San Francisco, where even the postcards feature fog, rarely sees temperatures over 90 degrees. Even more rarely does the coastal California city see such temperatures in October.
But on Tuesday, San Francisco hit 94 degrees.
The heat has prompted officials to issue an excessive heat warning through Thursday and to offer free pool admission in the city. In some inland areas, temperatures are soaring above 100 degrees.
This is a phenomenon known locally as “second summer,” said Dylan Flynn, a meteorologist at the National Weather Service in California, who noted that early fall was generally when San Francisco sees its highest temperatures. “But this one is more extreme,” Mr. Flynn said in a phone interview.
“It’s very rare to be in the 90s at all in San Francisco. But in October, it’s even more rare,” he added. “The last time we had 94 all the way in October was 2012.”
The worst of the heat is expected to last through Thursday, but high temperatures are expected through the weekend, said Mr. Flynn, who described the heat wave as the sort of “long duration” event that can raise the risk of illness, or even cause heat-related deaths.
Inland from San Francisco, temperatures have been even higher this week. The city of Santa Rosa hit 102 degrees on Tuesday, and San Rafael hit 105. In San Jose, it was 100 degrees. All three cities broke their previous records for Oct. 1, which were set during a heat wave in 1980, according to the National Weather Service.
The cause of this week’s heat is offshore winds, which are blowing from inland out to coastal areas, said Mr. Flynn, the meteorologist. Typically, San Francisco stays cool even when nearby inland cities like Sacramento are broiling, because of the cool Pacific and the breeze blowing in off the ocean, or what’s known as the marine layer.
“The only way for us to get these high temperatures on the coast is to have offshore wind — from inland out to sea,” said Mr. Flynn. “When we have that, that’s much dryer air. So this is a very dry heat,” he said.
Other parts of California have also seen temperatures way above the average for this time of year. And according to officials, the dry heat is introducing another concern, which is the heightened risk of fire in a season in which several major wildfires have scorched a total of a million acres in the state this year.
The National Weather Service measures the dryness of potential wildfire fuel, “and we’re basically as dry as it gets,” said Mr. Flynn.
Amid rising temperatures in Southern California, the Line fire, which has burned for nearly a month in San Bernardino County, flared over the weekend, when officials ordered new evacuations.
And in Shasta County, near the Oregon border, Pacific Gas & Electric warned it would shut off power to thousands this week, citing the danger of fire amid high winds and low humidity. In recent years, utility equipment in California has sparked fires in dry conditions.
In addition to the greater risk of fire have come the usual concerns that heat waves raise, specifically in parts of California like the Bay Area, where fewer people have air conditioning. Some coastal cities are not prepared for 90-plus degree days, and officials urged people to check on their older and disabled neighbors.
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