Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
935 PM PST Friday Dec 26 2014
..cold temperatures tonight and early Saturday with widespread frost
across inland locations...
Synopsis...widespread freezing temperatures are expected across
interior Monterey and San Benito counties late tonight into
Saturday morning. Widespread frost is likely elsewhere across
inland areas with patchy frost possible near the coast.
Otherwise...dry weather is expected to persist into the beginning
of next week. A weather system dropping south out of British
Columbia will bring blustery and cool conditions to the region
late Monday night into Tuesday...along with a slight chance of
showers with low snow levels. More freezing overnight low
temperatures are likely once again by the middle of next week.
Discussion...as of 9:05 PM PST Friday...primary forecast concern
in the short-term continues to be cold temperatures late tonight
and into early Saturday. Temperatures this evening are generally
running cooler than they were 24 hours ago and expect that trend
to continue overnight due to lighter winds. In general...expect
overnight lows tonight to be at least a couple of degrees cooler
than last night. This means widespread freezing temperatures
will occur across interior sections of Monterey and San Benito
counties late tonight into Saturday morning. A freeze warning has
been issued for the southern Salinas valley and interior mountains
of Monterey and San Benito counties from midnight through 8 am PST
Saturday. Other inland locations will see temperatures dip into
the upper 20s and lower 30s for a few hours around sunrise
Saturday...but freezing temperatures are not expected to persist
long enough to warrant freeze warnings outside of the warned area.
In any case...widespread frost will occur in most inland areas
late tonight...with patchy frost likely in low lying coastal areas
The airmass over our area will moderate somewhat over the next few
days and so overnight lows should trend slightly warmer on
Saturday night and Sunday night. Even so...there will still
be pockets of early morning frost in the coldest inland valleys.
The current synoptic pattern...and the pattern that is forecast to
persist well into next week...is one in which a high amplitude
ridge is locked in place off the West Coast of North America. This
pattern is what we saw for much of a cold and dry December 2013.
So look for continued dry and cool conditions through at least the
next 7 days.
A shortwave trough forecast to drop south out of British Columbia
late in the weekend is projected to carve out a deep upper trough
over the western Continental U.S. Early next week...forming a cutoff low
over south-central California/southern Nevada by late Tuesday. The GFS has
generally produced little or no precipitation across our area with this
system...while the European model (ecmwf) has forecast some scattered light
quantitative precipitation forecast...mainly on Tuesday and Tuesday night. If showers do occur
they likely won't produce much precipitation given where this system is
originating. However...snow levels will be quite low...as low as
2500 feet...and so any precipitation that does fall will fall as snow in
the higher hills. Any potential precipitation should end by late
Tuesday evening based on latest model data. Therefore...have
removed shower chances from the forecast for Wednesday and
As the upper low develops to our east on Tuesday and then drops
south into Southern California Tuesday night...conditions will be favorable
for strong and gusty north to northeast winds in the
hills...especially in the North Bay hills. The 00z GFS forecasts
925 mb winds of 50 knots across the North Bay from Tuesday through
early Wednesday. A mention of potential strong and gusty winds in
the hills has been added to the hazardous weather outlook for next
The Tuesday system will usher a reinforcing shot of cold air into
our region. Once winds subside we will likely see freezing temperatures
in the inland valleys once again. The most likely time frame for
freezing temperatures will be next Wednesday night (believe winds will
still be too strong on Tuesday night for widespread freezing
The longer range GFS has been showing a pattern shift out around
January 6...with the ridge shifting east and weakening...allowing
moist westerlies to arrive over California by late in the second week of
January. It's too early to have much confidence in this potential
pattern change back to wet weather...and will instead wait for more
consistency and buy-in from the other medium range models.
Aviation...as of 9:35 PM PST Friday...a cool dry airmass will
remain over the region providing clear skies across area
terminals. Light winds will persist through the forecast period.
Vicinity of ksfo...VFR.
Ksfo bridge approach...VFR.
Monterey Bay area terminals...VFR.
Marine...as of 9:00 PM PST Friday...light north winds are
anticipated to veer east to southeast tonight and Saturday. Winds
are forecast to turn northerly and strengthen Sunday night as a
weather system slides south across the state. Moderate north winds
will persist through the first half of the week. Northwest swell
will prevail through the weekend then gradually diminish.
... Warning...southern Salinas valley and interior
mountains of Monterey and San Benito counties.
Public forecast: dykema
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