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

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FXUS66 KMTR 210642

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1042 PM PST Tue Feb 20 2018

.SYNOPSIS...A cool and unsettled weather pattern is forecast
through this week and into next week as well. A pair of
dry systems are expected this week, both with the potential for a
dusting of snow down to around 2000 feet, and potentially as low
as 1500 feet. Another round of gusty winds is expected on
Thursday. A couple of wetter, more organized storm systems may
impact the region next week.


.DISCUSSION...As of 9:20 PM PST Tuesday...After a cold start to
the day, which included several record low temperatures, afternoon
highs were in the 50s. Today`s highs were anywhere from 5 to 10
degrees cooler than normal.

The persistent upper ridge that was aligned along the West Coast
for much of the first half of February is now well offshore along
150W, while a deep upper trough is centered over the western
CONUS. This synoptic pattern is forecast to persist for at least
the next 10 to 14 days. The models agree that a series of cold
weather systems will drop south along the West Coast, keeping our
temperatures cool and bringing periodic precipitation chances to
our region, along with low snow levels and occasional blustery

Evening IR satellite imagery shows widespread mid and high level
clouds spreading over California from the north. These clouds
will keep our temperatures tonight from dropping as low as they
did last night. So, even though the airmass over our region
remains very cool, temperatures should remain above freezing in
most areas tonight, except in some inland valleys across the
southern portion of our forecast area.

Current cloud cover is associated with a system approaching from
the north. This system`s surface cold front is evident on
satellite, extending offshore from Cape Mendocino at the present
time. The models agree that most precipitation with this system
will remain offshore as the front presses south overnight and into
Wednesday. The best chance of shower activity late tonight and
Wednesday will be across Sonoma County and in coastal areas
elsewhere. Where precipitation does fall, amounts are forecast to
be very light with generally less than a tenth of an inch
expected. Snow levels are forecast to be be between 2000 and 3000
feet through Wednesday.

A second system, currently located off the British Columbia Coast,
is forecast to quickly follow the first and potentially spread
scattered precipitation across our area on Thursday or Thursday
night. Confidence concerning both timing and precipitation
potential with this second system is low given model differences.
The 00Z NAM and 00Z GFS both indicate the highest chances of
precipitation will be on Thursday, but the NAM forecasts more
widespread precip compared to the GFS. The 12Z ECMWF is about 12
hours slower compared to the NCEP models, and doesn`t bring
showers into our area until Thursday night. In any case,
precipitation totals with this second system are anticipated to be
spotty and light for the most part. Most models indicate less
than a tenth of an inch except the NAM which forecasts as much as
a third of an inch over the Santa Lucias on Thursday. The Thursday
system is expected to be colder, with snow levels projected to
drop as low as 1500 feet. In addition, the Thursday system is
expected to generate blustery conditions, mainly near the coast
and in the hills where northwest winds may gust as high as 40 mph.
Winds with this system look similar to the Sunday-Monday system
which would mean that only a few isolated locations will
experience winds of advisory magnitude.

Once the Thursday/Thursday night system moves through we should
see a few days of dry weather late in the week and into the
weekend, along with a slight daytime warming trend. But clearing
skies will mean colder nighttime temperatures late in the week.
Many inland locations are expected to drop down to around
freezing on Thursday night and Friday night.

Longer range models indicate that the unsettled pattern will
continue into next week with at least a couple of systems forecast
to move south down the West Coast. The models agree that the
systems next week will contain more moisture and produce more
widespread and significant rainfall compared to this week`s
systems. However, it should be noted that since yesterday the
forecast tracks of next week`s systems has trended more onshore
with less over water trajectory. If this trend continues, expect
lighter precipitation amounts and lower snow levels next week.


.AVIATION...As of 10:40 PM PST Tuesday...Tricky forecast coming
up as an upper level shortwave approaches the area followed by
another shortwave for Wednesday night/Thursday morning. Models
showing a slight chance of showers in the SFO Bay Area between 12Z
and 18Z with this first system. The challenge will be how low the
cigs will be. Shower activity is usually associated with cumulus
clouds so the forecast is for cigs to remain in the 4000-5000 foot
level but cannot rule out lower cigs. Clouds should scatter out
for Wednesday afternoon but a similar challenge for the cigs is on
deck for Wednesday night.

Vicinity of KSFO...Cigs lowering to 5000 ft after 12Z with a few
showers. There is the possibility cigs could be lower, even into
marginal MVFR range. Clouds scattering out after 18Z.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR with increasing mid and high clouds
after 04Z.



Here are record low temperatures and the years they occurred for

Location          Wednesday

SF Bay Area

Healdsburg        28 in 1929
Santa Rosa        26 in 1913
Calistoga         25 in 2006
Kentfield         29 in 1953
San Rafael        33 in 1955
Napa              29 in 1955
San Francisco     36 in 1890
SFO Airport       34 in 1955
Half Moon Bay     31 in 2006
Oakland Airport   33 in 1955
Richmond          35 in 1981
Livermore         27 in 1955
Moffett Field     32 in 1955
San Jose          25 in 1897
Gilroy            31 in 1976

Monterey Bay Area

Santa Cruz        30 in 1955
Salinas           31 in 1970
Salinas Airport   30 in 1953
Monterey          36 in 1952
King City         23 in 1953

&& of 10:33 PM PST Tuesday...Light to moderate winds
with an improving sea state will continue through early Wednesday.
Northerly increase Wednesday and Thursday as another storm system
moves through through region. The gusty  winds will produce
hazardous conditions with steep wind waves and fresh swell.






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