Scientific Forecaster Discussion
fxus66 kmtr 181739
Area forecast discussion
National Weather Service San Francisco Bay area
939 am PST Thu Jan 18 2018
Synopsis...a cold front approaching from the eastern Pacific will
continue to bring rain to the North Bay early this morning and
spread southward across the rest of the region by late this
afternoon. Scattered showers will follow in the wake of the
frontal passage tonight and tomorrow as colder air settles aloft.
A few lingering showers may remain in the southern portion of the
region Saturday morning, otherwise dry conditions can be expected.
The next system is set to move in late Sunday and into Monday.
Discussion...as of 8:16 am PST Thursday...current satellite
imagery and surface analysis show the cold front moving into
northern California. Rainfall amounts overnight ranged from a few
hundredths to just over a quarter of an inch, mainly in the North
Bay. The highest rainfall amount was 0.31 inches in Monte Rio.
Temperatures this morning near normal in the upper 40s to low 50s.
Models still indicate chances of more steady light rainfall later
this morning across the North Bay, reaching the sf Bay area by
around 18-19z before spreading southward to the Monterey Bay
region late this afternoon. Once the cold front passes tonight
into tomorrow, showers will become more scattered and 850mb
temperatures will drop to around -2 degrees c. The resulting
instability behind the front will bring a slight chance of
thunderstorms late tonight and into tomorrow morning, mainly over
the coastal waters and coastal areas of the North Bay.
Forecast challenge for the day will be snow levels and a possible
light dusting of snow this afternoon/evening over higher
elevations in Big Sur. No major updates to the forecast planned at
this time. For further details please see the previous
Previous discussion...as of 3:15 am PST Thursday...latest
satellite imagery shows a cold frontal boundary approaching the
northern California coast this morning. Models forecast light rain
to begin spreading into the North Bay this morning then reach the
northern San Francisco Bay area by late morning then across the
rest of the sf Bay area by mid-afternoon and then into the
Monterey Bay area by late afternoon. Rain will turn to scattered
showers Thursday night and Friday after the front passes. The best
chance for showers will be Friday morning when the upper trough
is moving southeast across the coastal waters towards Southern
California. The flow will be northwest by then so shower chances
are a little higher along the coast.
Colder air will settle into the area with 850 mb temps dropping
from about 12 degrees c currently to -2 degrees c by Friday
morning. Cold air instability will bring a slight chance of
thunderstorms along the North Bay coast and the coastal waters
where the trough will be passing through. Snow level will drop as
low as 3500 feet by Friday but by then there will only be scattered
light showers so only a dusting of snow is expected. Rainfall
amounts from this system will range from 1/2 inch in the North
Bay...1/4 inch around the San Francisco Bay area...and 1/10 inch
from Monterey Bay area south. Up to 3/4 inch is possible in the
Lingering showers Friday night should remain over the waters.
Clearing skies will lead to a cold night with lows dropping to
the mid to upper 30s in the inland valleys.
Another system from the northwest will bring a chance of rain to
the North Bay Sunday afternoon and the San Francisco Bay area
Sunday night. The front washes out as it moves southeast with rain
only getting as far as the Monterey Bay area Monday morning.
Rainfall amounts could be locally moderate in the North Bay but
light elsewhere. The system is fast-moving so Hydro issues are
Medium range models show another fast-moving system moving through
around the middle of next week. But after that an upper level
ridge will build off the coast leading to an extended dry period.
Aviation...as of 9:39 am PST Thursday...for 18z tafs. Mixed bag
of IFR to VFR conditions observed across the district this
morning. Rain showers are expected to develop from north to south
through the day and will maintain scattered to overcast low clouds at the
terminals with MVFR or IFR ceilings.
Vicinity of ksfo...MVFR conditions have persisted the last several
hours at ksfo. As more rain showers develop through the day,
expect scattered to overcast low clouds to continue in/around the terminal.
Confidence remains low on rain shower timing, and medium for the
persistent cloud cover.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR conditions expected for the next
several hours before low clouds make it back to kmry and ksns.
Scattered rain showers expected by the mid/late afternoon.
Marine...as of 09:27 am PST Thursday...a long period west swell
will continue to move through the waters today. This swell will
contribute to hazards seas throughout the coastal waters, large
breaking waves along the shoreline, and high surf. A cold front
will pass over the waters later this morning, switching winds out
of the west or northwest by this afternoon or evening. Rain
chances will increase through the morning and will continue
through the weekend.
... Surf warning...caz006-505-509-529-530
Small Craft Advisory...mry Bay
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...rough bar advisory for sf bar
Public forecast: as/west pi
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