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Area forecast discussion 
National Weather Service San Francisco Bay area
852 PM PST Wednesday Dec 17 2014

Synopsis...showers will linger over the region through tonight
before a brief lull in precipitation on Thursday. Our next system
will bring rain back to the region late Thursday through
Friday...yet rainfall amounts are expected to generally be light. A
ridge of high pressure will then build over much of the state and
result in dry...seasonal weather conditions.

&&

Discussion...as of 8:47 PM PST Wednesday...the progressive middle-
latitude westerlies are sending yet another shortwave trough
eastward from the Bay area to the Central Valley this evening. A
line of showers developed during the day and moved over the area
resulting in hourly rain rates upwards of 0.30" to 0.40" over the
North Bay and rain rates generally 0.10" to 0.20" or less
elsewhere. Minor flooding issues remains a risk due to saturated
soils and above normal December rains. More rain is in the
forecast to arrive later Thursday into Friday. No updates are
planned for 1st or 2nd period forecasts as the present forecast
looks good with trends indicating gradually decreasing shower
chances later tonight into Thursday.

A shortwave ridge will be a visitor to the area Thursday providing
a very short term dry break over much of the forecast area. However
over the North Bay it's possible off and on spotty light rain
never really completely ceases as a low level warm air advection
pattern arrives (source for additional light rain development)
followed by the aforementioned ridge beginning to weaken as soon as
Thursday evening. Another weak trough then moves over the entire
area from late Thursday through late Friday with rain totals based
on 00z NAM model upwards of 1" over Sonoma...Marin and Napa
counties...upwards of 1/3" over San Francisco...San Mateo...Santa
Cruz and Santa Clara counties and 1/4" or less over the remainder
of the area. The next chance for dry weather is this weekend when
the eastern Pacific Ridge strengthens and edges closer to norcal.
The European model (ecmwf) and GFS forecasts go with dry weather through about the
middle of next week.

&&

Previous discussion...as of 01:58 PM PST Wednesday...lingering
showers continue to increase in coverage this afternoon as a weak
middle/upper level disturbance approaches the region. This will allow
for isolated to scattered showers to continue through the evening
with generally light rainfall amounts. Additional rainfall amounts
of 0.10" to 0.15" in urban areas with the coastal ranges picking
up as much as 0.25" to 0.50" of rain through tonight. A bit of a
lull in precipitation is expected on Thursday on the back side of
the exiting middle/upper level trough.

Chances for precipitation once again increase Thursday night into
Friday as the next Pacific system approaches the Pacific northwest.
This feature will push another frontal boundary into the North Bay
late Thursday night and then southward during the day on Friday.
Precipitable water values are forecast to be just above the 1 inch mark...yet the
middle/upper level support will not be all that strong. With that
said...only expecting widespread rainfall amounts to be around 0.50"
to 1.00" in the North Bay...generally below 0.50" over the San
Francisco Bay area and 0.25" or less south of the Santa Cruz
Mountains. A few showers may linger over the North Bay through
Saturday...yet a majority of the region is forecast to begin a
drying trend.

By early next week...a ridge of high pressure will build over the
eastern Pacific and bring dry conditions to the region. Daytime
temperatures will also warm slightly under partly cloudy conditions.
The ridge is forecast to remain in place through midweek and will
allow for some drying of the very saturated soils caused by the
much needed recent rainfall. The forecast models then diverge late
next week on the upper level pattern. The European model (ecmwf) and Gem push a
stronger middle/upper level system into the Pacific northwest with a
cold airmass dropping southward. Meanwhile...the GFS does break down
the ridge slightly...yet pushes the much colder air mass down into
the Central Plains. Overall...no major weather concerns anticipated
across the region for Christmas.

&&

Aviation...as of 4:00 PM PST Wednesday...the next band of rain
showers is on the way with borderline MVFR/VFR ceilings anticipated.
Light to moderate rain expected over the area terminals between
02z and 06z this evening. Showers will diminish overnight.
Abundant low level moisture is expected to bring lower ceilings
overnight. Confidence is moderate.

Vicinity of ksfo...borderline VFR/MVFR ceilings with light to moderate
rain expected across the Bay area through 06z tonight. Showers
will taper overnight. Southerly winds will prevail.

Ksfo bridge approach...similar to ksfo.

Monterey Bay area terminals...borderline VFR/MVFR ceilings with light to
moderate rain expected across the Monterey Bay area through 08z
tonight. Showers will taper overnight. Southerly winds will
prevail.

&&

Marine...as of 2:46 PM PST Wednesday...southerly winds are expected
to prevail this evening as a cold front passes over the waters. Winds
are anticipated to be light beginning Friday as high pressure
builds over the area between storm systems. A large...long period
westerly swell will impact the coast Friday afternoon lasting into
the weekend.

&&

Mtr watches/warnings/advisories...
... Pt to pt pinos 0-10 nm until 10 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm until 10 PM
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm until 10 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm until 10 PM
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm until 10 PM
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm until 10 PM
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm until 10 PM
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm until 1 am

&&

$$

Public forecast: canepa/rgass
aviation: CW
marine: Larry/mm

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