Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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339
FXUS66 KMTR 190601
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1101 PM PDT Fri Oct 18 2019

.SYNOPSIS...A weather system moving across far northern California
may bring light rain to the North Bay on Saturday morning.
Otherwise, dry conditions will persist through the forecast
period. Temperatures will warm slightly over the weekend, with
more robust warming forecast next week as offshore flow develops.

&&

.DISCUSSION...As of 8:50 PM PDT Friday...Temperatures today
remained slightly below normal as cool northwest flow persisted
across the region. Although temperatures were about the same as
yesterday, most locations felt warmer due to lighter winds.

Evening satellite imagery shows clouds increasing across northwest
California as a fast-moving system approaches from the northwest.
Models indicate that the bulk of the precipitation with this
system will remain to our north. However, the 00Z NAM and the
latest HRRR both forecast light rain reaching as far south as
northern Sonoma County on Saturday morning. A consensus of all
models indicate areas outside the North Bay will remain dry.
Clouds in the northern part of our area should clear by afternoon
as the weather system moves off to the east. Locally breezy
northwest winds will develop in the afternoon, mainly near the
coast.

An upper ridge building over the Eastern Pacific during the
weekend will slowly edge closer to California by Sunday. This
will result in very gradual warming over the weekend. More
significant warming is then expected starting Monday when light
offshore flow is forecast to develop and when the ridge builds
quickly over California. The ridge is then forecast to retreat
slightly offshore by midweek as a trough over western Canada drops
southeast over the northern Rockies and High Plains. Compared to
the ECMWF, the latest GFS digs this trough more sharply to the
west, thus setting up conditions conducive to the formation of a
strong surface high over the Great Basin by Wednesday night, and
potentially resulting in locally strong and gusty offshore winds
in the hills of the North and East Bay Wednesday night into
Thursday morning. However, the ECMWF forecasts a less amplified
upper level pattern and keeps the trough farther to the east, a
solution that would result in less winds in our hills at midweek.
How this pans out will be important from a fire weather
perspective (see Fire Weather Discussion below for details).
In any case, either model solution will result in warm and very
dry conditions persisting across our region through next week.

&&

.AVIATION...as of 10:55 PM PDT Friday...Moisture boundary that
brought clouds to the coast this morning has cleared out but there
are still a few clouds off the San Mateo coast that could spread
south into the MRY Bay Area late tonight. Models indicate in
increase in low level moisture Saturday morning after 15Z so there
is the possibility of cigs mid to late morning Saturday.

Vicinity of KSFO...VFR through Saturday morning. Models indicate
an increase in low level moisture after 15-16Z which could bring
cigs at or below 5000 ft.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Still a chance of MVFR cigs spreading
into MRY as early as 11Z otherwise VFR. Both Hi-Res and NAM
models bring in another batch of low level moisture Saturday
morning after 15Z so will have to watch for daytime clouds/low
cigs during the day on Saturday.

&&

.FIRE WEATHER...As of 3:05 PM PDT Friday...Two main fire weather
concerns over the next seven days. The first impact will be
focused tonight over the higher terrain of the Santa Lucia
Mountains. Hi-res WRF model continues to show a localized
offshore flow scenario tonight with N to NE winds 25-35 mph with
poor humidity recovery. The second impact will be focused on next
week. High pressure is forecast to build starting Sunday/Monday
and continue through much of the next week with a warming and
drying trend. More importantly, offshore flow is forecast to
develop on Monday and increase each day before peaking Wednesday
night/Thursday morning. Latest GFS continues to show a robust
15mb+ SFO-WMC...that being said the ECMWF isn`t as strong. None
the less, this situation next week needs watching. Even if the
strength of the wind doesn`t verify RH will be low by night number
three.  Stay tuned....

&&

.MARINE...as of 10:44 PM PDT Friday...High pressure off the
central California coast will keep gusty northwest winds over the
southern waters through Saturday. Winds will be lighter over the
northern waters. A frontal system will pass north of the area on
Sunday with high pressure building northward behind the front.
This will increase the northwest winds over the northern waters
early next week. The large northwest  swell will begin to subside
this evening with hazardous seas for  smaller vessels continuing
into this weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 AM
             SCA...Rough Bar Advisory for SF Bar until 3 AM

&&

$$

PUBLIC FORECAST: Dykema
AVIATION: W Pi
MARINE: W Pi
FIRE WEATHER: MM

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