140
FXUS66 KSGX 260522
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
922 PM PST Sat Jan 25 2025

.SYNOPSIS...
Scattered rain and mountain snow showers will become more
widespread Sunday into Monday. There is a slight chance of
thunderstorms for Sunday into Monday for the coast, valleys, and
mountains. Gusty south winds will increase today from the coast
to the deserts, with periods of gusty winds continuing into
Sunday. Dry and warmer weather is expected the end of the work
week. Another low pressure system could bring more chances for
precipitation next weekend.

&&

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

Key Messages:

- Scattered to widespread rain showers and mountain snow through
  Tuesday. There is a 25% chance of showers continuing into
  Wednesday, though where these showers would occur is still
  highly uncertain.

- Snow levels falling to 3500-4000 ft in San Bernardino County
  Sunday night into Monday night could result in travel impacts on
  I-15 through the Cajon Pass.

- There is a 15-20% chance of precipitation returning next
  weekend.

Evening update...

A very narrow band of light precipitation is moving through San
Diego County, with a second more robust band moving through Orange
County this evening. Despite their somewhat impressive look on
radar, rainfall rates within these bands are less than 0.10
inch/hour. Expect showers to become more widespread and intense
overnight with localized rainfall rates up to 0.30 inch/hour. Most
precipitation tonight will be confined to areas along and west of
the mountains and locally into the high deserts as the moist
boundary layer on the 00Z sounding only extends up to about 5000
ft and won`t deepen enough for any spillover into the lower
deserts by Sunday morning. As this moisture continues to deepen
and instability increases with the upper cold core moving closer
Sunday afternoon and Monday, expect showers to be able to make it
into the lower deserts. Otherwsie south to southwest windswith
gusts 40-50 mph will continue across the desert mountain slopes,
below the passes, and adjacent desert areas overnight, weakening
Sunday morning.

Previous discussion...
As the core of the low pressure system moves closer to Southern
California Sunday afternoon and evening, shower activity is
expected to become more widespread with increasing potential for
locally heavier showers. Generally, maximum rain rates for Sunday
afternoon into Monday morning will be 0.25 to 0.35 inches per
hour. There is a 10 to 15 percent chance of thunderstorm
development, higher chances closer to the coast with decreasing
chances towards the mountains. Where thunderstorms develop,
maximum rainfall rates may reach 0.50 inches per hour. For Sunday
afternoon through early Monday morning, the HREF has a 15 percent
chance of exceeding 0.50 inches per hour. Localized flooding is
possible, especially where thunderstorms/heavier showers occur.
Showers have the potential to be fairly widespread during the
Monday morning commute but are expected to become more scattered
through the day Monday. Isolated showers have the potential to
continue into Tuesday. Conditions should be mostly dry by
Wednesday, but there is some potential for wrap around moisture
from the low bringing areas of additional precipitation. The most
likely places of additional precipitation on Wednesday is the
mountains and deserts.

This system originated in northwestern Canada and will bring that
cold air with it as it travels south. Widespread mountain snow is
expected, with low snow levels. Initially snow levels are expected
to start around 5000-5500 ft this evening, but are expected to drop
to 4000-4500 ft for much of Sunday. Snow levels could fall as low as
3500 feet in the San Bernardino Mountains and as low as 4000 feet in
the San Diego County mountains Sunday night into Monday. During the
coldest part of the storm Sunday night into Monday, some snow may
accumulate at the top of the Cajon Pass and in portions of the High
Desert. For the High Desert, the most likely place for accumulating
snow is in the higher elevations in the Ord and Granite mountains.
There is a non-zero but less likely chance of accumulating snow near
the higher population centers in the High Desert. Snow accumulations
could range from 12-18 inches above 6,000 feet, 6-12 inches above
5,000 ft, and 3-6 inches above 4,000 feet. Up to 24 inches possible
above 7500 ft and up to an inch is possible at 3500 ft. Be sure to
check the latest road conditions if you are planning to head to the
mountains and through the passes, as snow covered roads may present
hazardous driving conditions. Light snow showers may continue into
Tuesday or Wednesday.

Daytime high temperatures will fall into the 50s west of the
mountains on Sunday and remain there through Wednesday before
warming into the 60s on Thursday. As the upper low moves off to the
east it will leave us in northeast flow aloft but keep an onshore
pressure gradient at the surface. As a result, fair weather will
return for Wednesday or Thursday through Friday with low clouds in
the coastal areas and valleys during the nights and mornings.
Temperatures will remain below seasonal averages into next weekend.
Another, weaker low pressure system could bring additional chances
of precipitation and gusty west winds next weekend, but confidence
is low due to the spread in model solutions.


&&

.AVIATION...
260430Z....An upper low pressure system will continue to affect the
region tonight and Sunday. BKN-OVC low clouds have moved to the
coastal mountain slopes this evening, with areas of terrain
obscuration. Bases are highly variable at 2500-5500 ft MSL with
several cloud layers in the region, and conditions will continue to
vary between MVFR and VFR for most sites through the period, with
locally lower conditions for the foothills and mountains.

-SHRA have developed offshore and moved into coastal regions, the
initial wave of which will move inland to the mountains through
about 06Z this evening. SHSN expected in the mountains above 4500-
5000 ft MSL. Showers will continue to move through the region in
waves tonight and Sunday, bringing locally lower CIGS 2000-3000 ft
MSL and VIS 2-5 SM. In the mountains where SHSN is expected and
clouds will be much lower to the ground, VIS will be 0-3 SM at
times. Strong S/SW winds with gusts 30-40 kts for the mountains and
desert slopes will continue through about 12Z tonight before
tapering off, though SW winds will continue to be breezy through
Sunday.

NWS San Diego is aware of limited communications of observations
from KTRM since January 8th. We are working with the FAA to address
this issue.

&&

.MARINE...
South-southwesterly winds will diminish tonight, but remain breezy
Sunday with gusts to 20 knots through early Sunday morning. A Small
Craft Advisory remains in effect for the nearshore waters through
Sunday morning and contains more details.

Rain showers will increase in coverage tonight, and continue at
times through Tuesday. Sunday evening through Monday morning,
heavier pockets of rain are expected, with even some embedded
thunderstorms possible, any of which could produce hazardous
conditions including locally gusty and erratic winds, lightning,
rough seas, and reduced visibility. Thunderstorm chances decrease by
late Monday morning. A Marine Weather Statement is in effect and
contains more details.

&&

.BEACHES...
Off-and-on rain showers will continue through the weekend into early
next week, and heavier pockets of rainfall are possible by Sunday
evening. There may be embedded thunderstorms which could lead to
gusty winds and lightning, with a 15% chance for a thunderstorm for
any point along the coast Sunday evening through early Monday
morning.

&&

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Storm Warning until 4 PM PST Monday for Riverside County
     Mountains-San Bernardino County Mountains.

     Winter Weather Advisory until 4 PM PST Monday for San Diego
     County Mountains.

     Wind Advisory until 6 AM PST Sunday for Apple and Lucerne
     Valleys-San Diego County Deserts.

PZ...Small Craft Advisory until 4 AM PST Sunday for Coastal Waters
     from San Mateo Point to the Mexican Border and out to 30 nm.


&&

$$

PUBLIC...SS/CO
AVIATION/MARINE/BEACHES...CSP