Strong high pressure over the Four Corners brought a hot and dry air mass to the Central Coast and Bay Area in mid June. Hot daytime temperatures for multiple days in the triple digits resulted in record breaking heat for the interior portion of the region.
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.
Strong high pressure over the Four Corners brought a hot and dry air mass to the Central Coast and Bay Area in mid June. Hot daytime temperatures for multiple days in the triple digits resulted in record breaking heat for the interior portion of the region. Higher elevation areas reported daytime highs in the 100s with overnight lows in the 70s and 80s. Several sites broke daily record highs on June 17th. The heat lasted into the start of that coming weekend and resulted in one fatality in San Jose. Additionally, persistent hot/dry conditions along with near record dry fuels allowed for the development of the Willow Fire in Monterey County.
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 Jun 17, 12:54 PM PST, 2 hours 6 minutes earlier.
Area Forecast Discussion
National Weather Service San Francisco Bay Area
154 PM PDT Thu Jun 17 2021
.SYNOPSIS...Today will be the peak of potentially life-threatening
hot temperatures across the interior as the center of a strong high
pressure system overlaps southeastern California. Inland temperatures
will be near record breaking highs today, then slowly ease with each
passing day Friday through the weekend. Meanwhile, onshore flow will
limit heat impacts along the coastline and around the bays. Gradual
cooling is likely this weekend and more so early next week.
&&
.DISCUSSION...as of 1:54 PM PDT Thursday...It's a hot one out
there for much of the Bay Area, especially away from the coast. A
scan of afternoon temperatures show triple readings across the
interior with 106 deg at Arroyo Seco, 105 deg at Pinnacles, 101
near Lake Berryessa, 100 deg at Livermore/Concord, and 99 deg at
Hollister. These sites could climb a few more degrees before the
end of the day. The closer one gets to the immediate coast the
extreme heat begins to take a nose dive. However, there are still
some impressive temperatures near the coast, but there is also a
large temperature spread across a small area. Monterey is a cool
70ish, but just a few miles inland temperatures are in the low
90s. San Francisco Peninsula is seeing temperatures in the mid and
upper 80s near the Castro and upper 60s near Ocean Beach.
Needless to say microclimates are in full effect. All that being
said, current heat related hazards remain in effect. In addition
to the heat, impressive relative humidity readings are being
observed across the interior with many stations below 15 percent
with a handful of single digit readings. Fortunately, winds are
overly strong and mainly a light breeze. Fire weather concerns
will continue this afternoon due to the hot, dry, and breezy
conditions.
There has been discussion about the chance for
showers/thunderstorms this afternoon and it being an outside
chance. It still appears to be the case, but KMUX radar is picking
up a few high based showers entering far S Monterey county. There
is so much dry air between the showers and the surface these
showers will likely be just virga.
For tonight, lower elevations will seem some minor relief with
lows in the 50s and 60s. Climb a few hundred feet and thermal
belts will be in full effect tonight. Overnight lows in the hills
will be in the 60s to lower 80s with no humidity recovery. Low
clouds are hugging the coast from Pt Conception southward. These
low clouds are forecast to track northward in the way of a
southerly surge. More on that later...
For Friday, the big dome of high pressure begins to weaken ever so
slightly as a weak shortwave trough skirts through far NorCal.
That being said, some minor relief will be felt along the coast
tomorrow. Despite temperatures technically dropping from Thursday
to Friday, they'll still be above normal for June. Interior
locations on the other hand will remain well above normal with
highs in the 90s to 105. Heat Advisories and Excessive Heat
Warnings will remain in effect with the exception of SF Bay
Shoreline and N Monterey Bay (Santa Cruz). The tricky part of the
forecast is the aforementioned surge. Models (HRRR/WRF) bring the
surge northward late Thursday night and early Friday to Big Sur.
The surge is then forecast to move north through the day, but as
it does it will be battling northwest flow over the waters.
