Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
947 PM PST Friday Nov 27 2015
Synopsis...dry and cool weather will continue through the
weekend with areas of nighttime freezing temperatures forecast
across inland areas. Temperatures will begin to moderate early
next week. There is also a chance of rain developing by the middle
portion of next week.
Discussion...as of 8:45 PM PST Friday...north winds brought a
drier airmass into our region last night and today. This drier
airmass allowed temperatures to climb a bit higher today compared to
yesterday and a few locations managed to make it into the lower
60s. Even so...temperatures remained below normal in most areas
even with a full day of sunshine.
The main weather story continues to be cold nighttime
temperatures. Winds persisted longer than expected last night and
these winds helped hold temperatures up...particularly in the North Bay
valleys. But temperatures did drop to freezing or below in most
inland areas from Santa Clara County southward last night. Four
minimum temperature records were set or tied early this morning
(see record event report for more details).
North winds remain locally moderate and gusty in the North Bay
mountains this evening...but winds in the valleys have eased and
so temperatures at 8 PM were running as much as 9 degrees cooler
than last evening in the North Bay valleys. Expect winds to be
lighter in most areas tonight which will likely result in cooler
overnight temperatures. Overnight lows are expected to dip into the middle
and upper 20s in the coldest inland valleys by daybreak Saturday.
Elsewhere overnight lows will mostly be in the low to middle 30s. A
freeze warning is in effect for all inland locations from late
tonight through middle-morning Saturday. A frost advisory is in
effect for coastal areas (except san francisco) during this same
The synoptic pattern is not forecast to change much through the
weekend. A cold cutoff low will remain centered over the Great
Basin while a high amplitude ridge persists just off the West
Coast. The temperature of the airmass aloft is forecast to warm
slightly through the weekend. However...with light winds and low
sun angle there will be no mechanism to scour out the cold airmass
in the low levels and so cold nights will persist through the
weekend. In fact...most model guidance indicates that Saturday
night will be a bit colder than tonight in most inland
locations...probably due to winds becoming light and variable by
then. For example...the NAM MOS guidance forecasts a low of 20
degrees at the Napa Airport early Sunday morning. So it's highly
likely that the freeze warning/frost advisory will be re-issued
for Saturday night/Sunday morning. Slight moderation in temperatures is
expected by Sunday night...but even so we will likely meet freeze
warning criteria for one more night.
The models agree that the high amplitude ridge offshore will begin
to weaken and shift eastward early next week. Previous model runs
had indicated that a weak Pacific frontal system would reach the
West Coast by late Monday and produce at least light precipitation
in our area. But recent model runs...particularly the 00z NAM and
00z GFS...show that system completely washing out offshore prior
to reaching the coast. Have therefore gone ahead and almost
entirely removed rain chances for late Monday and Monday night. We
should at least get an increase in clouds by late Monday and an
influx of warmer air as the flow aloft changes from north to west.
Thus...nighttime temperatures are expected to finally moderate and climb
above freezing in nearly all areas by Monday night.
The medium range models agree in bringing a reasonably robust
storm system into northern and central California by next
Wednesday night or Thursday and this appears to be our next best
shot at rainfall.
Aviation...as of 9:40 PM PST Friday...VFR conds expected to
continue through the night and on Saturday at all terminals as
the air mass remains very dry. Winds will generally be offshore
and light...though locally moderately strong and gusty overnight
in the vicinity of the North Bay hills.
Vicinity of ksfo...VFR. Light winds.
Ksfo bridge approach...VFR. Light winds.
Monterey Bay area terminals...VFR. Light winds.
Marine...as of 9:13 PM PST Friday...generally light winds and seas
will continue through Sunday evening...aside from some locally
gusty offshore winds and choppy seas near the coast from Half Moon
Bay northward late tonight into Saturday morning. Winds and seas
are expected to more broadly increase beginning late Sunday night
as a weak frontal system approaches from the west.
... Advisory...all coastal zones except San
freeze warning...all inland zones
Public forecast: dykema
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