Lake Arrowhead, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

914
FXUS66 KSGX 220841
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
854 PM PST Sun Dec 21 2025

.SYNOPSIS...
Dry conditions with above average temperatures inland will continue
into Monday. Periods of patchy fog expected on higher coastal
terrain and in the valleys tonight into Monday morning. An
Atmospheric River will bring widespread heavy rain and mountain
snow above 8000 ft late for Tuesday into Wednesday, with more
scattered showers continuing into Thursday (Christmas Day). Gusty
southerly winds will develop across much of the area, including
near the coast and over the coastal waters Tuesday into Wednesday.
There are additional chances for showers Friday through Sunday,
but with decreasing chances into Sunday.

&&

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

Key Points:

* Widespread moderate to heavy rainfall Christmas Eve with the
  potential for roadway flooding and localized flash flooding from
  the coast to deserts. Rapid rises in small streams and increased
  flow in main stem rivers expected. 27% chance of the San Diego
  River reaching minor flood stage.

* Snow levels are forecast to remain above 8000 ft into Christmas
  morning, increasing potential for rock and mudslides along
  mountain highways, and debris flows in and below burn scars,
  especially in the San Bernardino County mountains.

* Gusty southerly winds are expected across the area with 70-90%
  chance of wind gusts exceeding 35 mph along the coast Tuesday
  night into Wednesday. Make sure to secure outdoor furniture. Tree
  damage is possible.

* Additional chances of showers Thursday through Sunday. Snow levels
  are expected to lower to 6500-7000 ft by Friday morning, creating
  slick conditions on mountain highways.

This evening...the marine layer is about 500 ft shallower than
yesterday afternoon with weak onshore flow continuing. Low clouds
are forming in the coastal areas of San Diego County and
increasing in coverage. By Monday morning, most of the coastal
areas and western portions of the inland valleys will be covered
by low clouds. Locally dense fog will likely form on elevated
coastal and inland terrain. Temperatures this afternoon reached 80
degrees in some inland locations.

From previous discussion...
With a weakening high pressure ridge aloft, high temperatures on
Monday are expected to be 5 to 10 degrees above average for inland
locations, locally up to 15 degrees above average for the
mountains and deserts. Temperatures are expected to fall to near
or below average for the rest of the week.

An Atmospheric River is still on track to impact Southern California
on Wednesday. Periods of light to locally moderate rain may begin as
early as mid-day Tuesday with increasing chances of precipitation
into the overnight hours. Overnight Tuesday locally heavy rain may
develop, most likely on the coastal slopes of the San Bernardino
County mountains. The heaviest and most widespread rain is expected
through the day Wednesday, with the heavy rain moving northwest to
southeast from Orange to San Diego County deserts. After the
rain band passes, scattered showers are expected to continue with
locally heavy rain into Thursday (Christmas Day). Latest guidance
shows a significant decrease in IVT (atmospheric moisture) during
the day Thursday, bringing the potential for periods no
precipitation. Snow levels are expected to remain quite high
through the duration of the heaviest precipitation, but are
expected to lower later in the week. Please see the Hydrology
section below for more information on expected rainfall rates,
totals, and snow level information. In addition to periods of
heavy rain, gusty south winds are expected. Winds are expected to
increase on Tuesday afternoon, with the strongest winds expected
during the day Wednesday. Peak wind gusts of 60-70 mph possible on
the coastal slopes of the San Bernardino County mountains with
gusts 30-40 mph possible in the coasts and valleys.

There still remains some uncertainty in the upper level pattern for
Friday through Sunday. Global models continue to struggle with the
progression of the low pressure system that develops near the Gulf
of Alaska. The solutions with more ECMWF ensemble members show a
faster eastward movement of the low which would bring additional
chances of more widespread precipitation to the area sometime Friday
into Saturday, but drier conditions Sunday. Solutions with more
input from GEFS members show a slower eastward progression of the
low which would continue our precipitation chances into at least
Sunday. We will have to see how this evolves over the next few
days, but overall the rain for late into the week into the weekend
looks far less impactful, though snow could make travel difficult
in the mountains should the snow level lower enough.


&&

.AVIATION...
220330Z....Coast/Western Valleys...Very patchy low clouds with bases
800-1000ft MSL have begun to push into San Diego County, likely to
reach KSAN within an hour. Further spread up into Orange County will
occur by 06-08z. VIS is generally 3-6SM, lower where clouds
intersect terrain. Bases may fall a bit closer to 700-800ft MSL
through 10-12z before clearing back towards the coast by 16-18z.
Patchy low clouds develop again Monday evening, with similar bases
but will be a bit slower to move inland, likely after 02-04z Tue.

