Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1039 PM PST Friday Dec 6 2013
Discussion...as of 9:15 PM PST Friday...a cold front is dropping
south through northern California this evening. Precipitation
began to spread across the San Francisco Bay area in advance of
the front by late afternoon and has become more widespread this
evening. Cold air trapped in some of the valleys just to our north
resulted in some very low snow levels late this afternoon. Snow
was observed in Ukiah in Mendocino County at an elevation of 600
feet and snow was also reported at Angwin in northern Napa County
(elevation 1750 feet). Given these reports of low snow
levels...the decision was made to add the North Bay mountains to
the Winter Weather Advisory. Since that time...it appears that
warm advection ahead of the incoming cold front has managed to
scour out the cold air trapped at lower elevations and thus raise
the snow levels some. In any case...as the colder air arrives with
the front we will likely see snow levels drop back down.
Also...just got a report of snow falling at The Summit of Highway
17 in Santa Cruz County (1870 feet). So will continue with all
snow advisories for the time being. Looks now that most snow
accumulation will be above 2000 feet...but some light accumulation
is possible down to 1500 feet.
Some convective elements have been embedded in the precipitation band
just ahead of the cold front. There was one lightning strike near
Fort Bragg on the Mendocino County coast shortly after 5 PM this
afternoon. The Sonoma County Airport reported a heavy rain shower
at 7:30 PM this evening. No other lightning strikes have been
detected and the 00z NAM has backed off on the amount of
instability for the overnight hours. So will keep thunderstorms
out of the forecast. Periods of brief heavy rain continue to be a
possibility overnight...especially just ahead of the cold front.
Precipitation will taper off quickly once the cold front moves
through. Best estimates for cold frontal passage is between 2 and
4 am in the North Bay...between 4 am and sunrise for the rest of
the sf Bay area...and by middle morning Saturday for Monterey and San
Benito counties. Some locations in the North Bay have picked up
between a quarter and a half inch of rain thus far. Rainfall
totals by tomorrow morning are expected to range from a tenth of
an inch in some of the drier inland valley areas to three-quarters
of an inch in the coastal mountains. Snowfall accumulations above
2000 feet are expected to be in the 2 to 4 inch range...with
locally more in the Santa lucias.
Widely scattered showers may linger across the southern-most
counties into early afternoon on Saturday. But for the most part
all precipitation will end by midday and skies will clear from northwest to southeast.
Brisk northwest winds will develop behind the cold front late
tonight and continue through much of the day tomorrow with local
gusts up to 35 miles per hour likely near the coast.
The airmass behind the incoming cold front is very cold. The 00z
NAM shows 850 mb temperatures dropping down as low as minus 7.5
degree c by late tomorrow. That would be a colder airmass than what
produced our hard freeze the past couple of nights and even a bit
colder than the airmass that descended upon our region during the
freeze of middle-January 2006. Confidence is high that widespread
freezing temperatures will occur late Saturday night into Sunday
morning and a freeze warning has already been issued for our
entire County Warning Area...except San Francisco...from 2 am to 9 am Sunday. The
only question is just how far temperatures might drop. Northwest winds
will continue into Saturday evening but will drop off after midnight.
There will still be some moisture left over in the boundary layer
which might help hold temperatures up a bit. In any case...expect overnight
lows on Saturday night to mostly be in the 20s inland and in the
lower 30s near the coast...more than cold enough for a freeze
Sunday night could potentially be even colder than Saturday night.
The models forecast a dry shortwave trough to drop into northern California on
Sunday...turning the flow aloft from northwest to north and
advecting drier air into the region. Drier air should mean colder
temperatures. However...northerly flow aloft will pick up on Sunday night
and perhaps be strong enough to mix slightly warmer air to near
the surface...especially in the North Bay. Another complicating
factor is a system that is forecast to undercut the high amplitude
ridge offshore on Saturday night and approach the central California coast
on Sunday. The 00z GFS holds that system together long enough to
clip the southern Monterey County coast with some precipitation on Sunday
afternoon. If this were to occur...temperatures might be held up some
across the south on Sunday night due to increased moisture. But
the more likely scenario is what the 00z NAM shows...that the
shortwave dropping in from the north with deflect the system
approaching from the west and maintain a dry airmass over our area.
Will hold off on issuing freeze warnings for Sunday night until
confidence increases concerning the evolving weather pattern.
Daytime temperatures over the weekend will be well below
normal...and range from the middle 40s to lower 50s. Gradual
moderation of temperatures will occur next week. A shortwave ridge
will develop over our area early in the week which will allow
days to warm...but overnight lows in the valleys will likely remain
below freezing all the way through midweek. Increased onshore flow
from midweek on will finally result in moderating nighttime temperatures.
Also...precipitation chances will return by about Thursday as a weakening
system approaches the coast. Based on the 00z GFS...that system
will mostly dissipate by the time it reaches our area.
Aviation...as of 10:38 PM PST Friday...heavier showers and gusty
SW-west winds are moving across the Bay area at this time. The
surface trough and cold front are moving more or less in step with
each other across the area tonight. Still a lot of moisture
hanging around in the North Bay but drier (and colder) air will
soon sweep in from the north. As mentioned in the public
discussion...850 mb temperatures will cool off appreciably late tonight
through Saturday...bottoming out to -6c to -7c Saturday and
Deep northwest wind flow will set up very late tonight and through the
day Saturday. Jet stream winds will approach 130 knots over the
area and Sierra Nevada Saturday. Recent model forecast runs do
not show much in the way of low level northwest winds Saturday. Post
frontal sinking air and development of subsidence inversions
should help limit the amount of vertical momentum Transfer of
winds. MOS guidance for ksfo does show peak wind gusts to 20
knots near 21z Saturday...but other sites in the area do not show
that much wind.
Vicinity of ksfo...SW-W winds will gust to 20-30 knots until the cold
frontal passage. But as mentioned above...ksfo could see some wind
gusts returning Saturday as northwest winds align. VFR late tonight into
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...increasing rain coverage expected
tonight. Gusty winds possible around cold frontal passage.
... Weather advisory...North Bay mountains above 1500 feet.
Winter Weather Advisory...Santa Cruz Mountains and East Bay
hills above 2000 feet.
Winter Weather Advisory...mountains of Monterey and San
Benito counties above 2000 feet.
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm from 1 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 1 am
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 10-60 nm from 1 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm from 1 am
Small Craft Advisory...San Francisco Bay north of the Bay Bridge from 1 am
Small Craft Advisory...San Francisco Bay south of the Bay Bridge from 1 am
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm
Small Craft Advisory...mry Bay from 1 am
Public forecast: dykema
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