength Sunday morning across northern areas.
Winds have trended slightly higher in the latest model runs,
especially in the corridor from the Cajon Pass to the Santa Ana
mountains. This is currently favored to be a moderate Santa Ana
wind event, with peak gusts generally 40-60 mph, with isolated
gusts to 70 mph below the Cajon Pass and on the foothills of the
Santa Ana mountains. A High Wind Watch is in effect for the
coastal slopes of the mountains, much of Orange County, and
portions of the Inland Empire and San Diego valleys. Some of these
areas will likely be upgraded to a High Wind Warning with the
afternoon forecast package today.

The mid to upper level flow transitions to more northeasterly and
easterly late Monday through Wednesday. This will spread the Santa
Ana winds further south into the Riverside and San Diego County
mountains, though the winds look to be much weaker. Winds in the
Riverside and San Diego County mountains look to top out around
40-50 mph Monday through Wednesday.

As is typical with Santa Ana winds, drier air will be coming into
the region as well, dropping relative humidity values each
afternoon below 15 percent away from the coast, falling as low as
5 percent in portions of the Inland Empire and San Diego valleys. Humidity
recovery will be poor during much of this time, down to 15-30
percent each morning Monday through Wednesday in locations
affected by Santa Ana winds. The combination of winds and very low
humidity, especially with multiple days of poor recovery, will
lead to periods of critical fire weather Sunday and Monday, with
near-critical conditions continuing Tuesday and Wednesday as
weaker offshore winds prevail. Given the live fuel moistures still
remain above average due to our persistent marine layer and
previous rainfall, no fire weather products are needed for this
event.

In addition to the gusty winds and low humidity, temperatures will
warm up west of the mountains early to mid next week, while cold
air advection will keep the desert and mountains much cooler. West
of the mountains, highs will soar into the mid 80s each afternoon
Monday through Wednesday. Even some coastal areas could approach
80 degrees next week.

The pattern will shift on Thursday as troughing begins to set in
over the PacNW, resulting in the high weakening. This will shift
us into more of an onshore flow pattern, with cooler temperatures
and higher humidity.

&&

.AVIATION...
271530Z...Coast/Valleys...Two cloud layers with bases generally at
1500 feet and 3500 feet MSL with tops up to 4200 feet. These will
scatter out by 16-18Z. Areas of low clouds to develop in the coastal
basin after 03Z, first in San Diego County and later in counties
north, with intermittent cigs based at 1000-1500 feet MSL through 16-
17Z Saturday.

Mountains/Deserts...Mostly clear skies with unrestricted VIS through
Saturday morning.

&&

.MARINE...
No hazardous marine weather is expected through Saturday morning.
Gusty northwest winds strengthening over the outer waters Saturday
afternoon and night could produce conditions hazardous for small
craft. An abrupt shift to offshore winds early Sunday may produce
hazardous conditions at times between Camp Pendleton and San
Clemente Island for Sunday into Monday.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...High Wind Watch from Saturday evening through Monday evening for
     Orange County Coastal Areas-Orange County Inland Areas-
     Riverside County Mountains-San Bernardino County Mountains-
     San Bernardino and Riverside County Valleys-The Inland
     Empire-San Gorgonio Pass Near Banning-Santa Ana Mountains
     and Foothills.

     High Wind Watch from late Sunday night through Wednesday
     afternoon for San Diego County Mountains-San Diego County
     Valleys.

PZ...None.

&&

PUBLIC...Connolly
AVIATION/MARINE...MM