Excessive Heat in California, May 30 to 31, 2021

Name: Excessive Heat in California, May 30 to 31, 2021
Start: 2021-05-30T12:00
End: 2021-05-31T23:59
Location: California

May 30 to 31, 2021 · NWS STO · 9 events in 9 counties and zones

The toll

Deaths: 0
Injuries: 0
Property damage: None recorded
Crop damage: None recorded
Events: 9

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 STO wrote it up.

The first major heat event of the year brought widespread moderate to high heat risk Memorial Day weekend to Northern California. Some Valley locations reached over 100 degrees for the first time this year. Several new record daily high temperatures and one record monthly high temperature record were set in the area.

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 Sacramento, CA wrote its last area forecast discussion before the first event at May 30, 2:45 AM PST, 9 hours 15 minutes earlier.

Area forecast discussion, May 30, 2:45 AM PST, 9 hours 15 minutes before

Area Forecast Discussion
National Weather Service Sacramento CA
345 AM PDT Sun May 30 2021

.SYNOPSIS...
Very hot and dry weather early this week with record daily highs
possible on Memorial Day. Not as hot after mid-week.

&&

.DISCUSSION...
Clear skies cover interior NorCal early this morning except for a
bit of thin cirrus spilling through the eastern Pacific ridge and
some stratus along the western boundary of the Delta. Onshore
surface pressure gradient persists, but has weakened compared to
early Saturday and southwesterly gusts of 20-30 mph are limited
to the Delta. Current temperatures compared to 24 hours ago are
milder in the foothill and mountain thermal belts, but a little
cooler across the Central Valley where readings range from the mid
50s in the Sacramento area to near 70 at the north end of the
Sacramento Valley.

Upper ridge continues to strengthen across NorCal today which will
result in around 5-10 degrees of warming compared to Saturday and
highs close to 105 across the northern Sacramento Valley. The
Delta Breeze will diminish as the marine layer mixes and low-
level northerly flow develops. The big question is whether we'll
see enough residual onshore flow to keep the Delta, northern San
Joaquin Valley and Sacramento area just shy of seeing triple digit
heat today.

Heat is forecast to peak across most of the region on Memorial Day
when highs across most of the Central Valley will top out in the
105-110 degree range. Daily record highs (and perhaps a few monthly
records) will be in jeopardy. Exercise caution if planning outdoor
activities for the holiday.

By Monday evening, the marine layer is forecast to reestablish
along the coast leading to a return of the Delta Breeze. This will
help to cool temperatures back down to around the century mark
across the Sacramento and Stockton region on Tuesday while areas
further inland will remain very hot.

The edge will come off the heat further by Wednesday as both GEFS
and EC ensemble indicate the ridge will shift east into the
northern Rockies and Great Basin allowing for minor synoptic
cooling and a little more onshore flow.

&&

.EXTENDED DISCUSSION (Thursday THROUGH Sunday)...
Main forecast problem in the EFP is the timing of the upper trof
which brings cooler temps to Norcal, with some increasing
uncertainty in the magnitude of the Wrn trof later in the EFP,
some of which is due in part to model timing differences. GFS
members are more progressive (faster) than the ECMWF mainly
affecting the Fri/Fri nite forecast periods. In this scenario the
ECMWF would be cooler with possible unsettled wx over the Nrn mtns
while the GFS would mainly show a faster rebound in temps for the
weekend as the trof departs. Max temps will still run about 8 to
15 degrees above normal due to mild WNWly flow aloft from the the
Ern Pac high and possibly nearing triple digits again on Sat over
the Nrn half of the Sac Vly before cooling slightly again on Sun.
JHM

&&

.AVIATION...
Onshore flow has been strong enough overnite to spread the
elevated marine layer (Base 800 Tops 2600 ft AMSL) into the SE
Bay area and up into the KAPC area. Stratus deck will
erode/dissipate after 17z as the Ern Pac ridge builds into Norcal
and dry Nly winds to 12 kts prevail over Norcal. Conditions will
remain VFR over TAF sites with SCT-BKN250 occasionally spilling
over the ridge thru 19z Mon. With afternoon temperatures
surpassing triple digits over the Nrn half of the Sac Vly density
altitude likely to equal/exceed 3000 ft. Delta Breeze with gusts
to 25kts through the Carquinez Strait will weaken during the day,
but forecast to increase with gusts to 30 kts after 05z.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning from noon today to 8 PM PDT Tuesday for
Carquinez Strait and Delta-Central Sacramento Valley-Motherlode-
Mountains Southwestern Shasta County to Western Colusa County-
Northeast Foothills/Sacramento Valley-Northern Sacramento Valley-
Northern San Joaquin Valley-Shasta Lake Area / Northern Shasta
County-Southern Sacramento Valley-Western Plumas County/Lassen
Park.

&&

Event by event

9 excessive heat. Local standard time, as filed.

Excessive Heat, Shasta Lake/North Shasta County

When: May 30, 12:00 PM PST to May 31, 11:59 PM PST
Where: Shasta Lake/North Shasta County forecast zone
Reported by: ASOS

High pressure ridging brought a heat wave to Northern California over the Memorial Day weekend holiday. Temperatures reached upwards of 20 to 25 degrees above climatology. Several new record daily high temperatures and one record monthly high temperature record were set in the area. At the peak of the heat wave on the 31st, Redding reached 109 breaking both the daily and monthly high temperature records. Red Bluff reached 108, setting a new daily high temperature record. Downtown Sacramento hit 106 tying the daily high temperature record. Sacramento Executive Airport reached 105 setting a new daily high temperature record. There were three cooling centers open in Sacramento County, one in Yolo County, one in Butte County and one in San Joaquin County.

Excessive Heat, Northern Sacramento Valley

When: May 30, 12:00 PM PST to May 31, 11:59 PM PST
Where: Northern Sacramento Valley forecast zone
Reported by: ASOS

Excessive Heat, Central Sacramento Valley

When: May 30, 12:00 PM PST to May 31, 11:59 PM PST
Where: Central Sacramento Valley forecast zone
Reported by: ASOS

Excessive Heat, Southern Sacramento Valley

When: May 30, 12:00 PM PST to May 31, 11:59 PM PST
Where: Southern Sacramento Valley forecast zone
Reported by: ASOS

Excessive Heat, Mountains Southwestern Shasta County to Western Colusa County

When: May 30, 12:00 PM PST to May 31, 11:59 PM PST
Where: Mountains Southwestern Shasta County to Western Colusa County forecast zone
Reported by: ASOS

Excessive Heat, Northeast Foothills/Sacramento Valley

When: May 30, 12:00 PM PST to May 31, 11:59 PM PST
Where: Northeast Foothills/Sacramento Valley forecast zone
Reported by: ASOS

Excessive Heat, Carquinez Strait and Delta

When: May 31, 12:00 PM PST to 9:00 PM PST
Where: Carquinez Strait and Delta forecast zone
Reported by: ASOS

Excessive Heat, Northern San Joaquin Valley

When: May 31, 12:00 PM PST to 11:59 PM PST
Where: Northern San Joaquin Valley forecast zone
Reported by: ASOS

Excessive Heat, Motherlode/Camptonville to Groveland

When: May 31, 12:00 PM PST to 11:59 PM PST
Where: Motherlode/Camptonville to Groveland forecast zone
Reported by: ASOS

What these are

Excessive Heat: Heat at or above the local excessive heat warning criteria, or heat that caused deaths.

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

The same day elsewhere

Every storm in California in 2021 Every state in 2021 This day in weather history Warnings in California, May 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: 963146, 963147, 963148, 963149, 963153, 963154, 963150, 963152, 963155.