Frost Advisory 17, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco CA
1027 PM PDT Thu Oct 26 2023

...New AVIATION...

.SYNOPSIS...
Issued at 250 PM PDT Thu Oct 26 2023

Cold overnight lows expected through Saturday morning with patchy
frost possible in isolated, sheltered valleys of the North Bay.
This weekend, gusty offshore winds will set up across the region
which will result in elevated fire weather conditions. A Fire
Weather Watch is in effect for the North Bay interior mountains,
the East Bay Hills, the Eastern Santa Clara Hills, and the Santa
Cruz Mountains Saturday morning through Sunday evening.

&&

.UPDATE...
Issued at 857 PM PDT Thu Oct 26 2023

Skies are clearing as we head into a colder night. Lows in the
interior valleys are set to drop into the low 40s and upper 30s. The
overnight forecast team will make the final adjustments to overnight
temperatures and fine-tune Saturday morning's lows, while keeping an
eye on the frost risk. Additional attention will be payed to the
weekends fire wx risks, as higher resolution models begin to come
into range of the event.

&&

.SHORT TERM...
(This evening through Friday)
Issued at 250 PM PDT Thu Oct 26 2023

A cooler and drier airmass currently resides over the region as made
evident by the 12z OAK sounding, which shows 850 mb temps have
dropped 10 degrees in the last 24 hours. This in addition to light
offshore flow across the region has helped to eliminate the marine
layer inversion. Visible satellite shows clear skies across the
region but high clouds are approaching from the Pacific. Thanks
to the colder airmass, temperatures across the region today will
be running 5 to 10 degrees below seasonal norms with highs in the
upper 60s near the bay shoreline as well as the interior valleys.
Highs near the coast today will be in the upper 50s to low 60s.

Tonight will be another chilly night that will be a few degrees
colder than last night. The valleys of Sonoma County continue to
look like the coldest populated area in the cwa with lows dipping
again into the mid to upper 30s. Patchy frost will be possible
across sheltered valleys with lighter winds expected and clear
skies. It's not looking widespread or cold enough to issue the first
Frost Advisory of the season for the N Bay Valley zone tonight, but
some patchy frost is not out of the question for sheltered valleys
in the North Bay tonight. Another reason for not issuing a Frost
Advisory is the fact that there will be some high clouds that move
in over the Bay Area tonight, which will help to limit some
radiational cooling.

By Friday, the upper level longwave trough will start sliding south
into the Great Basin, which will set the stage for Saturday-Sunday's
fire weather concerns. Friday itself will feature rather benign
weather, though, with similar conditions to today but slightly
cooler.

&&

.LONG TERM...
(Friday night through next Wednesday)
Issued at 250 PM PDT Thu Oct 26 2023

Friday night through Saturday...

Colder temperatures expected again Friday night into Saturday
morning and forecasted to be slightly cooler than the previous
night. There will be more sites with temperature readings in the mid
30s Saturday morning and potentially more areas of frost across the
N Bay valleys. More confidence exists for Saturday morning to
potentially have the first Frost Advisory issuance, so stay tuned
for forecast updates. Less populated areas of the interior and
southern mountains (E Santa Clara Hills, San Benito Hills, S
Monterey County) could also see areas of frost Saturday morning.

Saturday through Monday...

Offshore winds are still on track to develop Saturday in wake of an
exiting (inside slider) trough and subsequent backdoor cold
frontal passage. Strong surface high pressure will build across
the Intermountain West Saturday night into Sunday, which will
serve to strengthen the offshore pressure gradient. SFO to
Winnemucca (WMC) forecasted offshore gradient is expected to be
impressively strong, reaching -14 mb per the GFS late Saturday
into Sunday.

As a result, dry conditions and north-northeasterly winds of 15
to 25 mph will develop first across the interior portions of our
forecast area (Napa Co, E Bay hills, and E Santa Clara Hills)
Saturday morning. Gusts of 35 to 45 mph possible across ridgetops.
By early Saturday afternoon, these gusty offshore winds will
spread to the Santa Cruz Mountains. The aforementioned regions are
now in a Fire Weather Watch which is valid from 5 AM Saturday
through 5 PM Sunday in coordination with neighboring NWS forecast
offices. See Fire Weather section below for a more detailed
analysis and overview of this upcoming offshore event.

By Monday, offshore winds linger but the speeds diminish greatly
as the SFO-WMC gradient diminishes. Upper level ridging builds in
and slightly warmer temperatures expected Monday, but still
looking near seasonable.

Tuesday-Wednesday...

