Scientific Forecaster Discussion
fxus66 kmtr 190615
Area forecast discussion
National Weather Service San Francisco Bay area
1015 PM PST Sat Nov 18 2017
Synopsis...dry weather will continue through the end of the
weekend, with mild days and cool nights. The next weather system
will bring rain chances late Sunday night and Monday, mainly
across the North Bay. There is a slight chance of additional
rainfall at times in the North Bay for much of next week, but
most of our area is forecast to be warm and dry through
Discussion...as of 9:00 PM PST Saturday...skies are mostly clear
this evening except for areas of high clouds - clouds which
helped produce a beautiful sunset earlier this evening.
Temperatures are running close to what they were last evening at
this time, and expect another chilly night with patchy frost in
the interior valleys, especially in the North Bay valleys and the
valleys of southern Monterey County. An early evening forecast
update included decreasing low temperatures for tonight in some
locations and adding patchy frost late tonight and early Sunday.
Little change is expected on Sunday. After a cool start, expect
highs mostly in the 60s, along with periods of high clouds.
Southwesterly flow over the eastern Pacific is directing a plume
of moist air towards the coast of the Pacific northwest this
evening. An upper trough off the coast of British Columbia is
forecast to sag slowly to the southeast through Sunday night and
help drive the moisture plume south into northwest California.
However, at the same time, an upper ridge off the coast of baja is
forecast to build northward. This ridge will block the southward
advancement of the trough and moisture plume. Increased moisture
will make its way as far south as the North Bay by late Sunday
night, and there may be sufficient warm advection coincident with
this moisture to generate rainfall in the North Bay late Sunday
night and Monday. However, the models have been showing a trend
towards more rapid strengthening of the ridge over southern and
central California. Thus, it now appears that only northern Sonoma
County and northern Napa County may see measurable rain on
Monday, with amounts of only a quarter of an inch or less. The
early evening forecast update included a reduction in pops for all
areas late Sunday night through Monday night. Rain chances will
end in all areas by late Monday night.
Our weather for the remainder of next week, including Thanksgiving
day, will be dominated by the upper ridge that is forecast to
strengthen over the southwestern U.S. Through midweek. This will
mean dry and mild weather for most of our forecast area, with
widespread high temperatures in the 70s expected from Tuesday
through Thanksgiving, and even some lower 80s likely across the
southern portion of our area. Meanwhile, moist southwesterly flow
between the upper ridge and a deepening trough offshore, will
continue to produce rainfall across northwest California. There is
a chance that the southern end of this precipitation may graze the
North Bay at times through the week. But the latest longer range
models indicate it's unlikely that even the North Bay will see any
rain from Tuesday through Thursday. According to the 00z GFS, rain
will redevelop in the North Bay late Thursday night into Friday.
But the 12z European model (ecmwf) maintains dry conditions through Friday.
Prospects for more rainfall are somewhat better by next weekend.
Aviation...as of 10:15 PM PST Saturday...the air mass is mainly dry
and stable supporting VFR tonight and Sunday. Few-sct cirrus clouds
tonight then high clouds thicken by late morning Sunday.
Vicinity of ksfo...VFR. Light wind tonight and Sunday.
Sfo bridge approach...VFR.
Monterey Bay terminals...VFR. Wind southeast 5-10 knots tonight and Sunday
morning then light west Sunday afternoon and evening.
Marine...as of 10:13 PM PST Saturday...high pressure off the coast
of central California will continue to generate light northerly winds
through Sunday evening. Winds switch to southerly by late Sunday night
or early Monday ahead of a frontal boundary. Winds over the southern
waters will remain light to moderate out of the northwest. Light to
moderate seas will continue through the weekend.
Public forecast: dykema
Visit US at www.Weather.Gov/sanfrancisco