Mentone, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

503
FXUS66 KSGX 201107
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
307 AM PST Sat Dec 20 2025

.SYNOPSIS...
Warm and dry weather through Monday with high temperatures
remaining 10-15 degrees above normal inland. Patchy fog will
continue near the coast and western valleys into early next week.
A low pressure system and associated weak to moderate Atmospheric
River will bring widespread heavy rain and high elevation mountain
snow late Tuesday into Christmas Day. Shower chances lower to
around 50-60% on Friday and 20-30% on Saturday.

&&

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

Key Points:

* High temperatures as much as 10-15 degrees above normal inland
  through Monday.

* Reduced visibility in fog for the coastal areas and western
  valleys each night and morning through Tuesday could result in
  locally hazardous travel conditions.

* Widespread moderate to heavy rainfall and gusty southerly winds
  Christmas Eve and Christmas Day with the potential for roadway
  flooding, rock and mudslides along mountain highways, and debris
  flows in and below recent burn scars. Rapid rises in small
  streams and increased flow in main stem rivers.

* Additional chances of showers Friday and Saturday with lower
  snow levels, creating slick conditions on mountain highways.

Marine layer low clouds have spread into the western valleys this
morning with only isolated dense fog being reported in the
valleys. Otherwise a few high clouds are moving over the region.
Low clouds and fog will clear inland by late morning, but could
linger along the coast through the afternoon.

Not much change in the weather over the next three days as we
remain under mostly zonal flow. An upper level high to our south
slowly shifts east through Monday, but the end result is little
change in heights and flow aloft becoming slightly more
southwesterly. A persistent marine layer with night and morning
low clouds and fog for the coastal areas and western valleys will
help moderate high temperatures in those areas, while inland
areas remain warm. Highs along the coast will be near normal,
around 5-10 degrees above normal for the valleys and low deserts,
and as much as 15 degrees above normal for the lower elevation
mountains and high deserts. Otherwise occasional high clouds will
pass overhead.

A big shift in the weather pattern will occur by midweek. An
upper level trough and associated Atmospheric River will amplify
off the West Coast. A short wave embedded in the trough lifts into
So Cal Tuesday night, ushering in the first surge of moisture and
widespread moderate to heavy precipitation. Current forecasts
from CW3E WRF models are showing IVT forecasts as high as 600-700
kg/m/s aimed at Southern California Tuesday night through much of
the day Wednesday. There are still differences in IVT from the
global ensembles, with the EPS having a 60% chance of a moderate
AR and a 10% chance of a strong AR in our forecast area, while the
GFS probabilities are a bit lower - around 30% and 0%
respectively. Even within each ensemble system, the spread in
rainfall amounts remains large - on the order of inches even in
the coastal areas, though the vast majority of members are on the
wetter end. This ample moisture combined with strong southerly
mid-level flow of 30-40 kt will lead to strong orographic lift and
considerably higher rainfall amounts along the south slopes of the
San Bernardino Mountains on Christmas Eve. See the Hydrology
section for potential rainfall amounts. Gusty south winds are
also expected across the coastal areas, valleys, and the San
Bernardino Mountains into the adjacent desert foothills. The snow
level will be above 8000 ft through Christmas morning.

For Christmas Day, ensembles begin to diverge on the evolution of
the trough and the trajectory of the next short wave. Around 30%
of the global ensemble members lift the short wave further north
into Central CA, resulting in lower precipitation amounts here,
41% dig the low further south with the next surge of moisture
aimed at So Cal for higher rainfall amounts, and the remaining 26%
somewhere in between. Then as we head into Friday and Saturday,
around 57% of the members have the trough progressing eastward
with precipitation coming to an end sooner, while the remaining
43% have another short wave diving down the back side of the
trough, causing it to amplify further and slowing the eastward
progression for more prolonged chances of showers. Regardless,
multiple days of precipitation, which will likely be heavy at
times, is bringing a risk of flooding and difficult travel
conditions around the holiday.

&&

.AVIATION...
201000Z...Coast/Western Valleys...Low clouds and fog spreading
inland, impacting coastal TAF sites with CIG restrictions. Low
clouds likely to reach up to 20 miles inland by sunrise and will
likely be vcnty KONT by 12Z. Cloud bases will be around 500 ft MSL
vcnty KSNA and about 1000 ft MSL vcnty KSAN, continuing to rise
after sunrise. Local VIS restrictions 1-3SM on higher inland terrain
...improving after 15Z. Expect partial clearing to the coast 17Z-19Z.

Otherwise...FEW-SCT high clouds with unrestricted VIS through
Saturday evening.

&&

.MARINE...
No hazardous marine conditions are expected through Tuesday morning.
Strengthening south winds late Tuesday in advance of a Pacific
storm. Winds and seas could become hazardous to small craft Tuesday
night through Wednesday.

&&

.HYDROLOGY...
For Wednesday morning through early Thursday morning, for the
coastal slopes of the eastern San Gabriel Mountains (Mt Baldy to
I-15), the chance for 3 inches or more of rainfall has increased
to 92 percent with a 69 percent chance for 5 inches or more and
an 30 percent chance for 6 inches or more. For Wednesday morning
through Thursday night, the chance for 8 inches or more of
rainfall has increased to 32 percent. This area tends to
receive the most rainfall in this weather pattern.

Probabilities for seeing rainfall totals over 2 inches Tuesday
night through Thursday night are listed below:

- OC/Santa Ana Mountains/San Bernardino Mountains: 80-90%
- Inland Empire: 60-90%, highest west
- SD Co Mts/Coast/Valleys: 50-75%
- Deserts: 15-25% (lower deserts), 40-55% (high desert)

Probabilities for seeing rainfall totals over 3 inches Tuesday
through Thursday night are listed below:

- OC/Santa Ana Mountains/San Bernardino Mountains: 65-85%
- Inland Empire: 35-60%, highest west
- SD Co Mts/Coast/Valleys: 20-55%
- Deserts: 5-15% (lower deserts), 20-25% chance (high desert)

Current deterministic forecast for Tuesday through Thursday night:
- Orange County: 3.5 - 4.5", highest north
- Inland Empire: 3-4", highest west
- San Diego County Coasts/Valleys: 2-3", highest north
- San Bernardino County Mountains: 4-10", highest west
- Santa Ana Mountains: 4-6"
- Riverside/San Diego County Mountains: 2-4"
- High Desert: 1.5-3", highest west
- Low Deserts: 0.9-1.5", locally 2.5" near the San Gorgonio Pass

Most of the precipitation across southwestern California through
Christmas morning is expected to fall as rain with snow levels
remaining above 8000 feet. Snow levels will drop some by later
next week, but exact levels and locations remains uncertain.

&&

.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...SS
AVIATION/MARINE...PG