Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
FXUS66 KMTR 050559
Area Forecast Discussion
National Weather Service San Francisco Bay Area
959 PM PST Sun Dec 4 2016
.SYNOPSIS...A mainly dry cold front will advance southward across
northern California tonight ushering in steadily colder air Monday
and Tuesday. The coldest air of this season so far will arrive
resulting in a period of unseasonably cold temperatures from early
to mid-week. By Wednesday morning, depending on whether or not
there is high cloud cover, a few locations in the Bay Area may
have minimum temperatures within a few degrees of tying their
records. As for specifics for possible frost/freeze headlines this
will be determined within the next day or two. For the latter part
of this week the forecast models are predicting a return to a more
moist Pacific flow with light to moderate rain possible Thursday
.DISCUSSION...as of 9:00 PM PST Sunday...Cloud cover is steadily
increasing ahead of an approaching cold front this evening.
Cooling and lift with the frontal boundary could produce spotty
very light rain or drizzle overnight, however it`s not enough to
warrant an update to the grids and zones at this time.
If you`re a fan of colder weather we`re looking at a fairly solid
surge for early to mid-week with cold air advection strongest from
the surface to lower levels Monday with another, deeper layered
surge on its heels Tuesday. Per recent NAEFS and GEFS model output
air temps Monday and Tuesday will be -1 to just shy of -2 standard
deviations from the 1000 mb level up through the 850 mb level.
NW-N winds particularly over the area hills and mountains later
Monday into Tuesday could be breezy to gusty as cold air advection
and a NW-SE oriented jet stream over NorCal result in large scale
sinking air from as high as approx 500-300 mb; right exit region
convergent jet stream winds will be over the forecast area. The
magnitude of cold air advection will temporarily cancel out the
compensating warming effect normally via adiabatic compression in
sinking air leading to a window of time Monday night and Tuesday
when mid-level subsidence inversions will be mostly absent allowing
for extensive vertical mixing and increasing chances winds momentum
transfer down to near the surface. Freezing levels will lower to
4-5 thousand feet Tuesday. If planning to be outdoors especially
if hiking and/or camping in the highest terrain of the forecast
area prepare for a pretty good winter-like chill. Winds should be
below wind advisory based on most recent NAM forecasting 850 mb
winds 20-30 knots Tuesday morning. If valley winds happen to de-
couple there could be localized low level wind shear Tuesday
Forecasting min temperatures Tuesday morning as well as Wednesday
morning will be challenging for a variety of meteorological reasons.
Mixing winds possibly holding temperatures up may be a factor Monday
night-Tuesday morning. Wednesday morning`s winds will likely be much
lighter or calm with what looks like a pretty good chance there`ll
be areas of near freezing to sub-freezing temperatures, coldest
away from the waters. For Wednesday, ECMX MOS guidance is within 3
deg of the record minimum temps for Oakland Airport (record is 32
in 2013) and Moffett Field (record is 34 in 2013). San Francisco`s
record min is 38 in 1972 and San Jose 29 in 1896. As for specifics
for possible frost/freeze headlines this will be determined within
the next 24-48 hours.
PREVIOUS DISCUSSION...Brief ridging will return on Wednesday with
highs in the 50s as high clouds begin to increase ahead of a warm
Confidence is increasing for warm advection rains to develop later
Thursday into Friday as a pacific jet approaches the region.
Early qpf numbers from the RFC shows 1-2.5 inches of rain
potential but at this time the best chances look to be focused
from the north bay southward to the Santa Cruz Mtns. Still a lot
of time to sort out timing and rainfall totals with this event but
the ecmwf has been consistent for at least the last four model
runs. With no strong cold advection not expecting heavy rain rates
but rather a prolonged period of soaking light to moderate rains
sometime later Thursday into Friday.
Low confidence beyond that with some type of break likely Friday
night into Saturday but then the models diverging on possible
continued systems later next weekend or early next week. Either
way no blocking ridges in sight and a nice moderately active start
to the rainy season.
.AVIATION...As of 9:48 PM PST Sunday...High clouds will continue
to stream across the region with cigs lowering to around 600-1000
feet overnight. A moist boundary layer will result in patchy low
cloud developing across the region overnight. Light and variable
winds expected through the overnight hours. Moderate confidence.
Vicinity of KSFO...Cigs expected to low to MVFR over area
terminals around 08z. Moderate west winds between 5 to 15 kt
expected through much of the forecast period. Strongest winds will
persist over KSFO. Moderate confidence with respect to the return
of the low clouds.
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals...IFR cigs expected through tonight. Cigs
expected to scatter out around 16z-17z Monday morning. Light and
variable winds will become westerly around 5 to 10 kt Monday.
.MARINE...as of 8:53 PM PST Sunday...Moderate northwesterly winds
to continue through midweek especially over the outer water as a
series of storm systems approach Northern California. A
northwesterly swell of 8 to 10 feet at 14 seconds will continue
through at least Tuesday.
.Tngt...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...Pigeon Pt to Pt Piedras Blancas 10-60 nm
SCA...Pt Arena to Pigeon Pt 10-60 nm
SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
SCA...Rough Bar Advisory for SF Bar until 3 AM
PUBLIC FORECAST: Canepa/RWW
Visit us at www.weather.gov/sanfrancisco
Follow us on Facebook and twitter at: