California's rainy season has been pushed forward by a month

California’s rainy season has shifted from November to December, prolonging the destructive wildfire season by nearly a month. This transition has taken place over the last six decades, according to a new study from the American Geophysical Union. 

The research does not definitively link the shifting rainy season to climate change. However, the results are consistent with climate models that predict warmer and drier autumn conditions in California.

Wildfires are the biggest hazard during California’s dry season from May to October. The start of the rainy season in November alleviates the threat of wildfire by making the plants too moist to ignite.

In recent decades, the rainy season has been arriving progressively later. Furthermore, the rainy seasom is expected to become shorter as a result of global warming

For the current investigation, the researchers analyzed rainfall and weather data in California over the past six decades. They determined that the official onset of California’s rainy season is 27 days later than it was in the 1960s. When the rain does finally arrive, the majority of it is limited to the months of January and February.

Study lead author Jelena Lukovi a climate scientist at the University of Belgrade in Serbia.

“What we’ve shown is that it will not happen in the future, it’s happening already,” said Lukovi. “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 findings suggest that California’s wildfire season will last even longer in the years to come. The state’s worst-ever wildfire season was in 2020. when nearly 10,000 fires burned more than 4.2 million acres of land.

A longer dry season will create more overlap between wildfire season and the inundation of Santa Ana winds that bring hot, dry weather to California in the fall. 

“It’s not just a matter of making the vegetation drier and keeping all else equal,” said UCLA climate scientist Daniel Swain. “You’re also increasing the number of opportunities for extremely dry vegetation and extremely strong offshore winds to coincide.”

The study is published in the journal Geophysical Research Letters.

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By Chrissy Sexton, Earth.com Staff Writer