Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

433
FXUS66 KMTR 181707 CCA
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
905 AM PST Sun Feb 18 2018

.SYNOPSIS...Blustery and much colder weather conditions will develop
across the region today in wake of a strong frontal passage. Post
frontal shower will be possible over the Pacific and along the coast
late tonight into Monday morning with snow levels dropping to or
below 1,000 feet. Widespread freezing temperatures are then expected
for inland areas once winds subside Monday night into Tuesday
morning. Cool and unsettled weather is expected to persist through
the end of next week.

&&

.DISCUSSION...as of 09:05 AM PST Sunday...A significant change in
the weather pattern is in the air, with mostly cloudy to overcast
skies and higher relative humidity, breezy to gusty northwest
winds, and a decreasing pressure tend.

This sudden shift is occurring ahead of the arrival a potent
storm system, a 520dm 500mb upper low centered over the Pacific
Northwest. Closer to home, a strong surface cold front ahead of
the parent upper low is moving north to south over the area --
bringing very light rainfall accumulations, low clouds, and breezy
northwest winds. This feature has already moved across the North
Bay and is presently transitioning from the San Francisco Bay
area towards the Monterey Bay region.

In the wake of this front, look for much colder temperature
tonight and through the coming days. Monday night/Tuesday morning
is currently on track to be the coldest period, with record low
temperatures forecast for many areas. A freeze watch is currently
in effect and will likely be upgraded to a freeze warning today.
In addition to the cold temperatures, snow levels are likely to
drop to 1000 feet or lower when the coldest post frontal air
surges into our area beginning tonight/early tomorrow morning.
Latest model guidance suggest snow accumulations are possible over
the highest elevations across the area, but primarily over the
Santa Cruz mountains, particularly along the Skyline corridor, and
over the majority of the Big Sur mountains.

In addition, 700mb temps <-16C, 850mb temps <-7C, 500-1000mb
thicknesses <522dm, a very shallow nocturnal/marine inversion
(0-200 feet), and model output are hints that there could possibly
be snow in the air/sleet as low as 100-200 feet, or even sea
level, around the Monterey Peninsula/Carmel Valley/Big Sur areas.
Any snow/sleet/wintry mixed precipitation could cause significant
travel hazards for those travelling over roadways such as: 35, 9,
17, 1 -- near the 68 exit between Monterey and Carmel and from
Garrapata southward, Laureles Grade, and other higher elevation
passes.

This storm systems main limiting factor is available moisture to
generate precipitation with. Overall, it is a rather dry storm
system, with only very light precipitation accumulations expected
today with the surface frontal boundary and only a scattered
amount of postfrontal rain and showers expected tonight/early
tomorrow. The short-term forecast models tend to agree that nearly
all precipitation will occur within proximity to the coast, both
on and offshore, which explains why other areas farther inland are
not forecast to see much (if any) precipitation despite cold
temperatures.

Models are showing a weaker shortwave trough passing through the
area midweek -- which would introduce additional cloudiness
(leading to warmer overnight temperatures/less risk of widespread
freezing Wed morning) and a slight chance of additional
precipitation for the area. Our main focus is on today/tonights
storm system but this next storm system should be watched closely
as it could bring around round of low elevation snow. See previous
discussion for additional information on the current forecast
package -- a new forecast package with updated information will
be issued early this afternoon.

.PREVIOUS DISCUSSION...as of 03:21 AM PST Sunday...A potent
mid/upper level low pressure system is currently dropping
southward into the Pacific Northwest early this morning. As this
system progresses southward, a strong cold front will push through
the San Francisco Bay Area later this morning and through the
Central Coast by early afternoon. As a result, blustery conditions
are forecast across the entire region this afternoon with daytime
temperatures a good 10 to 20 degrees cooler compared to
yesterday. West- northwest winds will become breezy to locally
gusty, especially along coastal areas where wind gusts of 40 MPH
will be possible. Mainly dry weather conditions are forecast with
the frontal passage, aside from a slight potential for isolated
showers in the coastal hills.

