Small Craft Advisory 255, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
955 AM PST Fri Dec 28 2018

.SYNOPSIS...Winds will gradually decrease today, but remain
breezy over the ridges. Dry conditions with seasonable highs and
chilly overnight lows will prevail through the weekend and well
into next week. Colder, wind-sheltered valley locations will
likely see overnight low temperatures at or below the freezing,
especially tonight and the first part of the weekend.

&&

.DISCUSSION...as of 9:55 AM PST Friday...Water vapor imagery
shows high pressure off the West Coast and a stationary upper
level low spinning over Northern Arizona this morning. As a
result, dry north northeast flow aloft continues to prevail across
Northern California, with just a few high clouds filtering across
the region at this time. Pressure gradients are continuing to
weaken at this time, with the SFO-ACV down to -4.8 mbs from an
early -6.1 mbs. That being said, we are still seeing some breezy
conditions out there, especially over the ridges.

These gusty winds helped keep temperatures warmer this morning than
we previously thought. Because of the warmer and breezy start to
the day, several sites are running a few degrees warmer than this
time yesterday. However, dew point temperatures are down
significantly, running 10 to 20 degrees lower than this time
yesterday. Therefore, expect temperatures to be similar to or
slightly warmer today, but still remain in seasonal ranges.

The primary concern with this forecast will be with the cold
overnight temperatures tonight. With the very dry airmass in
place, winds becoming light and variable, and clear skies,
temperatures will quickly drop after sunset. With dew points in
the 20s, it is not out of the questions that the colder wind-
sheltered locations will fall into the 20s. Therefore, a Freeze
Watch remains in effect at this time for the Southern Salinas
Valley and surrounding mountains. Will likely update with the
afternoon package. However, may need to expand it to Sonoma and
Napa Counties as well, where the colder valley locations will also
likely see temperatures drop to or below the freezing mark tonight.

Dry conditions will continue to prevail through the weekend and
into next week. Another inside slider will be another round of
gusty north to northeast winds to the region late Sunday into
Monday, and will need to watch to see if additional Wind
Advisories will be needed at that time. Daytime highs will remain
seasonal. Overnight temperatures will remain on the cold side, but
gradually warm a bit each night, as the atmosphere starts to
modify. Palmer

&&

.DISCUSSION...as of 03:20 AM PST Friday...Skies are clear this
morning as a dry northerly flow continues over the region. Gusty
northerly winds continue over the higher terrain with gusts of 20
to 40 mph common above 1000 feet, with the highest peaks gusting
higher. Winds in the lower terrain have subsided from their peak
Thursday afternoon, but remain breezy in some exposed locations,
especially near SF Bay where north to northwesterly winds of 10 to
20 mph have remained common overnight. The airmass is much drier
this morning, with dew points in most locations in the 20s to
lower 30s, a solid 10 to 20 degrees lower than yesterday morning.
Under clear skies, air temperatures have dropped into the 30s in
the wind- sheltered valleys, while temperatures remain in the 40s
to lower 50s where the winds are a bit breezier. Low temperatures
this morning will depend on how much the winds decrease through
the rest of the night. Still think most areas will at least drop
into the 40s, with 30s becoming more widespread in valley
locations as the night progresses.

The surface low to our southeast will continue to move away from
the region while high pressure slowly builds inland, reducing the
pressure gradient over the area. Expect the winds to continue to
gradually subside across the lower elevations this morning. Will
allow the wind advisory for elevations above 1000 feet to expire
at 4 am this morning as winds are expected to slowly decrease.
However, gusty winds will continue throughout the day across the
higher peaks. Locally breezy conditions in the lower elevations
will again occur this afternoon, but will not be as strong as
Thursday.

The cool, dry airmass will persist into Friday night, and winds
will become light across the lower elevations. Under these
conditions, expect Saturday morning to experience the coldest low
temperatures of the forecast period, and likely the coldest of the
season so far. Lows will drop into the 30s in many locations,
including the East and South Bay and much of the coast near
Monterey Bay. Lows in the 20s are expected in portions of southern
Monterey and San Benito Counties, where a Freeze Watch has been
issued. The North Bay valleys will also see temperatures near
freezing, with some isolated upper 20s possible as well. Lows
above 40 will be limited to the SF Peninsula and immediate coastal
areas.

The upper level ridge will flatten on Saturday as a shortwave
rides over the top of it. This shortwave will follow a similar
trajectory to the trough on Thursday, diving south into the Great
Basin on Sunday and Monday. Precipitation along a boundary will
fizzle out before reaching our area, and all precipitation near
the center of the trough will remain well to our east. Northerly
winds will increase again late Sunday and Monday behind the
trough. A reinforcing shot of cool, dry air will also move into
the region, keeping low temperatures cool early next week. A small
warming trend is likely towards the end of next week as the upper
ridge flattens and noses inland. Models depict a trough
approaching the area late next week, but weakening it with
precipitation not expected to make it this far south. In short,
dry weather will prevail until at least next weekend. ST

&&

.AVIATION....As of 09:30 AM PST Friday...For 18z tafs. VFR
conditions through the period for all terminals as a cool and dry
airmass remains in place. Locally gusty northerly winds will
continue this morning and diminish into the afternoon.

Vicinity of KSFO...VFR. Moderate gusty northerly winds will continue
into the early afternoon before gradually diminishing.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Winds expected to remain light
throughout the day.

&&

.MARINE...as of 08:18 AM PST Friday...Gusty northerly winds will
gradually diminish across the waters today and tonight. Winds will
remain light to moderate tomorrow before increasing once again
Saturday night into Sunday as high pressure builds offshore and
low pressure deepens over the Desert Southwest. Light to moderate
northwest swell will continue before a longer period northwest
swell arrives over the weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...SF Bay until 12 PM
             SCA...Mry Bay until 12 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 12 PM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm

&&

PUBLIC FORECAST: Palmer / ST
AVIATION: AS
MARINE: AS

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