Winter Weather Advisory 4, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
854 PM PST Wed Jan 27 2021

.SYNOPSIS...
Mostly dry tonight with increasing clouds into Thursday. Late
Thursday into Friday, widespread precipitation is expected as a
low pressures system moves across Southern California. The weekend
into early next week will be mostly dry with some warming. Around
Tuesday, another low pressure system will move inland along the
West Coast and could bring some precipitation to the area.

&&

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

.Evening Update...
Hello, evening readers. Taking a look at our latest storm system
on our doorstep, everything forecast-wise still looks on track
this evening. Diving into some of the higher resolution models,
the storm system may approach areas of the eastern LA metro a bit
earlier, most likely after 8 PM. After the system pushes through
SoCal, lighter showers will overspread much of the region. POPS
have been slightly adjusted a bit for timing differences. The
weekend looks fairly quiet, but we'll have to keep our eyes on
next week as we may see a cooler (and even wetter) pattern
develop by Tuesday.

.Previous Discussion (As of 146 PM Wednesday)...
The low pressure system that is expected to bring widespread rain
and mountain snow to SoCal Thursday evening into Friday, is
currently bringing widespread rain to northern and central
California. Lighter precipitation may make it to LA County early
Thursday but is unlikely to bring rain into Orange County or the
Inland Empire until Thursday evening. Temperatures tomorrow will
be a few degrees warmer than today but still 5 to 10 degrees below
average for this time of year. Much cooler conditions return on
Friday with temperatures 10 to 20 degrees below average.

Precipitation will become widespread and spread to the southwest
across California beginning Thursday evening. There is still some
uncertainty surrounding the onset of the heavy precipitation. HiRes
models are coming more into agreement that periods of heavy showers
could begin prior to 10 PM Thursday. Periods of heavy rain are
expected through into Friday morning, lingering into Friday
afternoon, or evening especially in San Diego County. An atmospheric
river will be feeding moisture into the system as it moves into our
area.

Rainfall is expected to range from around 1.25 inch to 1.70 inch in
the coast, valleys, and Inland Empire, 2 to 3 inches in the
mountains with 3 to 5 inches on south facing slopes of the San
Bernardino County mountains with locally greater amounts. High
desert areas are expected to receive around 0.50 inch to 0.75 inch,
with 0.25 inch to locally 0.50 inch in the lower deserts.

Runoff is expected to be greater due to heavier rainfall on soils
saturated by recent precipitation. With higher snow levels, snow
melt will add to the runoff, especially in the San Bernardino County
mountains. A Flash Flood Watch has been issued for all areas west of
the mountains below 5000 feet in elevation, including the burn
scars.

Snow levels will rise to 5500 to 6000 feet Thursday night, then fall
to around 4500 feet Friday afternoon. Snowfall of 4 to 10 inches is
expected above 5000 feet, 10 to 15 inches above 6000 feet, and 15 to
locally 30 inches above 7000 feet.

It's possible showers could linger into early Saturday morning, but
the rest of the weekend will be dry. Conditions will warm to near
average by Monday. The next chance for precipitation is around
Tuesday and Tuesday night as a trough of low pressure moves inland
along the West Coast.

&&

.AVIATION...
280420Z...SCT-BKN at/above 12000 ft MSL through Thu morning. Most
vis will remain above 6 miles through Thu. Clouds will increase Thu
afternoon with bases possibly lowering to 5000 ft MSL late. SCT-BKN
clouds could develop after 03Z Fri but impacts at TAF sites are
uncertain.

&&

.MARINE...
A storm system will move through coastal waters Thursday night with
increasing southerly winds, turning westerly Friday. Some gusts will
likely exceed 20 knots. A slight chance of thunderstorms Thursday
night into Friday could lead to hazardous marine conditions. No
marine weather hazards are expected Saturday through next Monday.

&&

.BEACHES...
Surf will build again Thursday ahead of the next storm. Sets to 7
feet are possible Thursday into Friday, mainly in southern San Diego
County but prevailing surf of 3-6 feet is expected. The bigger surf
will also bring a high rip current risk. There is also a slight
chance of thunderstorms Thursday night into Friday.

&&

.HYDROLOGY...
There is still some uncertainty surrounding the onset of the heavy
precipitation but it looks like it will begin late in the day
Thursday into Thursday night in Orange and San Bernardino Counties.
Precipitation will spread across southwestern California becoming
heavy at times into Friday morning. Showers will continue Friday
afternoon and end from the northwest Friday night. The ECMWF still
has ensemble members trending slightly slower in timing, extending
the duration of potentially heavy rain into Friday evening,
especially in San Diego County. There is a slight chance for
thunderstorms for Thursday night and Friday.

Rainfall is expected to range from around 1.25 inch to 1.70 inch in
the coast, valleys, and Inland Empire, 2 to 3 inches in the
mountains with 3 to 5 inches on south facing slopes of the San
Bernardino County mountains with locally greater amounts. High
desert areas are expected to receive around 0.50 inch to 0.75 inch,
with 0.25 inch to locally 0.50 inch in the lower deserts.

Runoff is expected to be greater due to heavier rainfall on soils
saturated by recent precipitation. With higher snow levels, snow
melt will add to the runoff, especially in the San Bernardino County
mountains. A Flash Flood Watch has been issued for all areas west of
the mountains below 5000 feet in elevation, including the Apple/El
Dorado, Bond, Blue Ridge, Silverado, and Valley burn scars.

Snow levels will rise to 5500 to 6000 feet Thursday night, then fall
to around 4500 feet Friday afternoon. Snowfall of 4 to 10 inches is
expected above 5000 feet, 10 to 15 inches above 6000 feet, and 15 to
locally 30 inches above 7000 feet.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Flash Flood Watch from Thursday evening through Friday afternoon
     for 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 Diego County Coastal Areas-San Diego County
     Mountains-San Diego County Valleys-Santa Ana Mountains and
     Foothills.

     Winter Storm Warning from 3 PM Thursday to 3 PM PST Friday for
     San Bernardino County Mountains.

     Winter Storm Warning from 6 PM Thursday to 3 PM PST Friday for
     Riverside County Mountains.

PZ...NONE.
&&

EVENING UPDATE...APR
PUBLIC/HYDROLOGY...CO
AVIATION/MARINE/BEACHES...PG