Name: Dense Fog in California, January 7, 2021
Start: 2021-01-07T01:00
End: 2021-01-07T10:20
Location: California

The toll

Deaths: 0
Injuries: 0
Property damage: None recorded
Crop damage: None recorded
Events: 5

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.

What happened

The episode as NWS HNX wrote it up.

A short wave trough pushed through central California during the evening of January 6, but skies cleared out and winds diminished behind it as high pressure built back into the area overnight. An area of dense fog formed from Turlock to Delano overnight as a result of clear skies, light winds and inversion conditions over the San Joaquin Valley. The fog dissipated by late morning from Fresno southward and lifted into a low cloud deck north of Fresno which persisted for much of the day.

What forecasters said that day

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 Hanford, CA wrote its last area forecast discussion before the first event at Jan 6, 3:34 PM PST, 9 hours 26 minutes earlier.

Area forecast discussion, Jan 6, 3:34 PM PST, 9 hours 26 minutes before

Area Forecast Discussion...UPDATED
National Weather Service Hanford CA
334 PM PST Wed Jan 6 2021

.UPDATE...Updated Air Quality Section.

&&

.SYNOPSIS...Calm, dry weather will continue with areas of
nighttime and morning fog possible through Thursday. Another weak
storm system will track through northern California later this
week and bring breezy winds and a slight chance of precipitation
to Yosemite National Park on Friday. Otherwise, much of this week
will be dry with slightly above normal temperatures. Dense fog may
become an issue in the valley by Sunday and for much of next
week.

&&

.DISCUSSION...Areas of fog formed over parts of the central San
Joaquin Valley through late morning. Some fog continued to plague
parts of the south valley through midday due to high cloud cover
moving through central CA. High cloud cover along the warm sector
of a trough should move east of the sierra this evening. Models
show precipitation remaining towards N CA. Think there will be
some patchy fog in parts of the valley tonight but more favorable
conditions towards the south valley as model cross sections show
stronger North winds and some low level cloud over the central
valley overnight. The forecast for fog will remain difficult
Thursday night as another trough brings stronger west flow aloft
and bands of high cloud cover. Precipitation threat remains mainly
over N CA with light amounts towards the Yosemite area on Friday
morning. The trough moves east through the Great basin Friday
night leaving stronger NW winds over central CA. Wind mixing
should mitigate the threat of fog over much of the valley Friday
night with some low clouds affecting the south SJ valley and
Tehachapi mountains due to the upslope wind flow. Flow turns
offshore for the weekend while a ridge builds inland from the
Pacific. Models show the ridge will continue to strengthen over CA
through mid week. This will result in warm and dry conditions for
much of central CA and bring more favorable conditions for fog
over the San Joaquin Valley for much of next week.

&&

.AVIATION...Local LIFR visibilities due to fog are expected in
the San Joaquin Valley 10Z through 19Z Thursday. Otherwise, VFR
conditions will prevail elsewhere over the central California
interior during the next 24 hours.

&&

.AIR QUALITY ISSUES...On Thursday January 7 2021...
Fireplace/Wood Stove Burning Status is: No Burning Unless
Registered in Fresno... Kern... Kings... Madera and Tulare
Counties. Further information is available at Valleyair.org

&&

.CERTAINTY...

The level of certainty for days 1 and 2 is medium.
The level of certainty for days 3 through 7 is high.

Certainty levels include low...medium...and high. Please visit
www.weather.gov/hnx/certainty.html for additional information
an/or to provide feedback.

&&

.HNX WATCHES/WARNINGS/ADVISORIES...None.

&&

public...MV
aviation....Andersen

weather.gov/hanford

Event by event

5 dense fog. Local standard time, as filed.

Dense Fog, Merced and Madera

When: January 7, 1:00 AM PST to 10:20 AM PST
Where: Merced and Madera forecast zone
Reported by: ASOS

Merced and Madera reported visibility below a quarter mile.

Dense Fog, Foggy Bottom

When: January 7, 3:50 AM PST to 10:00 AM PST
Where: Foggy Bottom forecast zone
Reported by: ASOS

Hanford reported visibility below a quarter mile. At 0430 PST, California Highway Patrol reported visibility between 200 and 500 feet on SR 198 and 4th Ave east of Hanford and was pacing traffic between Hanford and Visalia.

Dense Fog, Fresno

When: January 7, 3:50 AM PST to 10:18 AM PST
Where: Fresno forecast zone
Reported by: ASOS

Fresno and Reedley reported visibility below a quarter mile. At 0600 PST, a trained spotter reported 100 foot visibility in Sanger.

Dense Fog, Eastern San Joaquin Valley in Kern County

When: January 7, 4:15 AM PST to 5:35 AM PST
Where: Eastern San Joaquin Valley in Kern County forecast zone
Reported by: AWOS

Delano reported visibility below a quarter mile.

Dense Fog, Tulare County

When: January 7, 6:35 AM PST to 8:30 AM PST
Where: Tulare County forecast zone
Reported by: AWOS

Visalia reported visibility below a quarter mile.

What these are

Dense Fog: Fog reducing visibility to a quarter mile or less, filed where it caused accidents or disruption.

After NWS Directive 10-1605, the rules offices file Storm Data by.

The same day elsewhere

More

Every storm in California in 2021 Every state in 2021 This day in weather history Warnings in California, January 2021

Sources

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: 928008, 928009, 928010, 928012, 928011.