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000 
FXUS66 KMTR 171744
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
944 AM PST Sat Feb 17 2018

.SYNOPSIS...High pressure over the eastern Pacific will hold on for 
one more day resulting in seasonably mild temperatures across the 
region. Conditions will turn blustery and much colder on Sunday as a 
strong cold front sweeps through the region. There are also chances
of showers mainly across the Central Coast Sunday night into Monday
with snow levels forecast to drop below 1,500 feet. In wake of the
frontal passage, some of the coldest temperatures the region has 
experienced in years are forecast for early next week.

&&

.DISCUSSION...as of 08:50 AM PST Saturday... Today will be the
last day with near to somewhat above average temperatures
throughout the region. Temperatures this morning are running a few
degrees warmer compared to the same time yesterday. Afternoon
temperatures will be comparable to those observed yesterday, with
highs in the low 60s to mid 70s across the region. The high
pressure ridge responsible for these above average temperatures
will be deflected/retreat offshore and give way to a very cold
520dm 500mb storm system. 500mb heights and temperatures aloft 
will be approximately 3 standard deviations below normal (~00.27%
probability) with this storm system, an indication of its
cool temperatures and strength. 

The main impacts from this approaching storm system will first be
the very cold temperatures, with many areas on track to break 
record cold temperatures, as near to subfreezing temperatures
become widespread Monday night/Tuesday morning and Tuesday
night/Wednesday morning. Next, this trajectory of this system will
bring the cold core directly down the coastline, placing the
coastal waters and coast line on the windy side of this strong
storm system. As a result, sustained winds of 20 to 30 mph with
gusts of 30 to 40 mph will be possible along the coast later, as 
well as over the higher ridges and peaks around the area, during
the day Monday. Finally, while this system is relatively dry,
forecast models have been trending marginally wetter with each
run. 

The bulk of any precipitation is expected to come early Monday 
with the arrival of the cold upper core of the storm system and 
primarily be focused along the coast. This translated to a 
potential for some rare snow, but we are still fine tuning exactly
how much, where, and how low. 

700MB temperatures, 850MB temperatures, and 500-1000MB 
thicknesses are all trending marginally lower with each successive
model run, which translates to lower potential snow levels. Some 
forecast models are evening supporting "snow in the air" down to 
sea level Monday morning, with 700MB temperatures at 
-16C or lower, 850MB temperatures at -7 to -8C, and 500-1000MB 
thicknesses at 522-523dm. Models may be failing to take the 
relatively warm air of the marine layer (generally mid 40s to mid 
50s) that will be present along the coast, so there is still a lot
of fine tuning to do with the forecast as far as potential snow 
goes. The current forecast looks on track, if not a bit 
conservative, on the potential for snow -- with 2-6 inches in Big 
Sur, 1-3 inches in the Santa Cruz mountains, and less than an inch
over the coastal North Bay & inland higher terrain. 

The main forecast challenge of the day is to fine tune these 
winds, colder temperatures, and low snow levels. See previous
forecast discussion for more details on the current forecast
package.

.PREVIOUS DISCUSSION...as of 03:04 AM PST Saturday...Temperatures
across the region this morning are generally running a few 
degrees warmer compared to 24 hours ago. This is mainly due to an 
increase in surface moisture with dewpoint temperatures anywhere 
from 5 to 10 degrees higher than the same time yesterday. Despite 
this, sky conditions remain clear with mainly light and variable 
surface winds. For the rest of today, look for seasonably mild 
temperatures with most areas warming into the 60s and 70s as high 
pressure over the eastern Pacific holds in place for one more day.

