Winter Weather Advisory 21, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
913 AM PST Sat Dec 21 2019

.SYNOPSIS...
Upper level high pressure over Southern California will shift east
this weekend as an upper level trough moves in from the
northwest. This trough will establish itself over the U.S. West
Coast next week, sending Pacific storms into the area. One storm
will impact the area on Monday with a colder storm system arriving
Christmas night through Thursday. Light precipitation is forecast
between these storm systems. Precipitation will come to an end
next Friday with dry weather likely next weekend. However,
another Pacific storm could arrive by New Years Eve. Above normal
temperatures today will fall to well below normal during the
early to middle part of next week, before rebounding next
weekend.

&&

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

Morning Update:

No significant changes to the forecast this morning. Thick high
clouds streaming over the area today will keep sunshine filtered at
best.

.Highlights...

* Pacific storms arrive next week while temperatures lower
* Main impact periods: Monday then Christmas night-Thursday

An upper level ridge over Southern California will shift east
through Sunday. Temps today will again be above normal before
cooling early through the middle part of next week as an upper
level trough becomes established over the U.S. West Coast. South
winds will increase over the mountains and high desert on Sunday
ahead of the trough. Gusts to 50 mph or higher are possible along
the desert mountain slopes.

Precipitation could begin Sunday afternoon over Orange County and
SW San Bernardino/Riverside Counties, but more likely Sunday night
while spreading southeast. The bulk of the rain is expected to
impact SoCal on Monday with light showers lingering Monday night-
Tuesday. Rainfall amounts Sunday-Tuesday are forecast to be in
the 0.75-1.25 inch range from the coast to the mountains with the
deserts in the 0.20-0.40 inch range. Snow levels will be 5500-6000
FT Sunday night-Monday, falling to near 4500 FT late Tuesday. 4
to 8 inches of snow is possible above 6000 FT with this first wave
of precipitation late Sunday-Tuesday (bulk occurring on Monday).

Another storm will arrive during the middle part of next week from
the north and this will be a colder one. Trends have been later
with this storm and are now pointing to Christmas night through
Thursday for the brunt of the precipitation. This storm could
bring slightly more precipiation to SoCal with most places from
the coast to the mountains averaging one inch, but 1.5-2.0 inches
along the coastal mountain slopes. How much falls though will
depend on low trajectory which still is uncertain. Colder air
will move in from the north, posing more impact in terms of
mountain snowfall. GFS is colder with H7 temps down to -10C, but
ECMWF shows about -7.5C (as the closed low is further offshore).
NBM has snow levels at near 4000 FT and went with this given the
uncertainty in low track. Significant snow is possible with this
system with a foot of snow possible in the mountain towns above
5500 feet (unless low is too far offshore). Snow could fall as
low as the Cajon Pass and along I-8 through the San Diego County
mountains (low confidence).

Light precipitation will continue Thursday night before coming to
an end on Friday as the storm exits the area to the southeast.

These storm systems could pose a threat of localized urban
flooding. However, widespread flash flooding is not anticipated.

GFS and ECMWF ensemble mean show an upper level ridge building in
next weekend ahead of potentially another storm system arriving
by New Years Eve. This will result in dry weather conditions next
weekend with gradually warming temperatures, followed by perhaps
a return of more precipitation New Years Eve and Day.

&&

.AVIATION...

211615Z...SCT-BKN clouds AOA 20,000 ft MSL and unrestricted vis
through Sunday morning.

&&

.MARINE...
A brief increase in south to southeast winds expected late Sunday
night into Monday morning ahead of a cold front, with peak wind
gusts around 15-20 kts over the coastal waters. Winds will weaken
quickly after the frontal passage Monday morning. Winds may increase
again midweek as another storm moves through, though timing and wind
strength are still uncertain. Precipitation is likely at times
Sunday night through Wednesday, heaviest on Monday.

&&

.BEACHES...
A series of moderate to long period west-northwest swells will
generate locally elevated surf and strong rip currents this weekend.
Surf will mainly be 3-5 ft. Sets to 6 ft are possible late Saturday
into Sunday in San Diego County.

&&

.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...Gregoria/JMB
AVIATION/MARINE...Connolly