Scientific Forecaster Discussion
fxus66 kmtr 181205
Area forecast discussion
National Weather Service San Francisco Bay area
405 am PST Thu Jan 18 2018
Synopsis...a weather system approaching form the northwest will
spread rain across the North Bay this morning and then southward
across the remainder of the forecast area by this evening.
Scattered showers are forecast for Thursday night and Friday as
much colder air settles over the region. A few showers may linger
into early Saturday morning, but for the most part Saturday
should be a dry day. The next front will bring rain chances late
Sunday into Monday.
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 3:54 am PST Thursday...for 12z tafs...widespread
IFR/LIFR cigs & vsbys across the district this morning as an
upper level trough and cold front approach from the northwest. The
trough will introduce some rain and showers this morning. In
fact, ksts is already reporting -ra as of 1130z. Expect rain to
move north to south through the morning, becoming widespread by
this afternoon. Showers will linger this evening and into Friday.
S/SW winds will turn westerly this afternoon.
Vicinity of ksfo...LIFR to vlifr conditions will prevail through
at least 15z, perhaps later, with vsbys down to 1/2sm-2sm and
fog at times. Wet runways by late morning or early afternoon,
with low confidence on exact timing. Light winds.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...LIFR/vlifr expected through the morning.
Ksns still reporting clear skies. Will have to see if low clouds
make it through the Salinas valley later on this morning. Rain
will begin around 22z-00z Thursday afternoon. Light winds.
Marine...as of 02:35 am PST Thursday...a large long period
westerly swell will continue to impact the coastal waters through
the remainder of the week. This swell will bring hazardous seas to
the region as well as large breaking waves to the coastline
through Friday. 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...Sonoma County coast to Monterey
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: west pi
Visit US at www.Weather.Gov/sanfrancisco