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

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FXUS66 KMTR 060113

Area Forecast Discussion
National Weather Service San Francisco Bay Area
513 PM PST Mon Dec 5 2016

.SYNOPSIS...Look for continued cool conditions over the next
several days with areas of frost possible early Wednesday. A
weather pattern change will occur during the second half of the
week, with periods of rain likely from late Wednesday potentially
through the weekend.


.DISCUSSION...As of 2:00 PM PST Monday...Clouds have been very
stubborn today and are still through a good portion of our CWA. At
the same time higher level clouds are streaming across our region
from the northwest. The combination of clouds and cool NW flow has
kept temperatures on the chilly side today with many spots just in
the 50 to 55 degree range. A few additional degrees of heating can
be expected this afternoon before another round of cool overnight
lows return. This will be followed by even cooler air moving
across for Tuesday into Wednesday morning.

The overall forecast has not changed much from the previous
version. Still appears that the coolest readings will occur
Tuesday night in Wednesday with widespread 30s to lower 40s
forecast. Based off of the various models plus ensemble members,
look like this has less than a 20% chance of temperatures dropping
to the lower 30s in urban locations, so decided to not with any
additional frost or freeze products. Several locations that often
give a good indication for freezing conditions (Paso Robles and
Napa in particular) are not in the 20s suggesting that for the
rest of our area freezing conditions are unlikely.

Our focus will quickly shift to rain as a plume of moist air takes
aim on the coast starting Wednesday evening as a longwave trough
approaches the coast. PW values are forecast to possibly exceed
1.40" at the coast while values over 1.3" are even expected
inland. Compared to climatology those numbers are around the top
1% for the month suggesting that we could end up seeing decent
rainfall amounts.

The biggest challenge for the forecast is the varying solutions
currently displayed for the main location plus timing for the
plume. Right now the bulk of the models bring widespread rainfall
across our region on Thursday starting first in the North Bay then
working its way south through the day. During a portion of the day
rain rates will likely increase and moderate to even locally heavy
rain at time is possible. Rain will diminish Thursday night into
Friday with additional light precipitation expected later on Friday
through the weekend.

Current rainfall amounts have locally more than 3" over the Santa
Cruz Mountains with most urban locations and the bulk of the North
Bay in the 1-2" range. Lesser amounts can be expected for far
southern parts of the CWA plus San Jose to around Mountain View.

The storm track will shift back toward the north next week as a
ridge of high pressure begins to returns. Temperatures will slowly
moderate back to normal with dry conditions favored and most of
the rain confined to the North Bay.


.AVIATION...As of 5:12 PM PST Monday...A mix of surface to lower
level post cold frontal cooling, lower level inversions based
between 2500-3000 feet, and somewhat elevated surface dewpoint
temperatures presently in the upper 30s to 40s will likely cause
additional areas of MVFR cigs and possibly local IFR in fog to
form into Tuesday morning. Mid-high clouds are streaming in from
the W-NW with a hint of warm frontal inversion on 00z Oak sounding
at 14 thousand feet. A complex pattern, no doubt about it.
Meanwhile colder NW flow is now entering far NW California and
best guess is it arrives by early Tuesday morning over the North
Bay and approx mid to maybe late morning North Central Coast. Low
confidence winds forecast later tonight into Tuesday morning.

Vicinity of KSFO...W-NW winds 5-10 knots this evening. MVFR/VFR.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Light winds this evening, MVFR/VFR.

&& of 3:42 PM PST Monday...Moderate to locally strong
northerly winds will prevail along the central coast through
midweek. The gusty winds will produce hazardous conditions with
steep wind waves. A moderate northwesterly swell with a moderate
period 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...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm




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