Southern California is in the middle of a late-season heatwave, with the National Weather Service issuing long-duration heat advisories that it expects to run for at least another week.
Temperatures are forecast to hover near 100F in downtown Los Angeles and to spike well into the triple digits farther inland, raising the risk of wildfires.
“We're expecting temperatures will run some 15-20 degrees above normal – that's pretty significant,” said Richard Bann, a meteorologist with the NWS's Weather Prediction Center. His advice was brief: “Stay cool. Stay hydrated. Limit exposure to the sun.”
The sharpest figure came from V Kelly Turner, a professor of geography and urban planning at UCLA. “The extremely hot temperature Friday is rare,” she said. “Nearly 7F hotter than the hottest October 2 ever and about 30 degrees higher than historical averages around 75F.”
October heatwaves are not unusual in southern California. They are caused by ridges of high air pressure that park themselves above the region. This year they are also being influenced by an ongoing marine heatwave, which is in turn driving a strong El Niño, a year of above-average surface-water temperature in the Pacific.
There is one piece of good fortune. Current forecasts do not call for the strong winds that would fuel and spread wildfires, although the heat and low soil moisture still make them more likely.
Attribution of any single event is difficult, said David Neelin, an atmospheric physicist at UCLA who researches climate change. The direction of travel is not. “Overall, it's hard to escape that a warmer climate adds to the strength of heatwaves,” he said. “It adds to the probability of heatwaves and the likelihood of getting longer-duration heatwaves.”
The timing is awkward for the region's calendar of outdoor events, including the first game of the National League division series between the Dodgers and the Atlanta Braves on Saturday, as Edith de Guzman, a UCLA researcher on heat adaptation, noted. She said providing more shade was the “most effective way to keep people safe”.
“Climate change is making longer-duration heatwaves possible, while simultaneously increasing their frequency and intensity,” de Guzman wrote. “So we should prepare for more of this in the future.”
Southern Californians spent the summer wondering what had happened to their famed Mediterranean climate. The autumn has now provided an answer, and it is more precise than they may have wanted.
Consider what Professor Turner's figure actually records. Records of this kind are held by dates, not by weather: the hottest 2 October ever was, by construction, a 2 October. Friday beat it by nearly seven degrees, which means the new record for the date was set by a day behaving like a different one.
The gap from the average is the clearer measure. The date's historical average is around 75F. Friday was about 30 degrees above it. A day that misses its own average by thirty degrees is not a warm 2 October. It is a correctly performed day from somewhere else in the year, filed under the wrong heading.
The regional climate is classified as Mediterranean, a category defined by a hot dry summer and a mild wet winter. Southern California has now had the hot dry part twice this year.
This newspaper does not dispute the classification, which remains accurate in every particular. We observe only that the definition specifies the character of the seasons and is silent on how many of each a region is permitted.
The Department of Unnecessary Research draws attention to the wording of the advisory itself, which is issued for a long duration. The duration in question is the portion of autumn currently occupied by summer.
The National Weather Service expects the heat to last at least a week. The calendar expects October. Only one of the two is issuing advisories.