Northern California heat wave may go down as worst in history

The nearly weeklong heat wave blasting the West with record-setting temperatures could go down as the worst hot spell in Northern California for the month of September, and perhaps all time.

While the month is generally one of the state’s warmest, particularly in the Bay Area, the unprecedented heat is heightened by climate change, scientists say. The Earth’s warming has not only raised baseline temperatures but can help lock in extreme weather patterns, like the heat dome currently hanging over the state, and also create a feedback mechanism in which the heat propels drying, which propels more heat.

On Monday, never before seen temperatures were recorded in the East Bay, with Fairfield hitting 117 degrees, while the Sacramento area broke several monthly temperature records, including downtown Sacramento at 113 degrees. Daily records were set across the state.

Forecasters had expected Tuesday to be just as hot, if not hotter — and it delivered. New all-time high temperature records were set in Santa Rosa, 115; Napa, 114; and San Jose, 109, according to the National Weather Service. Livermore, 116, and Redwood City, 110, tied their records.

“This really is an extraordinary event, partly because it’s not over,” said Daniel Swain, a climate scientist at UCLA. “This will be the worst September heat wave on record certainly in Northern California and arguably across the state.”

The hot weather was already creating its share of problems. The state’s power supplies were being stretched, a handful of wildfires were given an unfortunate boost, and just plain misery was setting in for Californians not accustomed to the warmth and sometimes lacking air conditioning.

Swain said as bad as it is, the hot spell pales in comparison to the heat that bore down on the Pacific Northwest in June of last year. Temperatures in Oregon and Washington shot up 5 to 10 degrees above previous record highs. That would be the equivalent of the Sacramento area seeing temperatures in the low 120s, he said.

Still, depending on how the weather plays out over the next few days, it could be California’s worst bout of heat not only for the month but for all time, Swain said.

Of course, evaluating heat waves is not an objective science and depends on how such factors as temperature highs, geographic scope and duration are weighed. The current system, though, is exceptional on a number of levels, scientists say.

Patrick Brown, co-director of the climate and energy team at the Breakthrough Institute in Berkeley, points to the length of the hot spell as remarkable.

Tuesday was the sixth day that temperatures were significantly above average in Northern California, and the National Weather Service was anticipating “excessively hot conditions” through at least Thursday — though temperatures were expected to come down off the extreme highs of Monday and Tuesday.

“You sometimes get this self-reinforcing feedback that causes atmospheric circulation to stay here and sit for a long time, but it’s unusual for the system to sit here this long,” Brown said.

Also uncommon, he said, were warm overnight temperatures. Some parts of California, namely inland areas such as the San Joaquin Valley, didn’t drop below the 80s, which means residents aren’t getting reprieve from the heat. This is uncommon given the state’s relatively low humidity, Brown noted, and is the result of the compression of air and winds from the south that have accompanied the system.

High temperatures during the day, however, have not been as extraordinary, Brown said. This has led him to calling the heat wave exceptional for this time of year but not one for the ages.

“We’re not seeing all these all-time record highs fall,” he said. “There are some, but for it to be some really exceptional event, we’d expect to see the record daily highs falling to be widespread.”

Research has generally shown that climate change is contributing about 3 to 5 degrees to heat waves in the modern era.

“If the temperature is an all-time record, it doesn’t mean it couldn’t have happened without climate change, but there are boundaries with temperature and we’re moving those boundaries,” said Michael Wehner, a climate scientist at Lawrence Berkeley National Laboratory, who has helped pioneer the field of climate attribution. “We’re reaching the point, and we will soon get to that point, that some heat waves couldn’t have happened without climate change.”

Even if this heat wave doesn’t go down as the worst in Northern California history, Wehner said that such a bout of hot weather has a much higher likelihood of happening now than it ever has before.

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