Winter Storm Watch 4, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
930 AM PST Fri Nov 25 2016

.SYNOPSIS...
One more sunny mild day will occur today. Local fog will be possible
along the coast late tonight and early Saturday, with clouds
increasing Saturday. A trough of low pressure will bring a couple of
systems this weekend with rain possible late Saturday through Sunday
evening with several inches of snow in the higher mountains and
strong gusty winds at times. It will be cooler over the weekend,
with cool conditions continuing Monday under partly cloudy skies.
Starting Tuesday, it will be a little warmer, but temperatures will
still be a little below seasonal normals for the rest of next week.

&&

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

Skies were clear this morning, though it was chilly where winds were
light as dew points were low. A few morning lows include 14 at Big
Bear Lake, 32 at March Air Force Base and 34 at Ramona. There were
still a few locally breezy locations in/near passes and canyons under
the offshore flow, but peak gusts were 30-35 MPH from the east. These
winds should gradually decrease today, though local WRF does show
some gusts of 25-30 MPH from the east continuing early this afternoon
through Banning Pass. There could be some stratus/fog along the
immediate coast late tonight or, more likely, early Saturday, but
confidence is still fairly low at this point as moisture may be
slow to recover in the lowest 1000 ft above the surface.

A couple of lows, which were part of a trough, extended from 48N/132W
to 39N/135W with another low in the Gulf of Alaska. The first wave
will move through southern California Saturday evening with very
much a N-S orientation. There will be good moisture from the surface
to a little above 700 MB, though the deeper moisture will be short-
lived, mainly between about 3 and 7 PM. The winds in the 700-850 MB
layer will be southwest, generally 25-40 knots, so some orographic
precipitation will occur. 850 MB temperatures drop fairly quickly
with the first wave, with 1-3 deg C by evening, so generally 1-4
inches of snow should fall above about 5000-5500 ft elevation.
Rainfall amounts will be somewhat variable due to some convective
precip as 500 MB temps fall to -25 C during the early evening and 12Z
NAM boundary layer CAPE shows 500 J/kg then, but overall it should be
0.1 to 0.5 inches from the coast to the valleys and 0.5 to 1.0
inches along the coastal mountain slopes through Saturday evening. A
small amount of precip, mostly under 0.1 inches, could spill into
the deserts. A few showers should continue late Saturday night as
moisture remains at/below 800 MB, though the second wave from the
Gulf of Alaska low will arrive Sunday. Winds will be more westerly,
but moisture could still extend above 700 MB. Despite the colder
origins of the second system, temps aloft are progged to be slightly
higher than with the first wave, so the snow level could inch up a
few hundred feet, mostly 5500-6000 ft elevation, but another 1 to 4
inches of snow could fall. Rainfall amounts for Sunday should be
mostly 0.1 to 0.4 inches coast/valleys and 0.4 to 0.8 inches mountain
slopes with slightly higher amounts in San Diego versus the northern
mountains. Again, some precip could spill into the deserts, mostly
light. Winds will be increasing Saturday with some gusts over 50 MPH
in the afternoon on the mountain crests, desert slopes and into the
deserts. Strongest winds will occur with the second system Sunday
when local gusts could exceed 60 MPH on east slopes and into the
deserts, and a few gusts to 40 MPH could occur near the beaches.
Precip should end Sunday evening, and winds will decrease that
night.

Some residual moisture and a weak wave could bring an isolated shower
mainly to San Diego County sometime late Monday or Monday night, but
otherwise the upper low will shift east, and we will get a very weak
ridge moving through around Tuesday. Another short-wave will move
through Wednesday, but that will likely not have enough moisture for
precip. A trough deepens over the interior western US late in the
week and could cut off over Arizona/Sonora late in the week and
result in moderate or strong NE/offshore flow here. Temps will be
rather cool Monday, then warm some on Tuesday, but overall temps
should remain a bit below normal most of next week. There could be
local frost in the inland valleys a few nights next week when skies
are clear and winds are light.

&&

.AVIATION...
251625Z...Coast/Valleys...Unrestricted vis and mostly SKC through
26/0300 UTC. 26/0300-0900 UTC...Chance for dense fog/low clouds
developing within 10-15 sm of the coast, with possible impacts to
KSAN, KCRQ and KSNA. Confidence in fog/low cloud occurrence and
timing is low at the moment. Bases and tops would start out around
300-600 ft msl and 600-800 ft msl respectively. 26/0900-1500
UTC...Confidence increases in low cloud occurrence 15-20 sm inland
with bases rising to 800-1200 ft msl and tops rising to 1500-2000 ft
msl.

Mountains/Deserts...Unrestricted vis through Saturday morning with
SKC becoming SCT-FEW clouds AOA 20000 ft msl after 26/1200 UTC.

&&

.MARINE...
Two short-wave troughs of low pressure are expected to move through
the region late Saturday through Sunday, bringing rain and possible
thunderstorms at times. These disturbances will also create strong
west to northwest winds over the coastal waters starting Saturday
evening, and strengthening further Sunday morning (possibly reaching
near Gale-force for a brief time-period Sunday morning). Winds of 15-
25 kt with gusts 30-35 kt will be possible for both the inner and
outer waters. Also, strong northwesterly winds outside our area will
produce a fresh west-northwest swell from 285 degrees which will
combine with locally generated wind waves to create steep and
hazardous sea conditions, with combined seas peaking around 10-14
feet. A Small Craft Advisory will likely be needed for late Saturday
through early Monday.

Winds will gradually weaken Sunday night and seas will diminish late
Sunday night through Monday morning.

&&

.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...Maxwell
AVIATION/MARINE...Harrison