Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
913 am PST Sat Dec 20 2014
Synopsis...scattered showers will continue across our area today
and slowly diminish overnight as high pressure builds over the
eastern Pacific. High surf is expected along the coast with a
possibility of coastal flooding through this afternoon.
Discussion...as of 9:12 am Saturday...the current infrared satellite
satellite image is showing high clouds spilling over a ridge of
high pressure positioned over the eastern Pacific. Kmux Doppler
radar is showing showers associated with these clouds passing
along the coast. The 1200z koak sounding is showing a fairly
saturated environment through at least 8000 feet so any showers
passing by will reach the ground. Not much vertical motion is
present in the atmosphere so shower activity should be fairly
The 1200z gfs40 and nam12 have initialized well with the current
synoptic pattern. The 0000z European model (ecmwf) has also initialized well and
all 3 models remain in good agreement through at least the next 84
hours. The models all suggest that showers will stick around
through this evening with the greatest impact expected over the
North Bay area. With this guidance in mind will most likely bump
probability of precipitation up for the overnight hours since the current forecast package
is a little on the low side.
The models slowly build the east Pacific Ridge and slide it
closer to the coast through the weekend. This will bring an end to
the showers by Sunday evening with dry weather is then expected
through early Wednesday morning. The gfs40 and European model (ecmwf) are coming
into better agreement with a weak trough of low pressure pushing
into northern California Wednesday. This weak trough will bring a
possibility of showers to the North Bay area on Christmas evening. A
slight chance of showers will remain a possibility for the entire
forecast area on Christmas day. Keep in mind that any impact of
showers on Christmas day will be minimal.
Aviation...as of 4:15 am PST Saturday...abundant low level
moisture left over from the past system is bringing low ceilings to
the sfo Bay area. A few showers linger from the South Bay to the
mry Bay area. IFR ceilings expected to lift to MVFR after 17z and VFR
by 20z as the next approaching system provides some mixing. Rain
confined to the North Bay this afternoon and evening. Pressures
remaining stable so southeast winds staying below 5 knots through the
Vicinity of ksfo...IFR ceilings lifting to MVFR after 17-18z and VFR
after 20z. Light southeast winds 5 knots or less through the day.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR conditions with ceilings 4000 feet
or above. Isolated light showers through 16z.
Marine...as of 8:52 am PST Saturday...large long period swells from
a potent Gulf of Alaska storm will continue to impact the coastal
waters through the day. This swell will result in large breaking
waves across the San Francisco bar and offshore reefs and rock
outcroppings as well as hazardous seas offshore. Mariners are
urged to use caution while navigating the coastal waters through
today. Surface high pressure will build off the California coast
tonight into early next week resulting in increasing northwest
winds over the southern waters beginning Sunday.
... Surf advisory...entire coastline through this afternoon.
Coastal Flood Advisory...entire coastline through this afternoon.
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm until 3 PM
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm until 3 PM
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm until 3 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
until 3 PM
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm until 3 PM
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm until 3 PM
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm until 3 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm
until 3 PM
Small Craft Advisory...rough bar advisory for sf bar
Public forecast: Larry
aviation/marine: west pi
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