Scientific Forecaster Discussion
fxus66 kmtr 211735
Area forecast discussion
National Weather Service San Francisco Bay area
1035 am PDT Sat Oct 21 2017
Synopsis...a warming trend will begin over the weekend and
continue into early next week. Much warmer and drier conditions
are expected to develop by Monday and Tuesday. Slight cooling is
likely during the second half of the week, but temperatures will
remain well above seasonal averages.
Discussion...as of 08:05 am PDT Saturday...a cool start to the
morning with widespread temperatures being reported in the 40s
with a few middle/upper 30s in parts of the North Bay valleys.
Otherwise, mainly clear skies prevail region-wide with a few to
scattered high clouds streaming inland over the region. The
forecast remains on track for a slight warming trend this
afternoon before a more robust warm-up gets underway Sunday
through Tuesday as high pressure builds over the region. Please
see the previous forecast discussion below for additional details
as no major forecast updates are needed at this time.
Previous discussion...as of 3:30 am PDT Saturday...quiet night
across our region today with cool temperatures (mostly in the 40s
to the lower 50s) that are running 5 to 10 degrees cooler than
this time yesterday. Drier air is also in place with dew points in
the 40s. Only real question is if fog will form before sunrise.
Best bet is in the North Bay -- Santa Rosa is currently at 100% relative humidity
-- so will leave it in the forecast for that region. Today should
be generally sunny with slightly warmer temperatures as a ridge
of high pressure off the coast builds toward our region. Highs
will be in the mid 60s to the lower 70s.
Temperatures will continue to trend warmer into next week as the
ridge expands and builds into the southwestern conus. This has
been a change in the synoptic pattern that has been advertised for
over a week by the models. Latest guidance has 500 mb heights at
or above 595 dm for Monday and Tuesday. At the same time 925 mb
temperatures will be in the 25-30c range which would put it close
to some of the highest values ever for the entire month of
October. With winds forecast to be offshore, very warm
temperatures will make its way across our entire region --
including local beaches. Highs will be in the 80s to mid 90s.
Records will likely fall. Overnight lows will remain cool
providing relief from the daytime heat.
After that we see a large divergence with solutions. The GFS has a
system dropping into the Great Plains with the ridge/Rex block
focused more to the northwest of our area. On the other hand the
European model (ecmwf) shows that on Wednesday the ridge will flatten while a
system dives southward from British Columbia into the interior
northwest. This system will continue into eastern Nevada/Four
Corners region late on Thursday. If this solution verifies it will
lead to gusty northerly winds particularly for the North Bay
mountains and East Bay hills. European model (ecmwf) already has 925 mb speeds
forecast to exceed 30 knots. Although this is still a few days out,
we will be keeping a very close eye on it to see where the system
will end up. If the GFS is correct it will be a non-event.
No indication of rain through Halloween.
Aviation...as of 10:40 am PDT Saturday...VFR will prevail through
taf period with generally light variable am winds and light
onshore breeze PM winds. System impacting pacnw will spread some
high clouds into the area through the day.
Vicinity of ksfo...VFR. West winds around 11 kt after 21z.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR.
Fire weather...as of 5:30 am PDT Saturday...gradual warming
trend starts today with light northerly winds. This trend
continues into tomorrow with winds turning more to the northeast
with continued warming and drying. Another round of gusty offshore
winds is forecast for the Napa hills Sunday night into Monday
morning. Recent rain has helped but will need to watch the Sunday
night into Monday morning wind event closely. Long range trends
suggest warm to hot afternoons and very dry conditions with
periods of offshore winds for much of next week.
There is also concern about a potential northerly wind event for
the north and East Bay hills late Thursday into Friday.
Marine...as of 10:33 am PDT Saturday...light split flow winds
today with increasing northerly flow late in the weekend. A long
period northwest swell will continue to taper down through the
day. Hazardous conditions from squared seas will exist in most
offshore areas today.
... Arena to pt Reyes 0-10 nm from 11 am
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm from 11 am
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm from 11 am
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...rough bar advisory for sf bar until 3 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Public forecast: rgass
fire weather: Bell
Visit US at www.Weather.Gov/sanfrancisco