Patches of dense fog formed into the early morning of September 5th around the San Francisco Bay Area and down to Monterey. Observations ranged from 1/8 of a mile to 2 miles in affected areas.
Every event with a place to put it; 5 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS MTR wrote it up.
Patches of dense fog formed into the early morning of September 5th around the San Francisco Bay Area and down to Monterey. Observations ranged from 1/8 of a mile to 2 miles in affected areas.
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 Sep 4, 9:41 PM PST, 3 hours 34 minutes earlier.
Area Forecast Discussion
National Weather Service San Francisco Bay Area
1041 PM PDT Sat Sep 4 2021
.SYNOPSIS...A warming and drying trend will continue through Labor
Day as high pressure builds over the region. Temperatures trend
cooler with the return of a deeper marine layer by the the middle of
the upcoming week.
&&
.DISCUSSION...as of 9:48 PM PDT Saturday...Average temperatures
today varied within two to three degrees above and below the 30
year normals for Sep 4th. Highs today were 60s/70s coastside,
70s/80s bayside and 80s/90s inland. Good news, no heat products
are anticipated through Labor Day. The SFO-SAC pressure gradient
is 1.7 mb with onshore winds continuing this evening. Not a strong
pressure gradient field at present, however fyi, on the larger scale
the ACV-SFO, WMC-SFO (large errors), SMX-SFO pressure gradients are
underforecast by global and meso-scale model output.
Satellite imagery shows marine layer stratus and fog are developing,
the marine layer depth is compressed to 1,000 feet per profiler
data and mainly light onshore winds are continuing. Another night
of decent radiative cooling furthering the marine layer per 0.43"
total precipitable water (pw) on the 00z Oakland sounding, a drier
pw than usual for early Sep. Anti-cyclonic flow around a strong
500 mb 593 decameter high situated over SoCal will however slowly
increase the pw value to the max moving average (~ 1.40") for all
Oakland soundings by next Thursday night and Friday morning per
GFS forecasts, may play a key role in our weather since a mid-
latitude 500 mb trough will also approach our cwa during that
time. Global models disagree on the 500 mb trough's strength when
it nears our coast, but it bears watching and may need to update
blended model/grids. More than hints of showers/rain for our cwa
(and areas outside our cwa) from earlier GFS runs since earlier
this week, including recent GEFS (mean) plumes showing a few
hundredths measurable rain over our cwa late next week, while the
ECMWF is mostly scaled back on QPF for our cwa. A potentially
very early season collision of mid-lat dynamics meeting water
vapor availability could take place, especially interesting given
an ongoing active hemispheric wave number and strength of mid-
latitude westerlies. For the lead up, there's no indication of
surface based mucape on the NAM through 12z Wednesday, with large
disagreements between the GFS and NAM regarding mid level cape
(lowers forecast confidence), and the GFS indicates weak to no mid
level mucape even after 84 hours. With increasing water vapor
availability will have to closely watch the approaching 500 mb
trough, it may produce more in the way of showers vs. thunderstorms
since preceding conditions (onshore winds, marine layer) appear
unfavorable for a significant need of convective overturning, speaking
only for our cwa. It may be a different story outside our cwa.
Otherwise wildfire smoke continues tonight, Sunday through Monday
per 00z HRRR near surface smoke forecasts, coupled with morning
fog and haze lifting to hazy conditions during the day will reduce
the amount of insolation thus an offset to daytime highs. Made
minor downward adjustments to forecast high temps Sunday along the
immediate coast where stratus, fog, hazy and smoky conditions
will linger the longest; around the SF Bay Shoreline similarly
forecast highs may be locally too warm, however did not adjust
them quite yet, will let the midnight shift look over temperatures
and decide if needing adjustments. Another update, based on WRF-
ARW/GFS modeled QPF, included patchy light coastal drizzle in grid
forecasts for the immediate coast tonight and Sunday morning,
with patchy light coastal drizzle redeveloping Sunday night and
Monday morning.
&&
.PREV DISCUSSION...as of 01:54 PM PDT Saturday...A 592dm @ 500 mb high
pressure currently centered over southern California has resulted
in warming and drying trend across the region. As a result, the
marine layer has now compressed to around 1000 feet in depth with
low clouds hugging the coastline while mostly sunny conditions
prevail inland. However, smoke aloft has resulted in hazy
conditions over much of the region. The warming air mass aloft has
resulted in surface temperatures about 2-7 deg F warmer than the
same time yesterday.
