Thursday will feature the first triple digit readings of the year for parts of the Bay Area.
Baron/Lynx
Thursday will be the Bay Area’s hottest day of the year so far and, in San Francisco, likely the hottest since this year’s historic March heat wave that brought the city its first 90-degree March day on record.
A heat advisory begins at noon Thursday across much of the Bay Area. For the San Francisco Bay shoreline and North Bay, the advisory runs through Thursday evening. Farther inland, including parts of the East Bay and Santa Clara Valley, heat concerns may linger into Friday.
Much of San Francisco will warm into the low to mid-80s Thursday, with the warmth spreading across the bay toward Oakland and Berkeley. Inland valleys will run 15 to 30 degrees above normal, with the first triple-digit readings of the year likely in parts of the North Bay, East Bay and South Bay.
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The hottest spots will include Concord, Walnut Creek, Antioch, Livermore, Santa Rosa and portions of southern Santa Clara County, where highs may reach the upper 90s to around 105 degrees. A handful of climate stations may flirt with or exceed daily records for June 11.
This will not be a long-duration heat wave for everyone. But it also may not end at the same time everywhere.
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Along the coast in San Francisco and parts of the North Bay, temperatures should turn cooler Friday, though still above normal, as a weak sea breeze begins to return. But farther inland, especially in the East Bay and South Bay, the marine push may be too weak to bring meaningful relief right away. That means some inland communities should stay hot Friday and possibly remain well above normal into the weekend.
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A tense overnight fire setup in the hills
Early this morning, the Bay Area will face an isolated but potentially serious fire-weather threat in the hills.
A red flag warning is in effect from 11 p.m. Wednesday to 9 a.m. Thursday for the North Bay interior mountains and East Bay hills, where warm overnight temperatures, very low humidity and gusty offshore winds will overlap for several hours.
Red flag warnings are in place for the East Bay and North Bay hills from Wednesday night into Thursday morning. Warm temperatures, low humidity values and wind gusts up to 60 mph will create a brief period of critical fire weather in some spots.
Baron/Lynx
In those areas, particularly above 1,500 feet in elevation, temperatures are likely to stay above 70 degrees overnight Wednesday into Thursday morning, with relative humidity levels in the 20% to 30% range. Those conditions combined with gusts from 30 to 40 mph will create a period of elevated fire weather risk. Mount St. Helena may see wind gusts in excess of 60 mph.
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The highest-risk window will be over by 4 a.m. Thursday morning, but some risk will remain into the early morning. While winds are expected to ease after sunrise, humidity will remain critically low through Thursday afternoon, especially across interior parts of the Bay Area.
Why it’s hot, and why the cooldown will be uneven
The culprit is a strong but transient ridge of high pressure centered off the Pacific Northwest coast. The Bay Area sits on the southeast side of that ridge, where dry offshore wind flow pushes the marine layer offshore and allows temperatures to rise, even near the water.
The offshore high pressure ridge (dark red) will shift north towards the Pacific Northwest and British Columbia over the weekend. S.F. and the bayshore should cool down, but interior locations will remain hot.
WxBell
The ridge will shift north toward British Columbia by Friday, which should open the door for some onshore flow. But forecast models are not showing a strong marine surge. Instead, relief may arrive first along the coast and near San Francisco Bay, while inland valleys remain largely cut off from the cooling influence of the Pacific.
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That makes Friday’s temperature forecast more varied. San Francisco and areas near the bayshore should see temperatures fall back into the 70s. But inland areas like Walnut Creek, Livermore, Antioch and San Jose will still see daytime highs in the mid-90s.
Dangerous beach conditions
The heat will likely send more people toward the coast Thursday. But the ocean will not be calm just because the weather is warm.
A long-period southerly swell will continue to bring an increased risk of sneaker waves and strong rip currents to south- and southwest-facing beaches. The highest concern includes the North Bay coast, San Mateo County coast and Santa Cruz County beaches.
A beach hazards advisory statement is set to expire at 5 a.m. Thursday for the coastal North Bay including Point Reyes, the San Francisco Peninsula coast and northern Monterey Bay, but elevated rip current and sneaker wave risk will linger throughout the day.
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And long-period swells will keep the risk of sneaker waves and strong rip currents in place into the weekend.
Thursday breakdown
San Francisco: Thursday will be hot everywhere in the city. The marine layer will retreat back over the ocean and temperatures will quickly heat up. Highs will top out in the low to mid-80s downtown, in the Mission, SoMa and Mission Bay. A few eastern neighborhoods could briefly push into the upper 80s. Ocean Beach and the Richmond district should see highs in the upper 70s to low 80s. Overnight lows will stay warm, mostly in the upper 50s to mid-60s.
North Bay: The interior North Bay will be toasty on Thursday, with some daily records being challenged. The hills will start the day under elevated fire danger after a windy and dry night, especially in the North Bay interior mountains. Winds should ease after sunrise, but humidity will remain very low through the afternoon. Santa Rosa, Novato, Vacaville and Fairfield should all hit triple digits. Napa, San Rafael and Petaluma should see temperatures in the upper 90s. It should stay clear and warm overnight with lows ranging in the upper 50s in valleys to the 60s and low 70s in the hills.
East Bay: The interior East Bay will have some of the hottest weather in the region on Thursday. Concord, Walnut Creek, Pleasanton, Livermore and Antioch will likely reach the low 100s, with a few spots pushing higher. Oakland, Alameda, Berkeley and Richmond will warm into the mid-80s, with warmer pockets in the Berkeley foothills and away from the bay. The East Bay hills will also be under a red flag warning through Thursday morning, with gusty northeast winds possible overnight before winds ease during the day. Winds won’t be an issue overnight, but it will be warm again in the hills with lows in the 70s. Elsewhere, temperatures will fall back into the low 60s.
Pacific Coast and Peninsula: The marine layer should be weaker than usual on Thursday, allowing Pacifica, Daly City and Half Moon Bay to warm into the upper 70s to around 80 degrees. Across the Peninsula, South San Francisco, Burlingame and San Mateo should see highs in the mid to upper 80s. Redwood City should spike into the mid-90s. Beach hazards will be a concern along the San Mateo County coast, with sneaker waves and rip currents possible. Overnight lows will be mild, mostly in the upper 50s to 60s.
South Bay and Santa Cruz: San Jose, Mountain View, Sunnyvale and Cupertino should climb well into the 90s on Thursday, while Morgan Hill and Gilroy will likely top 100 degrees. Santa Cruz will be cooler, but still warm with highs in the mid-80s. The Santa Cruz Mountains will be warm and dry, with elevated fire concerns lingering through the morning. It will be clear and warm overnight with lows in the 60s.