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Area forecast discussion 
National Weather Service San Francisco Bay area
518 am PDT Friday Oct 31 2014

Synopsis...a significant storm system will drive a cold front
through central California today...bringing widespread rainfall to
the region. There is also a possibility of thunderstorms developing
this afternoon and evening. Showers will linger into Saturday with
dry conditions returning Sunday through much of next week as high
pressure builds over the region.

&&

Discussion...as of 04:15 am PDT Friday...a potent middle/upper level
trough off the Pacific northwest coast will continue to advance
eastward and drive a cold front through central California today.
Ahead of this boundary...a deep moisture plume with precipitable water values
between 1.00" to 1.25" continues to advect into the West Coast
with a decent rain band already showing up offshore (as seen via
kmux radar). Light rain currently over the North Bay will increase
through sunrise as the front approaches. Rain will then spread
southward and inland across the San Francisco Bay area during the
rush hour commute.

Frontal rains will then transition to showers by late this
morning as stronger middle/upper level dynamics push into central
California. Widespread showers will likely produce moderate to at
times heavy rainfall over the San Francisco Bay area down across
the central coast during the early afternoon. Instability is then
forecast to increase this afternoon/evening and will lead to the
possibility of thunderstorm development. The best support appears
to be south of San Francisco where the strongest short-wave is
prognosticated to push inland...however will keep a slight chance of
thunderstorms for the entire region given the strength of upper
level jet that will sweep across the state. With that said...the
heaviest rainfall amounts are forecast across the Santa Cruz
Mountains southward along the central coast where isolated
locations may pick up nearly 2 inches. Regardless...widespread
rainfall totals across the North Bay will range from 0.25" to
0.50" with 0.50" to 1.00" expected southward.

While the heaviest precipitation will taper off in wake of the
frontal passage this evening...showers will continue through the
night with an unstable air mass in place and abundant moisture
aloft. The models indicate an increase in shower activity Saturday
morning when another short-wave disturbance sweeps across the
region. Showers will then diminish Saturday evening as the
middle/upper level trough exits to the east.

While temperatures remain cool through the remainder of the
weekend...dry conditions return by Sunday. A slow warming trend is
then forecast through at least midweek with building high pressure
aloft. Forecast models then differ late next week with the
GFS/CMC keeping a ridge over the region...while the European model (ecmwf) brings
another Pacific system into the West Coast. For now...will keep
conditions generally dry with a slight chance of rain across the
North Bay.

&&

Aviation...as of 5:00 am PDT Friday...for 12z tafs. A storm
system just offshore of the Pacific coastline will shift onshore
through the day and spread rain and MVFR ceilings across the area.
Southerly winds ahead of the front will shift to the west-
southwest following the frontal passage between 18z and 21z. As
the front moves closer to the coast showers will begin to spread
across the region from north to south and winds will turn south to
southeasterly. Mainly MVFR ceilings expected with the exception of
ksts that will see IFR ceilings through much of the period. Rain is
forecast to begin over the North Bay around 14z this morning with
the heaviest rain coming between 16 and 20z. Wet runways and south
plan expected for the morning rush Friday.

Vicinity of ksfo...MVFR ceilings expected this morning as the front
approaches. Southeast winds will develop overnight with light rain
likely by 14z Friday. Would expect to start the day with southeast
winds and wet runways. Periods of moderate to heavy downpours
possible between 16z and 20z. Wind shift to the SW also
anticipated between 16z and 20z.

Ksfo bridge approach...similar to ksfo.

Monterey Bay area terminals...MVFR ceilings expected this morning as the
front approaches. Southeast winds will develop overnight with
light rain likely by 19z Friday. Periods of moderate to heavy
downpours possible between 18z and 22z. Wind shift to the SW
anticipated in the afternoon.

&&

Marine...as of 04:31 am PDT Friday...light wind and seas are
expected through late tonight. Winds will gradually increase early
Friday morning as a strong cold front approaches the northern
waters. Winds will briefly increase ahead and just behind the
frontal passage as rain moves from north to south through the
waters on Friday along with a slight chance of thunderstorms.
Behind the front lighter northwest winds are forecast Friday evening. On
Saturday stronger northwest winds and building seas to around 10 feet
will develop creating some hazardous seas for the start of crab
season.

&&

Mtr watches/warnings/advisories...
... Pt to pt pinos 0-10 nm until 3 PM
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm until 2 PM
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm until 2 PM
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm until 2 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm until 6 PM
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm from 9 am until 3 PM
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm until 2 PM
Small Craft Advisory...Monterey Bay from 11 am until 6 PM

&&

$$

Public forecast: rgass
aviation/marine: drp/CW

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