Otherwise...SCT-BKN high clouds with unrestricted VIS
continues through Monday.

&&

.MARINE...
No hazardous marine conditions through Tuesday morning. South winds
will strengthen late Tuesday in advance of a Pacific storm. Strong
winds and building seas are expected then through early Thursday
morning. Steep seas near 8-10 ft expected with wind gusts upward of
30-35 kts. Winds and seas weaken Friday.

&&

.HYDROLOGY...
While moderate to locally heavy rain may begin Tuesday evening
(especially for the mountains) guidance is coming into better
alignment that the heaviest and most widespread rainfall is expected
during the day Wednesday. Rain will move from northwest to
southeast through the day Wednesday, with scattered showers
lingering into the overnight hours and Christmas morning. Those
scattered showers may be locally heavy. Additionally there is a
slight chance of thunderstorms for Wednesday evening into Thursday
morning and heavy rain is expected where thunderstorms develop.
Peak rainfall rates are expected to range from 0.5-1 inch per
hour, with the potential to be locally higher where embedded
convective showers or thunderstorms develop. The coastal slopes of
the San Bernardino mountains are expected to receive a
significant amount of rainfall due to strong southerly flow aloft
(30-50 kt) enabling orographic enhancement of rainfall and the
warm airmass preventing snow formation below 8000 ft during the
heaviest precipitation.

For the San Bernardino/San Gabriel Mountains 4 AM Tue through 4 AM
Friday, the current deterministic forecast is:
- Cajon Pass westward: 8-14"
- East of Cajon Pass: 8-10"
Most of the rainfall is expected to fall on Wednesday.

For remaining areas 4 AM Tue through 4 AM Friday...
Probabilities of rainfall totals 3 inches or more:
- Orange County/Santa Ana Mountains: 60-85%
- Inland Empire: 30-75%, highest west
- Riverside/San Diego County Mountains: 25-65%
- San Diego County Coast/Valleys: 15-70%, highest north
- Deserts: 5-15% (lower deserts), 15-25% (high desert)

Probabilities of 4 inches or more:

- Orange County/Santa Ana Mountains: 40-65%
- Inland Empire: 15-50%, highest west
- Riverside/San Diego County Mountains: 20-45%
- San Diego County Coast/Valleys: 10-45%, highest north
- Deserts: less than 5% (lower deserts), 10% chance (high desert)

Current deterministic forecast for Tuesday through Friday morning:
- Orange County: 3.3-4.75", highest north
- Inland Empire: 3-4.75", highest northwest
- San Diego County Coasts/Valleys: 2.5-4", highest north
- Santa Ana Mountains: 4.5-6.5"
- Riverside/San Diego County Mountains: 3-6"
- High Desert: 1.5-3", highest west
- Low Deserts: 0.85-1.5", locally 2.5" near the San Gorgonio Pass

The higher rainfall amounts in the deterministic ranges above
will likely be more localized to where heavier, more convective
bands of rainfall develop.

Most of the precipitation across southwestern California through
Christmas morning is expected to fall as rain with snow levels
remaining above 7500-8000 feet. Snow levels could drop to 6500-7000
ft by Friday morning, potentially down to 5500-6000 ft by Saturday.
Confidence in snow levels is low as the spread/difference between
the NBM 25th and 75th percentile snow level is around 1500-2000
ft.

The San Diego River at Fashion Valley and the Santa Margarita River
at Ysidora are currently expected to reach Action/Monitor stage by
Wednesday evening. Based on current forecast, the San Diego river is
expected to peak at 8.2 ft early Thursday morning with the Santa
Margarita expected to peak at 10.9 ft late Wednesday night. There is
a 27% chance the San Diego river could reach minor flood stage by
Friday depending on the location and intensity of additional
rainfall.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Flood Watch from late Tuesday night through late Wednesday night
     for San Diego County Coastal Areas-San Diego County Deserts-
     San Diego County Mountains-San Diego County Valleys.

     Flood Watch from Tuesday afternoon through Wednesday evening for
     Apple and Lucerne Valleys-Coachella Valley-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.

PZ...None.

&&

$$

PUBLIC...PG
AVIATION/MARINE...Zuber