Satellite imagery early this morning shows a potent mid/upper level 
system just off of the coast of British Columbia. This system is 
forecast to push into the Pacific Northwest later today with a very 
cold air mass then forecast to drop southward down into northern 
California on Sunday. A strong cold front is expected to sweep 
through the San Francisco Bay Area early Sunday morning and through 
the Central Coast by early afternoon with strong cold air advection. 
Northwesterly winds ahead of the system are forecast to strengthen 
and become breezy to locally gusty late tonight into Sunday 
morning as the system quickly drops down the coast. The strongest 
winds are likely along the coastline and in higher elevations from 
late tonight into Monday afternoon. Wind gusts of around 40 MPH will 
be possible and may potentially reach up to around 45 MPH at times. 
With this said, have held off on posting a Wind Advisory at this 
time given the fact that wind gusts and the duration of the 
strongest winds may only marginally reach criteria. 

Regardless, blustery conditions are likely region-wide in wake of 
the frontal passage on Sunday as temperatures are forecast to drop 
10 to 15 deg F from this afternoon's highs. In addition to the 
drastic drop in temperatures, have added a slight chance of showers 
along the coastal ranges from the Santa Cruz Mountains southward as 
the latest short-range guidance suggest some light precipitation. 
However, widespread rainfall is not likely with the frontal passage. 
The forecast gets even more challenging heading into Sunday night as 
the lastest NAM and locally run WRF have become more aggressive on 
precipitation across portions of the region, especially from the 
Santa Cruz Mountains inland and southward down the coast. This 
combined with 850 MB temperatures falling to around or below -5 deg 
C, snow levels are likely to fall to or below 1,500 feet Sunday 
night into Monday. With this set up, cannot rule out 1 to 3 inches 
of snowfall in portions of the Santa Cruz Mountains and 2 to 6 
inches in the Santa Lucia Mountains. If snow showers do develop and 
become more widespread across this region, cannot rule out 
accumulating snowfall in areas such as the Diablo and Gabilan Ranges 
as well. Precipitation is then forecast to diminish by Monday 
evening/night. 

Another major impact to the region will occur on the nights/mornings 
following the frontal passage and when the winds subside. With 
temperatures aloft forecast to cool to near record values, look for 
widespread freezing temperatures Monday night/Tuesday morning and 
again Tuesday night/Wednesday morning. The ECMWF currently indicates 
Wednesday morning may be the coldest night the region has 
experienced in a while with lows dropping into the low/middle 20s 
for the coldest valley locations and low/middle 30s all the way to 
the coast. With this, a freeze watch/warning will likely be needed 
at some point this weekend.

Temperatures are then forecast to moderate some by late week yet 
will likely stay around or below seasonal averages. This will be the 
result of a broad mid/upper level trough over the western portion of 
the country. Both the GFS and ECMWF show signs of precipitation 
toward the end of the month, yet confidence is not high at this 
time and will need to be monitored in the coming days.

&&

.AVIATION...As of 9:45 AM PST Saturday. VFR today then pre- 
frontal weather conditions become increasingly favorable for 
stratus cigs to return tonight for all tafs. MVFR/IFR cigs 
develop at first then lift to MVFR cigs overnight. MVFR cigs mix 
out to VFR by late Sunday morning as a cold front sweeps south 
over the entire area. W-NW winds will become increasingly gusty 
later Sunday morning through Sunday evening.

Vicinity of KSFO...VFR today. Light wind until mid to late afternoon
today increasing to 10 to 15 kt into this evening. MVFR cigs will
develop by 06z. W-NW wind becomes increasingly gusty through 
Sunday.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR today. MVFR cigs then develop by 
06Z. Light winds, then increasingly gusty from the W-NW on Sunday.


&&

.CLIMATE...

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

&&

.MARINE...as of 09:39 AM PST Saturday...Gusty northerly winds will
push a cold front southward over the  California coastal waters
and bays Sunday morning ushering in  cold air. A combination of
gusty winds and arctic air will result in lowering the wind chill
into the 20s over the coastal waters  and bays from later Sunday
through Monday. The gusty winds will  also result in fresh swell
and steep wind waves.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm

&&

$$

PUBLIC FORECAST: DRP
AVIATION: Sims
MARINE: Sims

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