California’s Rainy Season Now Starts a Month Later Than It Used To

The start of California’s rainy season has been getting progressively later in recent decades, and now begins a month after it did just 60 years ago, shifting from November to December, according to a new study published in the journal Geophysical Research Letters. Scientists say the delay in the start of the rain has prolonged the state’s wildfire season and exacerbated water shortages.

Last year was California’s worst wildfire season on record, with nearly 10,000 fires burning more than 4.2 million acres.

“What we’ve shown is that it will not happen in the future, it’s happening already,” Jelena Luković, a climate scientist at the University of Belgrade in Serbia and lead author of the new study, said in a statement. “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 delay in rain is likely the result of changes in atmospheric circulation patterns, which are bringing more rain to Washington and Oregon while leaving California dry for longer. The trend is projected to continue. The extension of the dry season increases the overlap of California’s wildfire season and the influx of the Santa Ana winds, which bring hot, dry weather to the region in the fall and can fuel more intense fires.

The study did not directly link the rainy season shift to climate change, but the authors argue that the results are consistent with climate models that predict drier autumns for California in a warming climate.

“It’s not just a matter of making the vegetation drier and keeping all else equal,” Daniel Swain, a climate scientist at the University of California Los Angeles who was not involved in the study, told the American Geophysical Union about the new findings. “You’re also increasing the number of opportunities for extremely dry vegetation and extremely strong offshore winds to coincide.”