Strong Wind in California, December 27 to 28, 2018

Area Forecast Discussion
National Weather Service San Francisco Bay Area
850 AM PST Thu Dec 27 2018

.SYNOPSIS...Dry conditions with seasonable highs and cool
overnight lows will prevail through the weekend and well into
early next week. Breezy conditions are forecast for Thursday and
Thursday night, with locally strong and gusty winds likely in the
hills. The colder valley locations will likely see frost during
the late night and early morning hours, especially late this week
and into the early part of the weekend.

&&

.DISCUSSION...as of 08:45 AM PST Thursday...Morning weather
impacts to the Bay Area are geared toward dense fog and some
patchy low clouds. Livermore and Napa airports reported reduced
visibility of less than one mile due to fog. However, latest
trends and observations do show a gradual improvement. Expect this
trend to continue over the next hour or two with clear skies
developing.

For the rest of the day the weather story will be increasing
winds. Latest observations do show winds already ramping up over
the higher peaks: Mt St Helena 55 mph, Mt Diablo 40 mph, Rose Peak
30 mph. Winds will continue to increase this afternoon and tonight
as an upper low swings through the Great Basin and high pressure
builds behind it. A Wind Advisory is currently in effect for
locations above 1,000 feet. At this point, will leave advisory as
is with early forecast models (WRF/HRRR) keeping strong winds in
place.

No update is planned at this time.

&&

.PREV DISCUSSION...as of 03:35 AM PST Thursday...Shortwave trough
evident on water vapor imagery this morning continues to push
southeast into the Great Basin. Around the area, moisture has
increased in the low levels which has led to stratus formation in
the valleys, mainly from Alameda County south. These clouds have
small vertical depth and no precipitation is expected this
morning. Northwest winds have increased in exposed locations near
the coast and around the SF Bay, where wind speeds of 10 to 15 mph
have been common. Locations experiencing these northwest winds
such as San Francisco, Half Moon Bay, and Monterey, have been slow
to cool off with low to mid 50s as of 3 am. Surface dew points
are generally running up to 10 degrees higher than 24 hours ago.
This has also helped to keep temperatures from dropping very far,
with upper 30s in the coolest inland valleys and 40s elsewhere.

Surface high pressure building to our north, in combination with
a surface low to our southeast, is creating a strong surface
pressure gradient across the region. This gradient will
strengthen through the morning and lead to breezy to gusty
northerly surface winds. Winds will increase during the morning
and will peak in intensity during the afternoon. Winds near the
surface will subside on Thursday evening, but will remain gusty at
times over the higher terrain through Friday. Wind gusts from
20-30 mph are expected in the lower elevations, while in the hills
gusts up to 45 mph are anticipated with up to 55 mph possible
over the higher peaks. A wind advisory is in effect for all
elevations over 1000 feet across the CWA from 7 am Thursday until
4 am Friday. While not included in the wind advisory, gusts up to
40 mph over the lower elevations of northeast Contra Costa county
are also possible.

Cold air advection aloft is occuring this morning, and cooler air
will continue to filter into the area throughout the day behind
the trough. Additionally, the airmass will also become
increasingly dry and winds in the lower elevations will decrease,
setting the stage for cool overnight lows for the next several
nights. Patchy frost will likely develop in the wind- sheltered
valleys. The coldest night is expected to be Friday night, when
temperatures will fall into the 30s in many locations. Near-
freezing temperatures are expected in inland valley locations, and
temperatures could fall as low as the mid to upper 20s across the
southern Salinas valley.

Northerly flow will continue through the weekend as another trough
slides down the eastern side of the eastern Pacific ridge on
Sunday. Precipitation will the trough will fizzle out over
northern California before the remnants reach our area. Look for
continued dry conditions and another shot of increased northerly
surface winds on Sunday and Monday. Dry conditions will continue
into the new year with temperatures expected to warm slightly as
the upper ridge noses inland. Models continue to hint at a system
approaching the West Coast late next week, but continue the trend
of weakening it before it makes it into our area.

&&

.AVIATION....As of 04:05 AM PST Thursday...Ample low cloudiness
formed overnight from the SF Bay Area southward as a band of lower
level moisture moved into the district with a dissipating cold
frontal boundary. Clearing behind it has now progressed
southeastward to almost KSJC and the Monterey Bay terminals, and
expect VFR conditions to prevail across the area by around
midmorning as drier northwesterly winds develop. Only issue then
through the remainder of the day and evening is that these winds
look to become moderately strong and gusty in the afternoon.

Vicinity of KSFO...VFR. Northwest winds increasing to around 20
kt with gusts to 25 kt in the afternoon.

SFO Bridge Approach...MVFR ceilings far south end of approach
should clear within the next hour or so, otherwise VFR and similar
to KSFO.

Monterey Bay Terminals...MVFR ceilings expected to clear by
15-16Z then VFR through the remainder of the day and evening.
Locally gusty onshore winds during the afternoon.

&&

.MARINE...as of 02:51 AM PST Thursday...Surface high pressure
building inland to our north in combination with a low pressure
system moving through the Desert Southwest are resulting in strong
and gusty north to northwesterly winds over our waters. Expect
near gale to gale conditions today, then winds slowly decreasing
tonight and Friday. Winds could increase again Sunday and Monday
as the next system dives down through the Great Basin.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Wind Advisory...All elevations above 1000 ft
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Mry Bay from 12 PM
             SCA...SF Bay from 9 AM

&&

PUBLIC FORECAST: MM/ST
AVIATION: Blier
MARINE: Blier

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