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

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FXUS66 KMTR 011842

Area Forecast Discussion
National Weather Service San Francisco CA
1142 AM PDT Sun Oct 1 2023


Issued at 143 AM PDT Sun Oct 1 2023

Mostly cloudy skies this morning turning partly cloudy this
afternoon with continued seasonably cool temperatures as low
pressure exits to our east. High pressure builds Monday through
much of next week. Temperatures near normal Monday warm to well
above normal Wednesday through Friday with periods of light
offshore winds that will gradually increase fire weather concerns
for the second half of the week. The ridge stays in place through
next weekend before breaking down around Columbus Day.


(Today and tonight)
Issued at 143 AM PDT Sun Oct 1 2023

Low pressure that brought some rain showers to the region on
Saturday will continue to wobble eastward over Nevada. Cyclonic
circulation is on the wane with just a few lingering showers
across the region that will dissipate through sunrise. We have a
mix of cloud decks in the wake of the upper low with some
2000-3000 foot cloud decks around the Central Coast and some
lingering mid cloud decks across the Bay Area. It will take awhile
but as drier northerly flow develops today expect more sunshine
this afternoon with highs back into the upper 60s to mid 70s.

With more clearing tonight and the result of the recent rain could
some some pockets of valley fog develop tonight into Monday
morning for places like the 101 corridor in Sonoma Valley that are
protected from developing northerly winds across the Napa hills.


(Monday through Saturday)
Issued at 143 AM PDT Sun Oct 1 2023

Temperatures near normal Monday as high pressure begins to build
and dry northerly flow continues over the region, especially the
North Bay with about 4-8 degrees of warming compared to Sunday
afternoon highs.

On Tuesday afternoon a shortwave will move across Oregon and as it
kicks eastward the ridge will strengthen and a more favorable surface
pressure gradient will start to setup across Northern California
for inducing northerly flow down the Sacramento Valley eventually
passing over the Napa hills Tuesday night into Weds morning. This
will do a few things: drying airmass will lower humidities and
downslope flow will allow temps to warm back into the upper 70s
and 80s regionwide. The setup looks pretty textbook for a light to
moderate fall offshore wind event. During these scenarios favored
adiabatic downslope warming usually allows temps to trend on the
warmer side of model guidance. In addition, and this is picked up
by the EFI, temps near the coast and bays are similar and or warmer
than then the inland temps. Starting Tuesday night northeast winds
in the 10-20 mph range for the North Bay hills will keep the
airmass well mixed at night with mild temps in the hills that will
quickly warm as the sun comes out on Weds allowing temps to soar
well into the upper 80s and 90s across the region. Its possible
some record highs could be tied or broken Weds through Friday but
either way it will get hot during the afternoon. Will post the
record high data below in the .CLIMATE section. The first week of
October historically features some of our warmer temps as seasonal
dry offshore wind events historically occur. Looks like many of
the records this week occurred in 1930, 1980 and 1987 for proxy
patterns. Fire weather concerns will gradually increase with
hotter temps, periods of dry offshore winds (especially during the
overnight hours during the 2nd half of the week), lowering
humidity and a drying of the fine fuels all lining up with
climatology of peak fire season. Will note that fuels are only
forecast to be near normal dryness values but now the live fuel
components are approaching their minimums for the year. The
Extreme Forecast Index (EFI) is a good tool for sniffing out rare
event days and its not too excited about the upcoming week when
warm and dry weather is the norm and its not picking up on any
wind anomalies with the main focus on coastal warming which is no
surprise given the forecast synoptic pattern.

The heat should peak Weds through Friday then wane but still stay
warm and dry next weekend. Some hint of a trough passage as we
approach Columbus Day monday.  Heat products shouldn`t be needed
with cooling at night and a relatively dry airmass.


(18Z TAFS)
Issued at 1141 AM PDT Sun Oct 1 2023

Currently seeing VFR conditions across all terminals with the
exception of Monterey and Santa Rosa which have been slower to clear
out this morning. Santa Rosa is expected to clear shortly by about
19Z. Monterey on the other hand has higher uncertainty due to the
northeast feed that was occuring into the Salinas area; however, now
it appears that clouds are beginning to erode along the coastline
and the clouds that are present seem to be more terrain driven than
anything. Elsewhere, VFR conditions are expected to persist
throughout today. Ceilings are only expected to develop in Monterey
and Santa Rosa tomorrow, they will be MVFR. Winds are generally
pretty calm right now and they will continue a downward trend
throughout the TAF period with the exception of OAK and SFO which
will see an increase in winds this afternoon. Thanks to the recent
passing of the cold front, low level humidities remain elevated and
with that brings the potential for fog. Confidence right now is low,
but the potentially impacted terminals are Monterey, Salinas, and
Santa Rosa.

Vicinity of SFO... VFR conditions will persist throughout the TAF
period. Breezy west-northwest winds will build this afternoon with
maximum speeds between 10 and 16 knots. Winds will become light in
the evening.

SFO Bridge Approach... Similar to SFO.

Monterey Bay Terminals... Monterey still has an MVFR ceiling and
Salinas has just scattered out after seeing a feed from the
northeast. Winds will increase with the departure of the ceilings
with west-northwest winds peaking between 10 and 14 knots. Winds
will diminish in the evening and they will maintain that trend
through the end of the TAF period.


(Today through Friday)
Issued at 1141 AM PDT Sun Oct 1 2023

Northerly gale-force gusts north of Point Reyes weaken into the
afternoon today to become strong to fresh breezes. Moving into the
work week, high pressure develops, with winds decreasing to become
moderate. Waveheights continue to decrease through the week as
winds ease, creating more favorable conditions. Moderate period
(12-14 seconds) southerly swell persists through the week,
alongside shorter period northwesterly swell (8-10 seconds). New,
longer-period northwesterly swell moves in towards the middle of
the work week.



Record high data for this week:

Location       Oct 4th         Oct 5th     Oct 6th
Santa Rosa     101 1920       103 1933     102 1930
Kentfield       99 1987       100 1987      97 1930
Napa           100 1987       101 1987      96 1930
Richmond        99 1987        99 1987      91 1996
Livermore      106 1980       100 1980     100 2014
San Francisco  100 1987       102 1987      94 1992
SFO             95 1987        99 1987      92 1992
Redwood City   101 1980        99 1987      99 1987
Half Moon Bay   87 1953        82 2013      87 1972
Oakland City    92 2014        96 1987      93 1976
Salinas        100 1987       105 1987      95 1972
King City      106 1944       105 1933     100 1996


PZ...Gale Warning until 3 PM PDT this afternoon for PZZ570.




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