Residual moisture from a system that passed through the previous day allowed patchy dense fog to develop across the Bay Area. Visibility dropped down to 1/4 mile at the SF airport while runway visibility was reduced to 600 ft.
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The episode as NWS MTR wrote it up.
Residual moisture from a system that passed through the previous day allowed patchy dense fog to develop across the Bay Area. Visibility dropped down to 1/4 mile at the SF airport while runway visibility was reduced to 600 ft. This caused 22 aircraft holds for 669 minutes and 16 diversions at the airport.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS San Francisco Bay Area, CA wrote its last area forecast discussion before the first event at Jan 9, 3:40 AM PST, 2 hours 10 minutes earlier.
Area Forecast Discussion
National Weather Service San Francisco Bay Area
340 AM PST Sat Jan 9 2021
.SYNOPSIS...High pressure will build back over California, and
remain in place through next week. As a result, dry conditions
will prevail each day, with temperatures near or above normal for
the week. Near record temperatures could be possible on Thursday
and Friday of next week. Otherwise, expect patchy valley and delta
fog each night and morning, which may be locally dense at times.
&&
.DISCUSSION...as of 03:08 AM PST Saturday...The broad view over
the region shows high pressure build aloft. At the surface, a
slightly more complicated scenario early this morning. Residual
moisture from yesterday's passing front has led to some patchy
fog, some of it locally dense, this morning. Additionally,
northerly flow developed over the hills. The local WRF model had
this captured pretty well on previous runs and now it's
materializing. Winds over the higher terrain of the N and E Bay
are gusting 30-50 mph in the windiest locations. The northerly
winds are combating more widespread fog development. Therefore,
will not issuing a Dense Fog Advisory this morning as patches of
dense fog are vary transient and appear to not last too long.
Otherwise, high pressure will continue to take hold this weekend.
There is a front off the PacNW and the tail end of it moves
through NorCal Saturday night and early Sunday. Some of the model
guidance brings a few showers to the northern coastal waters, but
nothing over land at this point. Therefore, will keep forecast
dry. Temperatures will be above seasonal average today with highs
in the 60s. Southern areas will warm a few degrees more on Sunday
into the mid 60s, but northern areas (North Bay) may drop a few
with the weak FROPA. High pressure strengthens a tad more on
Monday with rebounding temperatures.
One change to the forecast is the introduction of showers on
Tuesday. Previous model runs did show a few showers north of the
Bay Area with another FROPA, but the 00Z runs have pushed the rain
chances farther south. Ensemble guidance have also increased POPs
across the North Bay early Tuesday. Updated forecast will now
include a few showers on Tuesday for Santa Rosa northward. Most of
the forecast area will remain dry, but a few showers will be
possible over far North Bay. In fact, areas to the south will
actually see an uptick in temperatures - mid 60s to low 70s.
Any showers end quickly Tuesday evening as high pressure to the
SW builds eastward. Near record high temperatures will then
develop Wednesday and Thursday across the region with Thursday
being the warmest. Highs will be in the upper 60s to mid 70s. A
few southern interior locations in Monterey/San Benito may even
hit 80 degrees. Above normal temps and dry conditions will persist
into next week.
&&
.AVIATION...As of 03:39 AM PST Saturday...For the 12z TAFs. High
pressure keeps the big picture mostly VFR through the TAF period;
however, residual moisture has reduced visibility at select
terminals, namely KAPC, KLVK, and KSNS early Saturday morning.
Clearing is expected after 17z, but would not be surprised if it
lingered a bit longer in the North Bay. Winds are light with
breezy onshore winds expected this afternoon. High clouds increase
once again overnight Saturday into Sunday ahead of the next
system, but VFR with light winds.
Vicinity of KSFO...Northerly flow is pushing a weak stratus deck
around 900 feet near the terminal. Might bounce the terminal
between VFR and IFR over the next couple hours. After 15z, VFR is
forecast with breezy onshore winds Saturday afternoon. Increasing
high clouds Saturday night as winds weaken.
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals...Patchy fog developing in the Salinas
Valley. Should mix out after 16z. VFR forecast through the
TAF period with breezier winds Saturday afternoon before weakening
Saturday night.
&&
.BEACHES...As of 3:30 PM PST Friday...Large breaking waves
prevail from a large long period northwest swell. Waves will
slowly diminish through Saturday before rebuilding on Sunday from
an even larger long period swell set to arrive. A High Surf
Advisory is in effect through early Saturday morning with another
hazard product likely needed for the next swell. Additionally, sea
surface temperatures remain cold, down in the lower to mid 50s.
Cold water drownings occur each year with these events but are
avoidable by remaining a safe distance from the coastline. Even
after the High Surf Advisory expires, continued vigilance at the
coast is advised.
&&
.MARINE...as of 02:31 AM PST Saturday...The current large
northwest swell keeps hazardous seas in place, particularly for
smaller vessels, providing enhanced waves near harbor entrances
through Saturday afternoon. Winds out of the north-northwest will
remain breezy with a few stronger gusts, especially along the Big
Sur Coast. The next large, long period northwest swell will then
arrive on Sunday increasing wave heights again making it hazardous
for smaller vessels.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
.Tday...High Surf Advisory...CAZ006-505-509-529-530
SCA...Pt Arena to Pt Reyes 0-10 nm until 3 PM
SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 PM
SCA...Pigeon Pt to Pt Pinos 0-10 nm until 3 PM
SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 PM
SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 PM
SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
SCA...Mry Bay until 9 AM
&&
PUBLIC FORECAST: MM
AVIATION: DK
MARINE: DK
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www.youtube.com/nwsbayareaThe national radar mosaic for the moment the first event began.
1 dense fog. Local standard time, as filed.
Visibility dropped down to 1/4 mile at the SF airport while runway visibility was reduced to 600 ft. This caused 22 aircraft holds for 669 minutes and 16 diversions at the airport.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in California in 2021Every state in 2021This day in weather historyWarnings in California, January 2021
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 944094.