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Area forecast discussion 
National Weather Service San Francisco Bay area
927 am PDT Tuesday Oct 21 2014

Synopsis...dry weather with seasonable temperatures are expected
both today and Wednesday. The next storm will bring a chance of
rain to the North Bay Thursday. Another system will bring a good
chance of rain to most of the district Friday or Saturday.

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Discussion...as of 09:21 am PDT Tuesday...the ongoing forecast
remains on track for today. Made only a few minor updates to
reflect current trends across the region. For more details...see
the forecast discussion below.

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Previous discussion...as of 3:00 am PDT Tuesday...generally clear
start to the day with fairly light winds and drier air has lead to
a cooler start. Temperatures have dropped into the 40s to lower 50s in
many spots (which is 6 to 12 degrees cooler than Monday morning at
this time). Highs are forecast to be close to what we saw
yesterday -- middle 60s to lower 70s in most locations. Similar
weather is forecast for Wednesday although with a slight increase
in 500 mb heights, highs should warm a few degrees.

By Thursday a shortwave trough associated with an area of low
pressure over the Gulf of Alaska will approach norcal. Rainfall
will generally stay to the north of our area, although some rain
could make it down into the North Bay on Thursday. Amounts should
be light although locally 1/3" is possible over the North Bay
mountains. Almost all of the guidance has rainfall amounts
dropping off to near zero down to around San Francisco, so
expectations are it will be very limited in area. Worth noting
that the Gem solution remains the most bullish with rainfall
amounts and coverage (even bringing some down to Monterey bay).
This is a big outlier from the other models (including how the
end of the NAM is trending by Friday morning), so will discount
it for now.

Things get more interesting for Friday and Saturday as the main
cold front associated with the low moves to the coast. It has
been fun to watch the evolution of the models with this system. A
few days ago, all indications were the rain would stay to our
north and the front would fall apart near our area. Run after run
has gotten wetter and now all of the main operational models show
a fairly impressive band of moisture moving through our area some
time between Friday night and Saturday. The European model (ecmwf) which had been
the driest of the solutions is now notably wetter than even just
24 hours ago. It brings 1/3"-1/2" around sf with locally more
than 2/3" over the North Bay. GFS is showing similar numbers
while the Gem brings 1-2" to the North Bay with around 1/2" to sf
and Monterey Bay. Still a few days out, so would like to see some
of the higher resolution models before advertising healthy
rainfall amounts. However, probability of precipitation were increased due to the better
model agreement. Winds could also be gusty ahead of the front
with 20-30 miles per hour possible especially for SW facing spots near the
coast.

Quiet weather returns for Sunday through at least Tuesday as the
storm track shifts back to the north.

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Aviation...as of 04:30 am PDT Tuesday...mostly clear skies this
morning in the wake of the cold front yesterday. Patchy high
clouds to move through later today. Light winds continue through
the day. High confidence.

Vicinity of ksfo...VFR through the forecast period. Mostly light
winds this morning...increasing to around 15 knots out of the west in
the afternoon.

Ksfo bridge approach...similar as ksfo.

Monterey Bay area terminals...VFR. Mostly light winds...offshore
in the morning before switching onshore in the afternoon.

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Marine...as of 09:21 am PDT Tuesday...northwest winds over the
coastal waters will increase today as high pressure builds over
the eastern Pacific. The strongest winds will be near the coast
south of Point Sur. Large moderate period northwest swell will
continue to move through the waters today before decreasing
tonight.

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Mtr watches/warnings/advisories...
... Bar advisory for sf bar
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm

&&

$$

Public forecast: rgass
aviation: ac
marine: Johnson

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