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Area forecast discussion 
National Weather Service San Francisco Bay area
239 PM PDT Tuesday Sep 16 2014

Synopsis...a cooling trend will continue through Thursday as an
upper level system impacts the region. Rain showers and a few
thunderstorms will also be possible Wednesday evening through
Thursday afternoon as the system moves inland.

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Discussion...as of 02:30 PM PDT Tuesday...the middle/upper level
trough over the northeastern Pacific continues to deepen this
afternoon as it slows moves toward the West Coast. This system
along with embedded short-waves will increase chances for
precipitation beginning Wednesday evening over the North Bay. In
addition...forecast models have increased the amount of
instability associated with this system. Thus...have introduced a
slight chance of thunderstorms over the North Bay region Wednesday
evening. As the trough moves inland and dips southward Wednesday
night...chances for showers will increase and spread down into the
Santa Cruz Mountains and over coastal areas of Monterey County by
Thursday morning.

Moisture from this system will continue to advect across the
region through Thursday afternoon with precipitable water values reaching as
high as 1.5" Wednesday night into Thursday morning. This will
continue to bring light rain showers and/or drizzle to much of
the region Thursday morning. By Thursday afternoon a few
thunderstorms will once again be possible as the middle/upper low
moves through the region and taps into increased instability. At
this time...the best chance for thunderstorms on Thursday
afternoon will be over the north and East Bay hills/mountains.
Overall rainfall amounts will remain light with a couple tenths of
an inch possible over the North Bay to a few hundredths of an inch
further south. Chances for precipitation will then diminish by
Thursday night as the upper low shift to our south and dry
offshore flow develops over the region.

With the aforementioned system approaching the region...surface
temperatures will be cooler Wednesday and especially Thursday
with increased clouds and cooling in temperatures aloft.
Temperatures will continue generally below seasonal averages
through at least Friday with the upper low forecast to meander
over Southern California. This feature will finally eject
eastward and allow for middle/upper level ridging to build over the
eastern Pacific early next week. Thus...dry weather conditions
will accompany a slight warming trend for the end of the forecast
period.

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Aviation...as of 10:45 am PDT Tuesday...there was very little in
the way of low clouds this morning with only small patches in the
north and easy Bay. Stratus will return tonight as an upper
trough approaches the area but warm airmass will keep stratus
from spreading inland until late.

Vicinity of ksfo...VFR through the Airport evening rush. MVFR
late tonight. Winds will be light becoming northwest 15-18 knots
after 22z.

Ksfo bridge approach...same as ksfo.

Monterey Bay area terminals...VFR through late tonight.

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Marine...as of 02:30 PM PDT Tuesday...a low pressure system
approaching the California coast will bring light southerly winds
to the area beginning Wednesday. Winds will switch to northwest
Friday as the low moves into the southwest. A moderate southwest
swell will continue through at least Thursday.

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Mtr watches/warnings/advisories...
...

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$$

Public forecast: rgass
aviation/marine: west pi

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