Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus66 kmtr 180537 

Area forecast discussion
National Weather Service San Francisco Bay area
937 PM PST Sat Feb 17 2018

Synopsis...blustery and much colder weather conditions will
develop on Sunday as a strong cold front sweeps through the
region. There is also a chance of showers from late Sunday into
Monday, mainly near the coast and in the Santa Cruz and Santa
Lucia mountains. Snow levels are forecast to drop as low as 1,000
feet and locally lower by Monday. Widespread freezing
temperatures are expected for inland areas once winds subside on
Monday night. Cool and unsettled weather is expected to persist
through the end of next week.

&& of 9:10 PM PST Saturday...warming that began
yesterday continued today. Afternoon highs today ranged from the
mid 60s to mid 70s and were anywhere from 5 to 10 degrees warmer
than normal. Today will be the last warm day our region will
likely see for quite some time as a major pattern change gets
underway tomorrow.

Evening satellite imagery shows a cold weather system over the
Pacific northwest with the leading edge of the cold air just
beginning to pass south of the California/or border at this time. This
system is forecast to continue plunging south overnight and a cold
front will sweep from north to south across our region on Sunday
morning. Much cooler and blustery conditions will develop behind
the front on Sunday. High temperatures on Sunday are generally
forecast to be from 10 to 20 degrees cooler than today. By late
afternoon northwest winds will likely gust as high as 40 mph along
the coast. The cold front is expected to be dry for the most
part, with only isolated light showers possible in the coastal
hills on Sunday.

As the cold upper trough continues to dig south on Sunday night,
the models indicate shower chances will increase, primarily in
coastal areas and in the Santa Cruz and Santa Lucia mountains.
Precipitation amounts are forecast to be light and rather
insignificant, but only if considering liquid precipitation. Snow
levels are forecast to fall as low as 1500 feet by late Sunday
night and then down to 1000 feet, or locally lower, by Monday as
one of the coldest air masses our region has seen in years sweeps
over our area. The NAM continues to forecast more precipitation
compared to the GFS or European model (ecmwf). The 00z NAM forecasts precipitation
totals as high as 0.40" in the northern Santa Lucia mountains of
Monterey County from Sunday night through Monday. This 0.40" of
liquid precip would translate into as much as 5 inches of snow
above 1500 feet in the Santa lucias. In the Santa Cruz Mountains
most models, even the NAM, forecast only about a tenth of an inch
of precip or less. The exception is the 12z local WRF model which
forecasts as much as 0.80" of precipitation in the Santa Cruz
Mountains on Sunday night and Monday. This much precip would
translate into several inches of snow at higher elevations and
with the projected snow levels we could easily see accumulating
snow at The Summit of Highway 17 and along Skyline Boulevard
(highway 35). Because the WRF is an outlier with much more precip
compared to other models, forecast confidence is too low to issue
a Winter Weather Advisory for the Santa Cruz Mountains at this
time. But this is something we will need to watch closely. We will
also need to consider winter weather advisories for the Santa
Lucia mountains. All other mountain zones are expected to see
little or no accumulating snowfall with this system. Rain and snow
showers are expected to end in all areas by Monday evening.

Northwest winds are forecast to remain locally strong and gusty
through Sunday night and Monday, especially near the coast. At
this time it appears winds will mostly remain below advisory
levels except for a few of the windier locations such as Point
Reyes. A Wind Advisory is not anticipated at this time.

Besides snow potential and winds, the other primary impact from
this system will be freezing temperatures. Overnight lows on
Sunday night are expected to remain above freezing in most areas
due to brisk winds and good mixing. By Monday night winds are
forecast to subside and skies will clear. This will set the stage
for the coldest night of this winter season, and widespread
freezing temperatures are expected across inland areas late Monday
night and Tuesday morning. Overnight lows on Monday night are
forecast to range from the mid 20s to lower 30s inland, and from
the mid 30s to lower 40s at the coast. A freeze watch has been
issued for all inland areas of our forecast area from late Monday
night into Tuesday morning. Several hours of subfreezing
temperatures are possible. Once the freeze watch is transitioned
to a freeze warning, we may also need to issue frost advisories
for some of our coastal zones.

Earlier it had appeared that Tuesday night could be as cold as
Monday night. However, recent models now show a system dropping in
from the north by late Tuesday night. Widespread cloud cover
associated with this system will help keep minimum temperatures up
above freezing in most areas on Tuesday night, except perhaps in
the far southern portion of our area.

Longer range models maintain a cool and unsettled pattern through
the rest of the week and into next weekend.

&& of 9:30 PM PST Saturday...low clouds are developing
along the coast ahead of an upper trough. MVFR cigs will spread
over the sfo and mry Bay area overnight with possible IFR for
cigs and vsby at sts. Increasing northwest winds will help to
scatter out the clouds by midmorning.

Vicinity of ksfo...MVFR after 08z. West winds to 15 kt through

Sfo bridge approach...similar to ksfo.

Monterey Bay terminals...MVFR spreading into sns after 09z.



Here are record low temperatures and the years they occurred for
Monday through Wednesday next week.

Location Monday Tuesday Wednesday

Sf Bay area

Healdsburg 28 in 1932 22 in 1922 28 in 1929
Santa Rosa 26 in 1932 26 in 1913 26 in 1913
Calistoga 25 in 1918 25 in 2006 25 in 2006
Kentfield 27 in 1932 27 in 1913 29 in 1953
San Rafael 32 in 1990 32 in 2006 33 in 1955
Napa 29 in 1990 28 in 1920 29 in 1955
San Francisco 38 in 1897 38 in 1897 36 in 1890
sfo Airport 36 in 1990 37 in 2011 34 in 1955
Half Moon Bay 34 in 1954 32 in 2013 31 in 2006
Oakland Airport 35 in 1955 34 in 2006 33 in 1955
Richmond 35 in 1990 35 in 1990 35 in 1981
Livermore 27 in 1956 28 in 1942 27 in 1955
Moffett field 35 in 1949 34 in 2006 32 in 1955
San Jose 32 in 1990 30 in 1897 25 in 1897
Gilroy 27 in 1964 30 in 1990 31 in 1976

Monterey Bay area

Santa Cruz 30 in 1990 29 in 1913 30 in 1955
Salinas 29 in 1990 31 in 2006 31 in 1970
Salinas Airport 29 in 1933 28 in 1953 30 in 1953
Monterey 35 in 1990 35 in 2006 36 in 1952
King City 25 in 1955 22 in 1953 23 in 1953

&& of 09:27 PM PST Saturday...low pressure dropping
south along the West Coast will result in increasing north to
northwest winds. Small craft advisories will transition into gale
warnings Monday night after a cold front pushes through. Winds
will slowly decrease Tuesday and Wednesday.


Mtr watches/warnings/advisories...
... Bay
Small Craft Arena to pt Reyes 0-10 nm
Small Craft Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm
Small Craft pinos to pt Piedras Blancas 0-10 nm
Small Craft Arena to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...sf Bay from 9 am



Public forecast: dykema
aviation: west pi
marine: west pi

Visit US at www.Weather.Gov/sanfrancisco

National Weather Service Glossary of Abbreviations