Johnstown, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

288
FXUS66 KSGX 191154
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
354 AM PST Fri Dec 19 2025

.SYNOPSIS...
Onshore flow will spread cooling inland through Saturday with
night and morning coastal low clouds and fog spreading inland into
the far western valleys at times over the weekend. Not much
change is expected for Sunday and Monday with high temperatures
for inland areas still 10 to 15 degrees above average. A low
pressure system from the west will bring increasing chances for
rain beginning on Tuesday. Moisture from an atmospheric river will
bring increasing chances for widespread heavy rainfall for
Wednesday into Thursday with decreasing chances for rain for the
final weekend in December.

&&

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

.SHORT TERM (Today through Sunday)...
Satellite imagery and surface reports shows widespread low clouds
and locally dense fog has returned over the inner coastal waters
extending locally inland along the coast. A Dense Fog Advisory
continues for areas near the coast in San Diego County through 9
AM this morning and for Orange County from the coast inland to
around Interstate 5 through 10 AM this morning.

Night and morning coastal low clouds and fog are expected to
spread a little farther inland each night through the weekend as
the marine layer slowly deepens. Visibilities won`t be as low near
the coast with the lower visibilities moving upward and inland to
near higher coastal terrain inland to the far western valleys.

Onshore flow will bring cooling of high temperatures for a few to
around 5 degrees today for the coast inland to the coastal
slopes of the mountains with a few to around 5 degrees of cooling
for Saturday for most areas with not much change for Sunday and
Monday.

High temperatures for Sunday and Monday will still as much as 10
to 15 degrees above average for inland areas with high
temperatures ranging from the mid to upper 60s near the coast to
the 70s for the valleys with the lower deserts around 80.

&&

.LONG TERM (Monday through Thursday)...
A low pressure system from the west will bring increasing chances
for rain beginning next Tuesday with moisture from an atmospheric
river bring increasing chances for widespread heavy rainfall for
Wednesday into Thursday (Christmas Eve and Christmas). High
temperatures will cool to around average for Wednesday and
Thursday.

For Tuesday night and Wednesday, there could be stronger south to
southeast winds across the coastal waters and adjacent coastal
areas with the ECMWF Extreme Forecast Index having winds between
the 70th and 80th percentile. Please see the Hydrology section
below for details on the rainfall potential through Thursday night
of next week.

&&

.AVIATION...
191050Z...Coast...Areas of dense fog and CIGS below 500 ft MSL will
continue along the coast and locally up to 10 mi inland through the
morning. Confidence is low in fog moving into KCRQ due to weak
offshore/easterly flow there, and KSAN vis likely varying between
1/4 SM and 3 SM as they remain on the edge of the fog bank. Fog
clearing to the coast 15-18Z, latest in Orange County. Areas of low
clouds with bases 400-700 ft MSL spreading back inland after 20/02Z
with vis 2-5 SM, locally down to 1/4 SM again on the coastal mesas
and higher coastal terrain.

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

&&

.MARINE...
Areas of dense fog reducing visibility to 1 NM or less this morning,
mainly over the inner coastal waters. Visibility will improve by
late morning. Otherwise, no hazardous marine conditions are expected
through Tuesday morning, then gusty south winds developing Tuesday
evening with a Pacific storm. Conditions could become hazardous to
small craft at times Tuesday night and Wednesday.

&&

.HYDROLOGY...
A low pressure system from the west will bring increasing chances
for rain beginning next Tuesday. Moisture from an atmospheric
river will bring increasing chances for widespread heavy rainfall
for Wednesday into Thursday. More than 60 percent of ECMWF members
now have values of Integrated Vapor Transport exceeding 750
kg/m/s directed into southwestern California centered during the
day on Wednesday with the ECMWF control run having around a 21-hour
duration of IVT exceeding 500 kg/m/s and the ECMWF ensemble mean
having around an 18-hour duration exceeding 500 kg/m/s.

For Wednesday morning through Wednesday night for Lytle Creek on
the coastal slopes of the eastern San Gabriel Mountains, the
chance for 3 inches or more of rainfall has increased to 71
percent with a 35 percent chance for 5 inches or more and an 11
percent chance for 6 inches or more. For Wednesday morning through
Thursday night, the chance for 8 inches or more of rainfall at
Lytle Creek has increased to 13 percent.

NBM deterministic rainfall for next week through Thursday night
for northern Orange and southwestern San Bernardino Counties
ranges from 3.5 to 4 inches near the coast to 6 to 9 inches on the
coastal slopes of the mountains to the west of the Cajon Pass.
Amounts decrease to the south to 1.5 to 2 inches near the coast of
central and southern San Diego County to 2 to 3 inches for the
mountains of central and southern San Diego County. For the high
desert, 2 to 3 inches is expected with 0.5 to 1.5 inches for the
lower deserts.

Most of the precipitation across southwestern California through
Christmas Thursday is expected to fall as rain with snow levels
remaining above 7000 feet.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Dense Fog Advisory until 9 AM PST this morning for San Diego
     County Coastal Areas.

     Dense Fog Advisory until 10 AM PST this morning for Orange
     County Coastal Areas-Orange County Inland Areas.

PZ...None.

&&

$$

PUBLIC/HYDROLOGY...17
AVIATION/MARINE...SS