Rainy season begins a month later now than in 1960
California’s annual rainy season now is starting nearly a month later than it did 60 years ago, a new study published Thursday revealed, an ominous trend that is making the wildfire season longer.
Historically, November has been a wet month that usually ended the wildfire risk across the state. But increasingly it is dry, creating conditions that worsen the risk of massive late-season fires such as the Camp Fire in November 2018 that killed 85 people and destroyed the town of Paradise or the Thomas Fire in December 2017 that burned more than 280,000 acres in Santa Barbara and
Ventura counties, leading to a mudslide the following month that killed 23 people.
The start of the state’s winter rainy season now is 27 days later than it was in 1960, according to the study that was published in Geophysical Research Letters, a journal of the American Geophysical Union.
“What we’ve shown is that it will not happen in the future, it’s happening already,” said Jelena Lukovi, a climate scientist at the University of Belgrade in Serbia and lead author of the new study. “The onset of the rainy season has been progressively delayed since the 1960s, and as a result the precipitation season has become shorter and sharper in California.”
The study, co-authored by John Chiang, a professor of Geography at UC Berkeley, analyzed daily and monthly precipitation data from 407 National Weather Service weather stations across the state from 1960 to 2019. It is the first to confirm what many scientists, and fire fighters, have believed was happening.
“There was a time when the public could essentially let their guard down from wildfire,” said Isaac Sanchez, a battalion chief with CalFire, the state’s primary firefighting agency. “But that time doesn’t exist anymore, especially in certain parts of the state.”
The change, believed to be related to the warming climate, means that trees, brush and grasses are dried out and prone to burning for more months each year.
It also means that those dry November conditions are occurring during a time of year when major wind events — Santa Ana winds
in Southern California and Diablo winds in Northern California — typically occur, which can quickly help spread wildfires.
“Already dry vegetation becomes that much drier,” said Daniel Swain, an atmospheric scientist at UCLA. “The level of dryness really does dictate the kind of fires you see — how hot they burn, how quickly the winds can push them. It’s not just that we are adding a month to fire season, we are adding a month to the worst part of fire season.”
The study found that
the total amount of precipitation in California hasn’t been dropping. It’s been compressed into a narrower time frame, mostly between December and March.
That study’s findings are consistent with climate change computer models that have been predicting in recent years that as the Earth continues to warm, California will be likely to see more extreme swings in weather, including sharper, deeper droughts from hotter temperatures, and wetter, more soaking winter storms because warmer conditions increase the amount of water vapor that storms carry.
The Earth’s average global surface temperature has risen 2.16 degrees Fahrenheit since the late 19th century, largely due to climate change from the burning of fossil fuels. Last year was the hottest year recorded since 1880, when modern temperature records began. The seven hottest years on Earth in the past 140 years all have occurred since 2014. And the 10 hottest have come since 2005, according to scientists at NASA and NOAA, the parent agency of the National Weather Service.
What has happened over the last 60 years to make for a drier autumn in California is essentially that summer weather patterns over the Pacific Ocean have persisted for longer into the year. Ridges of high pressure have remained longer. They can divert the jet stream north, moving storms away from California. When the high pressure ridges break down, storms, like the atmospheric river event that