Rip Current and High Surf in California, December 6 to 7, 2018

Area Forecast Discussion
National Weather Service San Francisco Bay Area
259 PM PST Thu Dec 6 2018

.SYNOPSIS...Fair weather is forecast Friday through early Sunday
as high pressure moves ashore from the eastern Pacific. A
weakening storm system arrives Sunday night into Monday bringing
the possibility of light rain.

&&

.DISCUSSION...as of 03:00 PM PST Thursday...A shift in the
weather pattern is underway this afternoon as a narrow, high
amplitude ridge aligned just offshore of the western CONUS
coastline begins to shift inland over the California/Oregon
border. Meanwhile, the upper low responsible for bringing
unsettled weather to the region yesterday has shifted towards the
southeast to focus on Southern California today (bringing some
significant impacts there, including closing the I5 Grapevine pass
due to winter weather, causing a flight from Oakland to Burbank
to skid on the runway, and more). The 558dm upper low is now
centered between the Channel Islands and Catalina and is forecast
to continue trekking towards the southeast through the day.
Visible satellite imagery shows gradual clearing in the wake of
this feature with a few holdout clouds over Pinnacles National
Park early this afternoon. Diurnal heating and cool air aloft has
enabled the development of weak vertical convection in the form of
cumulus clouds primarily across the southern zones.

Daytime temperatures are running in the upper 50s to mid 60s,
which is near seasonal normals for this calendar date, and
generally 3 to 8 degrees warmer than this same time yesterday.
Offshore winds on the northern flank of the upper low and to the
east of an advancing ridge have taken hold across the Bay Area,
with light to locally breezy N to E winds being reported at most
major airports through the day. Near the coast, winds will shift
to become weak onshore by the late afternoon and evening while
the offshore winds inland will become nearly calm. These
environmental conditions will allow for a stronger nocturnal
inversion and increased likelihood for local areas of fog or
frost to develop. The risk for fog/frost is highest in the North
Bay valleys, particularly to the north and west of Santa Rosa.
Overnight lows will drop into the 40s for areas near a coast or
shoreline, while areas farther inland will see the thermometer
drop in the mid to upper 30s. For the deepest inland locations,
near to slightly below freezing temperature will be briefly
possible around sunrise Friday.

Friday and Saturday will see fair weather conditions as the high
pressure ridge gently comes ashore and moves overhead. The apex of
the ridge will be focused directly over the San Francisco Bay Area
Friday afternoon however only minor day to day variations in the
afternoon temperature is forecast through the next several days.

The forecast models models have generally agreed to back off on
the next storm system in the queue, slated to arrive late Sunday
into early Monday. The energy associated with this feature looks
increasingly disorganized and weaker overall and as a consequence
the main frontal precipitation band is expected to fall apart as
it moves inland over the North Bay. A secondary pulse of light
rain showers may be possible as the base of the parent trough
swings over the extent of the Northern California/Southern Oregon
border, however, the bulk of this shower activity will remain to
our north and east. Otherwise, expect increasing cloud cover ahead
of this system by early Sunday with warmer overnight lows for
Sunday night in response to uptick in moisture across the
atmosphere.

Models continue to be in some alignment by mid week next, showing
the development of an extremely weak ridge late Tuesday into
early Wednesday. A quick moving system will once again skirt
across the Oregon/California border, teasing the North Bay with
slight rain chances. A broader ridge setups later Wednesday
through early Friday but this may just set the stage for the
buildup of a more developed storm system to sweep into California
by next Friday and Saturday. That said, the next best chance for
widespread rain is later next Friday. Ensemble model members
disagree on the strength of this storm system at this time, with
a broad range in estimates. Suffice to say, we will keep a close
watch on this and any other upcoming storm system through the
coming days.

&&

.AVIATION...as of 9:38 AM PST Thursday...High clouds emanating
from the low pressure system currently located off the southern
California coast will continue to stream across Monterey and San
Benito Counties through this afternoon. As this system moves south
and east the high clouds will be pulled farther south. A few
patches of mid level clouds around 3k to 6k linger this morning
but are expected to burn off by this afternoon. Otherwise mostly
clear skies and light winds anticipated through the remainder of
the forecast period. Patchy fog will once again be likely over
KSTS Friday morning.

Vicinity of KSFO...VFR conditions with generally light
northeasterly winds through the morning before turning
northwesterly this afternoon and evening. Light easterly flow will
return overnight with scattered high clouds anticipated.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions with passing high clouds
anticipated through the day. Winds will remain light and variable
through much of the forecast period.

&&

.MARINE...as of 9:00 AM PST Thursday...Generally light to
moderate northerly winds will develop in wake of the exiting
system and persist through late week. A long period west to
northwest swell will also move through the coastal waters today
into Friday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...NONE.

&&

PUBLIC FORECAST: DRP
AVIATION: CW
MARINE: CW

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