Strong high pressure building over the region in early to mid July brought warm to hot temperatures across the interior. Daytime highs rose into the 100s with locations across the East Bay up to 110 degrees between July 9th and 11th.
Every event with a place to put it; 6 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 building over the region in early to mid July brought warm to hot temperatures across the interior. Daytime highs rose into the 100s with locations across the East Bay up to 110 degrees between July 9th and 11th. Overnight lows remained warm, particularly across higher elevations where temperatures were still in the 70s and 80s in the early mornings. Additionally, prolonged dry conditions resulted in near record dry fuels for July. This along with breezy afternoon winds prompted the Diablo Fire in Contra Costa 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 Jul 9, 3:49 PM PST, 3 hours 11 minutes earlier.
Area Forecast Discussion
National Weather Service San Francisco Bay Area
449 PM PDT Fri Jul 9 2021
.SYNOPSIS...Dangerously hot conditions will impact interior and
higher elevation locations around the San Francisco Bay Area and
Central Coast this weekend. Onshore flow will provide limited
relief to areas immediately adjacent to the ocean or bay.
&&
.DISCUSSION...as of 02:26 PM PDT Friday...Stout high pressure
continues to dominate the SW US today. Ridging from this high
pressure is building toward the central California coast, causing
a heat up across the region. This ridge is strong enough to cause
nearly a complete elimination of the marine layer. There are low
clouds showing just off the coast and covering Point Reyes this
afternoon, but are expected to remain there or farther west. As
the ridge builds westward, it is possible no marine layer develops
at all overnight. Despite the marine layer not being seen,
onshore flow has been showing a bit this afternoon with winds in
the 10-20 knot range along the immediate coast. This has helped
places like downtown SF already see their max temp for the day in
the mid 70s. This will also help to cap heating in other areas
that are directly impacted by this onshore flow. That said, if you
go up a few hundred feet, you'll be out of the influence and heat
will dominate. Other areas around the Bay Area, such as interior
Contra Costa are already well into the 100s.
Going into tonight, look for a fair night in areas with the
onshore wind influence. However, like last night, get above the
onshore flow and the heat will remain on as heat in the column
above about 500 feet is simply amazing. It will be interesting to
see what the next few upper air soundings show for 850mb temps. If
we hit 31C @ 850mb, we would break a record for that level and
would speak clearly to the heat that is present. Above about
500-700 feet, expect the overnight lows to not drop below 75F, and
really many areas may not drop below 80F in the mountains.
Saturday continues to be the day to watch. This will be the day
the ridge has the farthest westward extent with some good
strength. At this time we could see the onshore winds even die out
for time, which means significant warming for most all areas. It
would not be surprising to see SF top 90F if the onshore flow
shuts off for a time on Saturday. Outside of the immediate coast,
Saturday will be down right hot. Nearly all interior areas will
eclipse 100F and outlying areas will be pushing 105F and even
110F. There are a few hot pockets that may brush 115F.
Overall, this is a dangerous heat event and needs to be taken
seriously. Stay hydrated and check on each other often.
By Sunday we'll see a slight decrease in temps, but still many
locations will be well above 95F. It won't be until midweek that
we really start to drop toward more seasonable conditions.
To nerd out a bit, temps were done mainly with the NBM 75th
percentile and some manual adjustments. This was done as the
ensembles and the extreme forecast index supported going higher
than many deterministic members and the 50th percentile just did
not cut it.
Seek cool areas. Be careful if you head to the ocean where the
water is cold and rip currents exist year round.
&&
.AVIATION...as of 04:51 PM PDT Friday...For the 00z TAFs. VFR. A
shallow marine layer is being observed at the coastal profilers
this afternoon, with a 600-800ft depth right along the immediate
coast. This has helped to promote onshore winds along the coast,
gaps, and passes, that will gradually weaken overnight. Otherwise,
expect only a few clouds to roll in to some of the terminals,
with VFR conditions lasting through the end of the TAF period and
over the next couple of days as a high pressure sits overhead.
Vicinity of KSFO...VFR. West to northwest wind 15 to 20 knots,
with a few gusts to 20-25 knots this evening. No significant
marine layer expected to develop tonight and will observe lighter
winds through the overnight and into tomorrow morning.
KSFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals...VFR. A very shallow marine layer is
will continue into the overnight, which will help to promote some
SCT/FEW skies along the Monterey Peninsula and immediate coast,
but are not looking at large-scale marine intrusion tonight. Have
added the possibility of some coastal BR at MRY owing to the
shallow depth, but skies will remain VFR through the end of the
TAF period and over the next couple of days.
&&
.CLIMATE...
Record Maximum Temperatures:
Date: July 9 July 10 July 11
-------------------------------------------------------
Santa Rosa (Co-Op) 100,2002 103,1961 113,1913
Kentfield 104,1985 104,1961 112,1913
Napa (Hospital) 106,1961 108,1961 106,1961
Richmond 92,1985 91,1959 94,1983
Livermore (Co-Op) 108,1905 109,2002 111,1961
San Francisco (Dtown) 90,1985 92,1959 92,1983
San Francisco Airport 94,1959 97,1959 93,1983
Redwood City 103,1961 104,1961 104,1961
Half Moon Bay 72,1985 78,1959 74,1959
Oakland (Dtown) 89,2002 95,1983 98,1983
San Jose Airport 99,2002 97,2002 103,1913
Gilroy 107,2002 109,1961 110,1961
Santa Cruz 99,1985 105,1959 102,1913
Salinas Airport 92,2002 92,1983 87,1934
Monterey (Office) 92,2002 90,1961 90,1983
King City 108,2002 101,1964 107,1983
&&
.MARINE...as of 04:43 PM PDT Friday...Winds remain out of the
northwest and predominantly gusty over the northern waters.
Northwest winds gradually diminish across the southern waters
through Sunday. Seas remain predominately wind driven. This will
be accompanied by weak to moderate period northwest and southerly
swells.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
.Tngt...Excessive Heat Warning...*CAZ507-510>512-516>518*
Heat Advisory...*CAZ506-513*
SCA...Pt Arena to Pt Reyes 0-10 nm
SCA...Pt Arena to Pigeon Pt 10-60 nm
&&
PUBLIC FORECAST: BFG
AVIATION: Diaz
MARINE: Garcia
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www.youtube.com/nwsbayareaThe national radar mosaic for the moment the first event began.
7 excessive heat, 1 wildfire. Local standard time, as filed.
Mesonet (PG&E) station at Ida Clayton Road reported overnight minimum temperature of 87 degrees F.
Parkfield RAWS station reported a high temperature of 112 degrees F and remained above 100 degrees until 8 pm PDT.
Fort Hunter Liggett RAWS station reported a high temperature of 114 degrees and remained in the 80s and 90s well into the night.
Byron ASOS reached a daytime high of 113 degrees F and remained in the 100s until 8 pm PDT. Temperatures were still in the 80s by midnight.
Bradley RAWS station reported a high temperature of 109 degrees and remained in the 80s and 90s well into the night.
Cazadero CWOP station reported high temperature of 109 degrees F.
The Diablo Fire broke out in the morning on the 11th south of Byron. Breezy afternoon winds contributed to the growth of the fire. The fire was contained later that evening https://abc7news.com/byron-fire-contra-costa-county-grass-wildfire/10879543/.
Mesonet (PG&E) station at San Jose Mines Rd had a high temperature of 104 degrees and remained in the 80s and 90s late into the night.
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, July 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: 966974, 966981, 966978, 966983, 966976, 966971, 966985, 966975.