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Area forecast discussion 
National Weather Service San Francisco Bay area
916 am PDT Sat Aug 29 2015

Synopsis...much cooler weather is forecast for this weekend into
next week. Outside of a slight chance for rain along the Sonoma
coast this morning...dry weather is expected.

&& of 09:10 am PDT Saturday...a frontal boundary
that brought light precipitation to a good portion of the North
Bay this morning continues to push inland. Many areas in this
region picked up a few hundredths of an inch...while drier
conditions persist south of the San Francisco Bay area. Once this
boundary pushes further inland later this morning...chances of
precipitation will diminish. Temperatures region-wide will also be
much cooler today compared to the past few days given the cooler
air mass associated with this frontal passage. Many spots will be
a good 10 to 15 degrees cooler this afternoon. No changes are
needed to the ongoing forecast at this time.

&& of 5:40 am PDT Saturday...radar continues to show a
lack of any radar returns across the North Bay or coastal waters
and with the hrrr now greatly backing off on rainfall decided to
remove mention of rain for most of our area. Did keep a slight
chance for coastal Sonoma County in case some light rain finally
hits that area. For the rest of the County Warning appears that we will
stay dry through the day.


Previous of 3:20 am PDT Saturday...coastal radars
have picked up echoes through the overnight hours although to this
point virtually all have been north of our County Warning Area. Latest run of the
hrrr continues to be in good agreement with previous runs and
brings light rain across the North Bay this morning and
potentially down into the sf Bay area as a cold front moves
across. Will be interesting to see how well the hrrr ends up
doing. Earlier this year we found that in some cases it nailed the
rainfall for the day while other times it would bring rain across
while the ground truth was far drier.

More notable than the potential for light rain will be much
cooler weather this weekend as the marine layer deepens while the
flow at the surface becomes more westerly. At the same time the
strong ridge of high pressure centered near The Four Corners will
progress to the east as the longwave trough advances to the coast.
Inland spots will generally see the biggest drops compared to
yesterday with highs as much as 15 to 20 degrees cooler. Coastal
temperatures will generally be 5 to 10 degrees cooler. The frontal
passage will also trigger breezy conditions with locally gusty
westerly winds.

Synoptically the next 10 days are shaping up with the longwave
pattern fairly stagnant...trough over the west with a ridge to the
east. This will translate to temperatures at or below normal. Any
rainfall is expected to fall north our our region. For what it's
Worth...the 16 day GFS shows dry weather after today for our
entire area.

&& of 4:45 am PDT Saturday...low clouds that came in
earlier this evening is mixing out ahead of an approaching frontal
system as several area metars are currently reporting scattered clouds.
Satellite image shows low clouds associated with the front
just offshore. Thus widespread ceilings will return to the area after
14z but ceilings will be higher at or above 1500 feet. Clearing takes
place by afternoon as the back edge of the front moves through.
Breezy Post-frontal west winds possible in the afternoon.

Vicinity of ksfo...MVFR ceilings by 14z clearing by 21z.
Confidence is moderate.

Ksfo bridge approach...similar to ksfo.

Monterey Bay area terminals...IFR ceilings will lift to MVFR ceilings
after 14-15z. Ceilings are expected to further lift and mix out by
around 18z. Confidence is moderate.

&& of 4:30 am PDT Saturday...a long period west swell
has arrived and will continue to advance on west facing beaches
through the weekend and into early next week. These long period
swells can produce hazardous rip currents that can quickly pull
swimmers out to sea. The highest risk of hazardous conditions will
be this weekend when onshore winds weaken and the period increases
from 15 to 18 seconds. Swimmers caught in a rip current should
swim parallel to the coast to escape the rip currents.

&& of 09:10 am PDT Saturday...for today... a frontal
system will move through the area. Northwest winds will increase
in the southern waters this afternoon and over the northern waters
Sunday behind the front. Additionally... a long period west swell
continues to move through the region into early next week.


Mtr watches/warnings/advisories...
... Pinos to pt Piedras Blancas 0-10 nm from 3 PM



Public forecast: rgass/Bell
aviation: west pi
marine: drp

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