Scientific Forecaster Discussion
fxus66 kmtr 122343
Area forecast discussion
National Weather Service San Francisco Bay area
343 PM PST Thu Dec 12 2019
Synopsis...rain chances increase again Friday into Saturday
with temperatures turning colder Sunday into Monday. An unsettled
and wet pattern will likely return by middle of next week.
Discussion...as of 1:50 PM PST Thursday...very mild conditions
across the district with afternoon readings ranging from the upper
50s to mid 60s, warmer than yesterday at this time by as many as
10 degrees. Radar continues to show a few light showers passing
across the Bay area, with Concord, Hayward and navato reporting
drizzle or light showers in the latest surface observations. This
shower activity is expected to decrease overnight.
By late Friday night, an increase in moisture will spread across
northern California for renewed chances of light rain or drizzle.
The North Bay is expected to see the highest chances, then light
rain chances spread into the southern portion of the district by
Saturday morning. A few light showers are also possible on Sunday
as a second weak impulse moves through.
Conditions will dry out and temperatures will turn cooler Sunday
into early next week as a colder air mass moves over the area from
the north. Looking for more widespread 30s and 40s for overnight
lows from Saturday night into Monday night. Will need to watch for
the potential for areas of frost or even freezing temperatures in
the region's coldest interior valleys early next week.
While the longer range ensembles still indicate a return to
unsettled and wet conditions with the potential for more
widespread rainfall across the region during the extended forecast
Aviation...as of 3:40 PM PST Thursday...abundant low-level
moisture continues to keep MVFR cigs over most of the area with
isolated light rain or drizzle. Weak upper level ridging will act
to compress the low-level moisture down to the surface tonight
which will result in IFR cigs and reduced vsbys. Could see dense
fog in the North Bay valleys and around the mry Bay area later
tonight. By late in the period a shortwave will bring deeper
moisture into the area. This should lift the cigs and bring more
widespread light rain.
Vicinity of ksfo...MVFR cigs lowering to IFR after 08z. Bases
currently 1500-2000 ft lowering to 700-900 ft. Isolated showers
possible at any time but more likely by Friday evening. Winds
variable through the period with speeds less than 10 kt.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals... mostly MVFR cigs this afternoon lowering
to IFR after 06z. LIFR with vsbys as low as 1/2 mile between 12z
Marine...as of 03:21 PM PST Thursday...currently, winds are light
out of south-southwest and will begin to shift west and be from
the northwest early this evening. By Friday night, northwest
winds will increase as the next chance for rain increases. The
large, long-period northwest swell will continue to prevail and
impact the waters through Saturday. Hazardous conditions for
small craft vessels will exist with this swell due to steep large
... Surf advisory...caz006-505-509-529-530
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm
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...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Public forecast: Sims
aviation: west pi
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