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Area forecast discussion 
National Weather Service San Francisco Bay area
434 am PST Thursday Dec 25 2014

..much cooler weather on Christmas day and on into the weekend...

Synopsis...clear skies and cool temperatures will prevail across
the district on Christmas day with winds continuing to subside
during the morning. Dry weather is expected to continue through
the weekend and into at least the beginning of next week. Most
locations will see their coldest temperatures so far this season
over the next two nights...with overnight lows forecast to be at
or slightly below freezing in the inland valleys along with some
late night and morning frost.

&&

Discussion...as of 3:35 am PST Christmas day...a cool and dry air
mass has swept across the district in the wake of wednesday's fast
moving cold front. Temperatures around the bays are now generally
in the middle 40s to lower 50s range...a few degrees cooler than at the
same time yesterday morning. Latest Bodega Bay profiler data
however shows temperatures at the 1000 meter level having fallen
from 18 degree c to 6 degree c over the past 24 hours...with reports from
RAWS sites up in the hills indicating even more dramatic decreases
in dew points.

Aside from a few well-protected inland valley locations...boundary
layer winds and associated mixing...along with still a bit of an
onshore component to the surface flow...will keep today from getting
off to too chilly a start. And then the day itself will be
sunny...dry and cool...and locally a bit on the breezy side...with
afternoon highs at lower elevations generally in the middle to upper
50s.

Beginning tonight however...we'll start to experience more of an
impact from the dramatic change in air mass that has occurred...with
winds turning more offshore and further diminishing. Conditions will
then be nearly ideal for strong radiational cooling...especially in
the inland valley locations where overnight lows will drop to around
freezing by dawn on Friday. And thus for the first time this fall
and winter season...frost will occur in some of these areas.

Daytime temperatures will generally recover well back up into the
50s by Friday afternoon...but then lows Friday night into Saturday
morning look to be even colder. Believe there will be some
possibility of frost even in parts of the San Francisco Bay
shoreline forecast zone itself. Saturday itself looks to be very
similar to Friday...sunshine and afternoon temperatures in the
50s.

In the longer range...the high amplitude longwave upper level ridge
that is presently building off the West Coast appears to remain
entrenched. But latest longer range GFS and European model (ecmwf) model output
indicate steepening of the ridge and a series of upper level
disturbances propagating southward from British Columbia down its
front side. The first of these looks to brush through northeastern
California in the Sunday afternoon to evening time frame with
perhaps just a few clouds spilling into our area. But the subsequent
one...around Monday afternoon and evening...appears both potentially
stronger...and with successive model solutions trending farther
westward as it moves southward into California. In fact...just
received 06z deterministic GFS solution even GOES as far as
developing it into a cut-off upper level low right on the central
California coast along with some precipitation over our area around
Tuesday. This is still a long ways out but will need to be watched
as this is the basic set-up for a potential low elevation snow event.

&&

Aviation...as of 4:30 am PST Christmas day...strong northerly
flow over the area will keep VFR conditions today. North to
northwest winds becoming gusty after 18z.

Vicinity of ksfo...VFR. Northwest winds to 10 knots increasing to 15
knots gusting to 20 knots after 18-19z.

Ksfo bridge approach...similar to ksfo.

Monterey Bay area terminals...clouds have cleared from the coastal
hills. VFR.

&&

Marine...as of 4:30 am PST Thursday...high pressure off the West
Coast combined with low pressure over the Great Basin will keep
moderate northerly winds through this evening. Winds will decrease
Friday and Saturday as the low moves east and the high weakens.

&&

Mtr watches/warnings/advisories...
... Pt to pt pinos 0-10 nm until 4 PM
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm
Small Craft Advisory...rough bar advisory for sf bar

&&

$$

Public forecast: blier
aviation/marine: west pi

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