As the upper level trough continues to dig to the south, a very cold
air mass will advect southward across the region late tonight into
Monday morning. The forecast models continue to show post frontal
showers developing over the Pacific and along coastal areas from
late tonight into Monday morning as well. While widespread
precipitation is unlikely, snow levels will fall to or below 1,500
by tonight and potentially as low as 1,000 feet Monday morning.
Thus, the chances for accumulating snowfall have increased for the
Santa Cruz and Santa Lucia Mountains from late this evening into
Monday morning. The latest high-resolution models indicate the
potential for 1-3" in the Santa Cruz Mountains with the highest
elevations picking up nearly 5". Meanwhile, the Santa Lucia
Mountains will likely receive the greatest snowfall where 2-6" will
be more likely with isolated amounts upwards of 8". A dusting to
light snowfall amounts will also be possible over portions of the
North Bay Mountains, East Bay Hill as well as the Diablo and
Gabilan Ranges if showers become more widespread. Light rain
showers are also likely to impact portions of the Central Coast
including the Monterey Peninsula from late this evening into
Monday morning. Given 850MB temperatures forecast to drop below -5
deg C across much of the region, cannot rule out rain/snow mix
even at sea level. Any wintry precipitation will make traveling
extremely hazardous tonight into Monday morning. Thus, folks are
urged to drive with extreme caution if traveling.

Aforementioned breezy conditions are forecast to subside Monday
night into Tuesday morning as a very cold air mass settles in over
the region. Thus, look for widespread freezing temperatures inland
with frost possible all the way to the coast. Forecast temperatures
are expected to fall into the 20s for the coldest inland valleys and
higher elevations with lower 30s likely elsewhere. These conditions
will likely be the coldest the region has experienced in years. A
Freeze Watch remains in effect for inland areas Monday night into
Tuesday morning. While the cold air mass is forecast to remain in
place through midweek, the latest models show another mid/upper
level short- wave dropping southward across the region by
Wednesday morning. This would bring an increase in cloud cover
region-wide and very slight potential for precipitation to the
northern portion of the region. If this does occur, temperatures
would likely moderate enough to prevent another widespread freeze
across the region Tuesday night into Wednesday morning (which was
previously forecast to be the coldest night under a clear sky).
Thus, forecast confidence remains low and folks are encouraged to
stay up-to-date with the latest forecast information.

Medium range models maintain a cool and unsettled pattern over much
of the West Coast through late week and into next weekend. However,
there are difference in the solutions during this time frame with
respect to chances of precipitation and exactly how cool/cold
temperatures will be as weak systems pass over the region. Thus,
will need to closely monitor conditions in the coming days.

&&

.AVIATION...As of 4:57 AM PST Sunday...An approaching cold front
has resulted in the development of local MVFR and IFR cigs along
the coast, over the East Bay and South Bay and into the Salinas
Valley. Elsewhere it`s VFR. The cold front is steeply sloped with
north-northwest winds pushing it southward to the north Central
Coast by 18z this morning. Skies will trend toward VFR and winds
will become gusty from the west-northwest. Both winds today into
Monday as well as a chance of rain and snow showers late tonight
into Monday will be primarily due to increasing boundary layer
instability. The SFO-SAC pressure gradient will reach nearly 4 mb
this afternoon, but the ACV-SFO gradient is forecast to remain
nearly flat today into Monday morning then increasing to nearly 6
mb later Monday afternoon. The coldest air in many years will be
ushered in beginning today, freezing levels will lower substantially
today through Monday.

Vicinity of KSFO...MVFR cig forecast to hold til 17z though latest
satellite imagery shows partial clearing in the vicinity of KSFO.
May need to amend toward scattered clouds depending how the cloud
cover behaves. West winds 20 to 30 knots, possibly a few higher gusts
at times today into this evening.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...IFR cigs this morning lifting to MVFR
cigs later this morning. Gusty west-northwest winds today. Tempo
clearing of cigs, but cigs will return this evening and overnight
leading to instability showers of rain possibly snow toward 12z
Monday and thereafter. The 06z NAM model forecasts the -8C 850 mb
isotherm over the Monterey Peninsula Monday morning, that`s
impressively cold for this area, supportive of snow precip-type
at sea level.

&&

.CLIMATE...

Here are record low temperatures and the years they occurred for
Monday through Wednesday this 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

&&

.MARINE...as of 08:24 AM PST Sunday...Gusty northerly winds will
push a cold front southward over the  California coastal waters
and bays this morning ushering in the  beginning of what will be
the coldest air felt in many years. The wind chill will make it
feel like it`s in the 20s over the coastal waters and bays later
today into Monday night. Instability will  result in gusty
northwest winds and a chance of rain and snow  showers. Gusty
winds will generate fresh swell and steep wind waves. Another cold
front will swiftly move southward over the coastal  waters late
Wednesday night reinforcing the cold air through late  this week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Mry Bay
             SCA...SF Bay from 9 AM

&&

$$

PUBLIC FORECAST: DRP
AVIATION: Canepa
MARINE: Canepa

Visit us at www.weather.gov/sanfrancisco

Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/nwsbayarea
www.twitter.com/nwsbayarea
www.youtube.com/nwsbayarea



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.