High Surf Advisory 5, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
930 PM PST Sat Dec 26 2020

.SYNOPSIS...
Returning onshore flow will spread cooling and higher humidity
inland this weekend, with a return of coastal low clouds and fog
tonight and Sunday morning. A Pacific storm system will move through
southern California late Sunday night through Monday night and bring
occasional rain with snow in the mountains, with a slight chance of
thunderstorms. Gusty winds will also accompany the storm system. Dry
but chilly weather will return Tuesday, with a gradual warming trend
the second half of next week.

&&

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

Evening Update:

The only update this evening was to adjust the sky cover tonight and
Sunday morning. Abundant low clouds are forming along the coastal
strip, especially in San Diego County, and will continue to increase
overnight with the increasing onshore flow. No other changes were
needed this evening and the previous discussion, below, remains our
latest thinking on the upcoming significant storm system.

Previous Discussion, issued at 200 PM PST:

A weak trough of low pressure will move through northern
California/northern Nevada today and help bring more onshore flow to
SoCal, opening the door to low clouds and fog near the coast
tonight. The marine layer should deepen rapidly, so the stratus
should make it into the valleys by Sunday morning, and Sunday will
be rather cool and also increasingly breezy in the mountains and
deserts.

The storm system will dig southward off the California coast Sunday
and arrive in our region late Sunday night. Right now, it looks like
there could be three waves with the system which could prolong the
steadier precipitation, resulting in a bit higher total
precipitation amounts than previously thought, though maximum
rainfall rates are staying about the same. Some orographic
precipitation could start late Sunday evening in the San Bernardino
County Mountains due to moist southwest flow, but the first
wave/cold front should move through early Monday morning. A second
wave will likely move through Monday afternoon with the trough axis
finally moving through Monday evening. Actually, there could be a
fourth wave on the back side of the upper trough Tuesday, but by
then the atmosphere should be dry enough to preclude precipitation.
This will be a colder system with Gulf of Alaska origins, so the
snow level should fall to 4000-5000 feet Monday morning, with lowest
levels in San Bernardino County. Not only will the air be cold at
850 MB (a little below 0 deg C) but it will be even colder relative
to normal higher up, including 500 MB temps around -27 or -28 deg C,
and that will make the atmosphere rather unstable, so we have a
slight chance of thunderstorms Monday into Monday night. Some of
these convective showers or thunderstorms could be briefly heavy,
prompting concerns for the fresh burn scars. See Hydrology section
below for more details. Snowfall could reach over a foot in some
mountain locations, mainly above 6000 feet, due partly to the
moisture reaching above the -10 deg C level, where dendritic snow
crystal growth can occur, which is favorable for heavier fluffier
snow. Wind gusts from the southwest/west should locally reach 50 MPH
along the mountain crests and some desert slopes Sunday night and
Monday, but there could be a period of time Monday when wind gusts
could reach 25 MPH.

All of the operational models show the storm system clearing the
area sometime on Tuesday, with showers lingering longest over San
Diego County, possibly into early Tuesday morning. While Tuesday
will be quite chilly, with highs possibly staying below 60 deg F in
many areas, a transient ridge will follow in NW flow aloft for
warmer and dry weather around Wednesday into Thursday. Another
trough could move by to the north or northeast Thursday Night Friday
and could bring some deepening of the marine layer or possibly even
coastal drizzle, but some ensemble solutions suggest offshore flow
developing instead, especially if the upper trough tracks more
inland. Ensemble solutions diverge substantially starting Friday, so
confidence of the forecast from then on.

&&

.AVIATION...

270520Z...Coast/Valleys...Areas of low clouds with bases 700-1300 ft
MSL will continue to fill in along the coast and spread 15-25 sm
inland overnight. Local vis 3-5 SM in BR/HZ in the valleys after
08Z. Bases gradually lifting to 1000-1500 ft MSL by 14Z, scattering
out 17-19Z. Low clouds will begin to spread back inland around
28/00Z with bases closer to 1500-2000 ft MSL. Isolated light showers
could begin as early as 28/05Z, becoming heavier and more widespread
late Sunday night.

Deserts/Mountains...Increasing clouds at/above 20,000 ft MSL and
unrestricted vis through Sunday evening.

&&

.MARINE...
An approaching storm system late Sunday into Monday will bring a
combination of gusty winds, high seas, and rain to the coastal
waters. Combined seas of 10 ft or higher are expecting beginning
Monday afternoon as northwest swell builds, along with north-
northwest winds gusting 20-25 kt. Rain showers will develop Sunday
night, continuing through Monday when there will be a slight chance
of thunderstorms. There is the potential for waterspout development
on Monday. A secondary swell could keep seas above 10 ft through
Tuesday.

&&

.BEACHES...
Building west-northwest swell from 285 degrees expected late Sunday
into Monday, with period 12-15 seconds, and will be followed by a
larger swell from 285 degrees late Monday/Tuesday. Surf will begin
to build in on Sunday, with surf of 3-6 feet. Surf will be 4-7 feet
with sets to 8 ft Monday and Tuesday, locally up to 9 to 10 feet in
southern San Diego County. Additionally, there will be a slight
chance of thunderstorms/lightning at the beaches on Monday.

A Beach Hazards Statement remains in effect in anticipation of high
surf, dangerous rip currents, and the possibility of lightning.

&&

.HYDROLOGY...
At this time, overall QPF expectations with the incoming storm early
next week look moderate, as a far as winter storms go. The amount of
available precipitable water will mostly be 0.50-0.75 inch, which is
plenty for moderate rain and snowfall, but keeping any heavy amounts
more isolated. No mainstem river flooding is expected, and any small
stream flooding will likley be short-lived and not widespread do to
the dry conditions overall. This will limit significant run-off to
poorly drained areas, and near heavier convective showers and
thunderstorms.

Upslope flow will enhance amounts over the mountains, but since it
has been so dry, much of this will likely be absorbed, except in and
below those recent burn scars where rainfall rates in isolated
shwrs/T-storms may be sufficient for debris flows in these sensitive
areas. This will come better into focus as we get closer to the
event. For now, the best chance for rainfall rates that could reach
the USGS thresholds would be later Monday when the atmosphere
becomes most unstable. Since the snow levels will be 4000-4500 feet,
that will limit the potential of debris flow where burn scars extend
above that elevation.

With snow-levels down to 4000-4500 FT, snowfall should be a major
impact for travelers in the mountains. For now 3-6 inches of snow
looks to be widespread above 5000 FT, with areas of 8-12 inches
above 5500 FT, locally higher in San Bernardino County.

For rainfall...The showery nature of this event will make total
amounts highly variable and spread out over time. Smoothed estimates
at this time are...0.50-1.00 inches west of the mountains...1.00 to
2.00 mountains...and 0.15 to 0.35 deserts. However, local higher
amounts could occur if there is substantial convection.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Storm Watch from Monday morning through late Monday night
     for Riverside County Mountains-San Diego County Mountains.

     Winter Storm Watch from late Sunday night through Monday evening
     for San Bernardino County Mountains.

     Beach Hazards Statement from Sunday morning through Wednesday
     morning for Orange County Coastal Areas-San Diego County
     Coastal Areas.

PZ...NONE.

&&

PUBLIC/HYDROLOGY...Maxwell/JMB
AVIATION/MARINE/BEACHES...SS