Therefore, didn't fully bite on lowering temperature dramatically
for Santa Cruz and Watsonville, but definitely has bust potential
if the surge moves faster/stronger. Friday night will be another
active thermal belt night with mild and dry conditions in the
hills.
Further cooling is expected on Saturday with increased onshore
flow. At this point, will not have any heat hazards issued.
Interior locations will be more in the 90s with a few lower 100s.
More widespread cooling is expected on Sunday with mostly 90s
interior. Coastal clouds will likely make a return as well.
Next week will becomes more interesting as the high pressure ridge
exits to the east and a trough/upper low develops off the coast.
The trough will bring temperature back to near normal and in a few
locations below normal. What a swing from this week to next week.
Monday and Tuesday highs will be in the 60s/70s coast and 70s/80s
inland. Medium range models are event hinting at some light precip
on Tuesday, but at this rate it looks more like drizzle.
&&
.AVIATION...as of 11:15 AM PDT Thursday...For the 18z TAFs. High
pressure ridge bulging into the area today. Main impact will be
brief HZ around sunrise each morning briefly reducing visbys to
4-8SM. CIGs will be limited to high clouds from moisture rotating
clockwise around the ridge. Winds light inland and breezy onshore
near the coast, locally gusty in the afternoon near coastal gaps
such as KSFO. A couple of items worth watching: possibility of
elevated convection rolling in from SoCal and the development of a
southerly surge tonight into tomorrow morning.
Vicinity of KSFO...VFR with passing high clouds. Light terrain
driven (offshore) winds in the morning becoming breezy/gusty
onshore winds by the afternoon. Patchy HZ/BR into sunrise Friday,
otherwise VFR. Will need to watch for the development of a
southerly surge along the coast.
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals...VFR with passing high clouds. Light
onshore winds today. For tonight, a southerly surge is forecast to
develop late tonight into tomorrow morning but is expected to
bypass KMRY given the SSW trajectory, thus only FEW-SCT very low
clouds and patchy morning HZ/BR expected Friday Morning.
&&
.MARINE...as of 11:02 AM PDT Thursday...Northwest winds will
prevail across the waters today, locally stronger gusts up to gale
force in the outer waters north of Point Reyes, and diminishing
yet breezy in the southern interior waters. A southerly wind
reversal is expected to occur along the immediate Big Sur coast
later today into tomorrow and may also result in low visibility
conditions. Seas remain mainly wind driven with northwest waves
at 8 to 10 seconds along with a marginal southerly swell.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
.Tngt...Heat Advisory...CAZ508-529
Excessive Heat Warning...CAZ507-510-511-516>518
Heat Advisory...CAZ506-512-513-528
SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
GLW...Pt Arena to Pigeon Pt 10-60 nm until 3 AM
SCA...Pt Arena to Pt Reyes 0-10 nm
SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 AM
&&
PUBLIC FORECAST: MM
AVIATION: DRP
MARINE: DRP
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www.youtube.com/nwsbayareaThe national radar mosaic for the moment the first event began.
4 heat, 1 wildfire. Local standard time, as filed.
Santa Rosa climate station reported 104 degrees breaking a previous daily record of 101 in 1922.
Livermore station broke a daily record at 107 degrees previously 104 in 1961.
Daily record broken in Gilroy of 106 previously 105 in 1961.
County medical examiner reported that a man died in San Jose due to hyperthermia after collapsing due to heat exhaustion. The man collapse on the 17th and later died on the 19th. Exact time unknown.
Record dry fuels along with building high pressure and a hot airmass allowed for the development of the Willow Fire in the Ventana Wilderness in Monterey County. The fire was fully contained on July 11th and burned over 2800 acres https://inciweb.nwcg.gov/incident/7549/.
As the offices recorded each death: direct deaths were caused by the weather itself, indirect ones by something it set in motion.
| Event | Date | Age | Sex | Where | Kind |
|---|---|---|---|---|---|
| Heat, Santa Clara Valley Including San Jose | 2021-06-17 | 51 | Male | Outside/Open Areas | Direct |
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, June 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: 961454, 961455, 961457, 961453, 961458.