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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 190158

Area Forecast Discussion
National Weather Service San Francisco Bay Area
558 PM PST Sun Feb 18 2018

.SYNOPSIS...Blustery and much colder weather conditions have 
developed 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.

&& of 2:15 PM PST Sunday...A significant change in 
the weather pattern is in the air, with mostly cloudy to overcast 
skies and higher relative humidity, much cooler temperatures (5 to
15 degrees cooler than this time yesterday), gusty northwest 
winds (peak gusts so far today 55mph at Altamont Pass, 51mph at 
Mount Diablo), a decreasing pressure tend, and some light 
precipitation (generally less than 0.10") earlier today. This 
sudden shift is due to a relatively dry but very cold 521dm 500mb 
upper low descending into California from western Canada. While 
the parent upper low remains situated over Washington state early 
this afternoon, the child surface cold front is sliding southward 
across the area today and is currently passing over the Monterey 
Bay/Big Sur region -- after already passing through the San 
Francisco Bay Area this morning. 

In the wake of this front, much colder temperatures are
anticipated from 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 
including downtown San Francisco (38, a record going back to 
1897). A freeze warning is currently in effect for tonight from 
midnight to 9am for all inland areas (excluding the shorelines) of
the San Francisco Bay area and Monterey Bay area. Please be sure
to prepare your pipes, cover your plants, bring in your pets, and
check to make sure your extended family have enough warmth

In addition to the cold temperatures, snow levels are likely to 
drop to 1000 feet or lower when the coldest post frontal air mass
surges into our area later tonight. Latest model guidance suggest
light 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. Accumulations of 2 to 4 inches are 
possible in the Santa Cruz mountains and 4 to 8 inches in the Big 
Sur mountains, each locally higher with heavier showers. This snow
will be generated by convective snow showers which will create
spotty accumulation amounts, with locally higher and lower amounts
depending on where these showers move. A winter storm advisory is
in effect from midnight tonight to 10am Monday morning for both 
of these locations due to the hazardous travel conditions 
anticipated from any wintry precip tonight/tomorrow morning.

In addition, 700mb temps <-15 to 16C, 850mb temps <-5 to 7C, 
500-1000mb thicknesses 522 to 524dm, a very shallow 
nocturnal/marine inversion (0-200 feet), and some model output 
indicate a very real possibility that there may be snow in the 
air/ mixed with rain as low as 200 feet around the Monterey 
Peninsula/Carmel Valley/Big Sur areas. 

Overall, this storm system will be relatively dry because the
majority of the air mass is continental in nature. The short- 
term forecast models tend to agree that nearly all precipitation 
will occur within proximity to the coast as some moister air is 
entrained into the system from off the coast. This explains why 
other higher elevation areas farther inland are not forecast to 
see much (if any) precipitation despite cold temperatures. 

Models are showing a pair of weaker shortwave troughs 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.  


.AVIATION...As of 5:30 PM PST Sunday...Cold northwest flow 
settling over the area with scattered cumulus clouds. A few 
showers are developing offshore and coming onshore around Monterey
Bay. This should continue through this evening with possibly an 
increase in activity after 10Z from the MRY Bay Area south. Winds 
gusting as high as 30-35 kt through at least 04Z.

Vicinity of KSFO...VFR. West winds 25 kt gusting to 35 kt through
at least 04Z. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...A few passing showers in MRY early
tonight but mainly VFR. MVFR cigs with shower activity increasing
after 10Z.



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

&& of 8:41 AM PST Sunday...Gusty northerly winds will 
push a cold front southward over the California coastal waters and
bays this morning. Winds will increase by tonight resulting in 
gale conditions, especially over the outer waters. 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 gusty winds and hazardous conditions.


     .Tngt...Winter Weather Advisory...CAZ512-517
             SCA...SF Bay
             SCA...Mry Bay
             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
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm




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