The warming trend will continue Sunday and into Labor Day as the
ridge aloft strengthens over the region. Looking for more widespread
mid/upper 90s across the interior with the region's warmest spots
reaching between 100-105 deg F. With this, looking for Heat Risk in
the moderate to locally high category across the region's far
interior while onshore flow greatly lessens impacts across the major
population centers. Thus, no heat products are anticipated at this
time. Additionally, there remains some uncertainty on the exact
strength and position of the mid/upper level high as it battles with
a cut-off low pressure out over the Pacific. Regardless, upper 60s
to 70s will be common near the coast both Sunday and Monday with a
shallow marine layer likely to hold strong. This will bring low
clouds back to the coastline and locally into the adjacent valleys
with some patchy fog also possible during the late night and early
morning hours. A great note from the overnight forecaster: "Atop the
marine layer, warm overnight conditions will persist with poor
humidity recoveries, though they should remain sub criteria for
significant concerns. That said, given that it is a holiday weekend,
fuels are very dry, and that most fires are human caused, please be
cautious and mindful of the hot and dry conditions and avoid any
activities that may lead to no wildfire starts."
Another feature worth keeping an eye on is moisture in the mid-upper
levels advecting northward between the aforementioned low pressure
offshore and high pressure inland. This may result in some high-
based cloud cover, especially out over the Pacific from Sunday
afternoon into early Monday. Otherwise, not expecting any major
impacts at this time.
The forecast ensembles are indicating the development of mid/upper
level troughing along the West Coast by midweek. Thus, look for
temperatures to cool slightly back to near seasonal averages by
midweek with the potential for additional cooling late in the
week. This pattern will also support the deepening of the marine
layer and return of more widespread night/morning clouds in the
interior valleys.
&&
.AVIATION...as of 10:41 PM PDT Saturday...For the 06z TAFs. The
marine layer is still around 1,000 ft AGL per profiler data.
Satellite imagery shows stratus along the coast and making its way
down the Salinas Valley. VFR conditions currently north of the
Monterey Bay with IFR/LIFR at KMRY/KSNS. Patchy fog along the
coast and in the valleys possible late tonight into early tomorrow
morning. Low confidence on how far inland stratus will reach
around the SF Bay, but MVFR/IFR cigs possible late tonight into
tomorrow morning. Latest HRRR smoke model shows some smoke
lingering over the area through the rest of the taf period. May
see some HZ/FU lower slant-range vis particularly across the
North/East Bay. Light winds overnight, onshore in the afternoon
10-15 kt, locally higher at KSFO.
Vicinity of KSFO...VFR into late tonight. Expecting stratus to
move into the East Bay overnight, low confidence as to whether or
not low clouds will cross over into the terminal. Light winds
overnight, increasing again in the afternoon W/NW at 15-20 kt.
KSFO Bridge Approach...Possible low clouds in the vicinity 10z-17z
Sunday.
Monterey Bay...Stratus has filled into the bay and is moving down
the Salinas Valley. IFR/LIFR conditions overnight and through mid
to late Sunday morning (~16z-18z). Patchy fog also possible late
tonight into early Sunday morning. Light winds overnight with
moderate sea breeze in the afternoon.
&&
.MARINE...as of 8:59 PM PDT Saturday...Light to moderately breezy
northwest winds continue across the waters tonight with the
breeziest conditions in the northern outer waters. Winds will
increase tomorrow afternoon in the outer waters and near coastal
jet regions, including the Big Sur coast and south of Pigeon
Point. Typical gusty winds will also funnel through the Golden
Gate gap. Seas will remain predominantly driven by steep, short
period northwest waves at 6 to 7 seconds. Light southerly swell
will continue through the week along with a developing northwest
swell that will reach our coastal waters by Labor Day.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
.Tday...None.
PUBLIC FORECAST: Canepa
AVIATION: AS
MARINE: Lorber
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www.youtube.com/nwsbayareaThe national radar mosaic for the moment the first event began.
5 dense fog. Local standard time, as filed.
Santa Rosa Airport ASOS reported visibility down to a quarter of a mile.
Napa County Airport ASOS reported visibility down to a quarter mile on and off during this time period. Weather sensor also reported fog.
Santa Cruz AWOS station reported visibility down to a quarter of a mile.
KPIX webcam on Salesforce Tower in downtown San Francisco as well as satellite imagery and other public reports showed fog in downtown San Francisco. Conditions began to improve mid to late morning.
Monterey Airport ASOS reported visibility down to a quarter of a mile.
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, September 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: 979068, 979071, 979079, 979041, 979086.