Upper flow pattern becomes more zonal/westerly, which may open up
the door for some precip to move through. Will continue to
monitor.

&&

.AVIATION...
(06Z TAFS)
Issued at 1026 PM PDT Thu Oct 26 2023

High confidence that clear skies and VFR conditions prevail
through the TAF period. Generally light winds through the night,
with northwesterly flow at up to 10-15 knots developing Friday
afternoon.

Vicinity of SFO... VFR through the TAF period. West-northwest winds
diminishing through the night, light winds through Friday morning
before northwest winds gusting up to 10-15 knots build in the
afternoon.

SFO Bridge Approach... Similar to SFO.

Monterey Bay Terminals... VFR through the TAF period. Light drainage
winds overnight and into the morning, with breezy west-northwest
winds up to 8-12 knots Friday afternoon.

&&

.MARINE...
(Tonight through next Tuesday)
Issued at 857 PM PDT Thu Oct 26 2023

North to northwest winds weaken in the late night, but remain
strong along the southern coastal waters into early Friday. North
to northwest winds increase again elsewhere across the coastal
waters late in the week. Gusty winds will diminish on Sunday
leading to calm conditions into the next work week.

&&

.FIRE WEATHER...
Issued at 250 PM PDT Thu Oct 26 2023

Offshore wind setup slated for the weekend will bring elevated
fire weather conditions to the North Bay, East Bay hills, and the
Santa Cruz Mtns. Increasing confidence in the track of an upper
low through the Great Basin will cause a surface high pressure to
build across the interior regions Saturday. Ensemble guidance
advertising the SFO-WMC pressure gradient in the -12 to -14 mb
range during the time from about noon Saturday through noon
Sunday, which is approaching the 99th percentile. This set up
promotes widespread offshore flow with high confidence. Wind gusts
along ridgetops expected to be in the 35-45 mph range, with
moderate confidence that a few of the favored spots such as Mt.
St. Helena and Mt. Diablo could see gusts to 50 mph. Minimum RH
during the afternoons Saturday and Sunday will be in the 15-20%
range across much of the area as the downsloping offshore winds
increase. Overnight recoveries aren't much better between 25-45%.

The limiting factors with this event are related to the fuel
conditions and the degree of stability in the valleys. In regards
to the fuels, there is less concern in the more dense 100 and 1000
hour fuels as much of the moisture is still retained from recent
rains. Much more concern lies in the finer, more flashy fuels,
such as grasses and shrubbery. Stability presents some degree of
uncertainty to the wind forecast in valley areas. These areas are
still expected to be breezy through the weekend, but not as high
as their adjacent mountains and hills. If the valleys are able to
hold on to their moisture a little longer with weaker flow near
the surface, the atmosphere would remain largely decoupled and
the highest gusts would remain in the higher terrain. The worst
case scenario would be the stability completely eroding allowing
the highest gusts to mix down into the valley floors. We will be
keeping a close eye on this over the coming shifts and update the
forecast as needed.

&&

.CLIMATE...
Issued at 512 PM PDT Thu Oct 26 2023

Record low temperatures at the long term sites:

Station                 Oct 27th                Oct 28th

Santa Rosa              28 in 1919              28 in 1946
Kentfield               34 in 1919              34 in 1923, 1919
Napa                    34 in 1918              31 in 1946
Richmond                43 in 1984, 1970        41 in 1956
Livermore               30 in 1946              30 in 1946
San Francisco downtown  47 in 1984, 1908, 1893  46 in 1971
SFO Airport             42 in 1954, 1951        40 in 1954
Redwood City            39 in 1931              33 in 1946
Half Moon Bay           37 in 2022              37 in 1946
Oakland downtown        46 in 2010, 2004, 1975  44 in 1975
San Jose                37 in 1893              35 in 1946, 1894
Gilroy                  30 in 1967              30 in 1967
Salinas Airport         34 in 1931              32 in 1946
King City               30 in 1970, 1929        29 in 1984, 1970

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Fire Weather Watch from late Friday night through Sunday
     afternoon for CAZ502-504-512-514-515.

PZ...Small Craft Advisory from 3 PM to 9 PM PDT Friday for PZZ535.

     Small Craft Advisory until 3 AM PDT Saturday for PZZ565.

&&

SHORT TERM...McCorkle
LONG TERM....McCorkle
AVIATION...DialH
MARINE...Murdock

Visit us at www.weather.gov/sanfrancisco

Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/nwsbayarea
www.twitter.com/nwsbayarea
www.youtube.com/nwsbayarea