Arrowhead Highlands, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

648
FXUS66 KSGX 021828
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
1000 AM PST Thu Jan 2 2025

.SYNOPSIS...
Dense fog will linger in parts of Orange County through late this
morning. Otherwise, fair and warmer weather will prevail today under
a ridge of high pressure and weak offshore flow. Areas of coastal
low clouds and locally dense fog will occur tonight and early
Friday. Onshore flow will develop Friday into Saturday morning, with
cooler weather and more clouds. High pressure and offshore flow
will return Saturday afternoon and continue through early next
week and bring gradual warming. A trough of low pressure moving
south over the western United States may bring stronger Santa Ana
Winds around Tuesday or Wednesday.

&&

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

Fog was stubborn from I-405 west to the coast in northern Orange
County at 10 AM, with sunny skies elsewhere. Inland Valleys were
already warming up, with Ramona being 74 degrees as of 10 AM.
Earlier this morning, we were seeing temperature differences around
30 deg F between cold calm valley floors and slightly elevated
adjacent terrain, such as 35 vs 65 around the San Pasqual Valley in
San Diego County. Highs should reach 80 in the warmer valleys early
this afternoon.

Onshore flow will return late tonight and Friday as the upper ridge
moves inland and an upper level trough approaches the West Coast.
The marine layer will remain rather shallow tonight, with dense fog
likely in coastal areas, possibly spreading into the coastal mesas.
A coastal eddy could develop early Friday but probably will not
have much influence on the fog as the flow will mostly be over the
ocean. The transition to a trough pattern will bring several degrees
of cooling west of the mountains Friday. As the trough axis
passes Friday night, gusty southwest to west winds will develop
over the mountains and deserts with local gusts 30-45 mph on
the wind prone desert slopes and through/east of the passes. The
marine layer will deepen Friday night with the passage of the trough
with low clouds spreading into the valleys.

We will make a quick flip back to weak offshore flow Saturday
afternoon as surface high pressure builds over the Great Basin
behind the departing trough. High temperatures will actually be
near normal Saturday, but that is short-lived as continued weak
offshore flow and upper ridging to the west bring highs back to
several degrees above normal Sunday and Monday.

An upper level ridge will gradually amplify off the West Coast,
while a trough of low pressure moves south over the western US.
There is a great variability in the strength and timing of the
trough with around a 40 percent chance of some type of closed low
forming in the southwest, according to the cluster analysis of the
ENS/GEFS/GEPS. Some of these solutions suggest the closed low
getting far enough west (SoCal) to draw in moisture and result in a
colder atmosphere as well, though nearly all solutions have strong
surface high pressure over the Great Basin and northern Rockies with
moderately strong MSLP gradients to SoCal. Most likely we will have
a moderate to strong Santa Ana wind event in the Tue-Thu time frame,
with most favorable conditions for strong winds around Wed. See the
fire weather discussion below for more info on probabilities.

&&

.AVIATION...
021745Z...Coast...Low clouds and fog have retreated offshore in most
areas this morning, with the exception of a patch of fog in northern
Orange County, which will break up and clear through about 19Z this
morning. Low clouds with bases around 300-600 feet MSL with areas of
vis 1-5 SM redevelop and move ashore after 03Z and into western
valleys up to about 10 miles inland by 12Z Friday. Low clouds may be
in and out of sites inland from the immediate coast. Most areas will
see clearing to the beaches again 16-18Z Fri, though northern Orange
County could again see some persistent clouds through about 19Z Fri.

Otherwise...FEW-SCT clouds at/above 25000 feet MSL and unrestricted
vis today and tonight.

&&

.MARINE...
Patchy dense fog expected this morning with visibility 1 NM or less.
Visibility will improve in the next few hours (8-10 AM PST).
Otherwise, no hazardous marine conditions are expected through
Monday.

&&

.FIRE WEATHER...
Local east winds with gusts to 30 MPH will continue through this
afternoon, with some relative humidities around 10 percent for
elevated fire wweather conditions. Onshore flow will bring higher
humidity Friday through early Saturday, albeit with local wind gusts
over 40 MPH through San Gorgonio Pass and on some of the desert
mountain slopes. Weak offshore flow will develop late Saturday and
continue through Monday with most wind gusts remaining at/below 30
MPH and relative humidity mostly remaining above 15 percent. There
is a high risk of stronger Santa Ana winds Tuesday and especially
Wednesday as a trough of low pressure moves south through the
western United States, with a 90 percent chance of at least
local wind gusts of 40 MPH or greater and a 60 percent chance of
local wind gusts of 60 MPH or greater. There is some question as to
how low the humidity will be as some model guidance suggests a
closed low developing which could bring in moisture, but there is
a 60 percent chance of widespread daytime humidity below 15 percent
Wednesday. Conditions could continue Thursday. Elevated to (more
likely) critical fire weather conditions are expected Tuesday
through Thursday of next week.

&&

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.

&&

$$

PUBLIC/FIRE WEATHER...Maxwell
AVIATION/MARINE...CSP