Busy California airports used by millions of travelers could face flooding risks during the upcoming holiday season thanks to sea level rise and a record-breaking El Niño climate pattern , a new report warns.

While recent levee improvements have reduced the risk at Oakland International Airport, San Francisco International Airport - built on marshland - is vulnerable to flooding of 3 feet or more. That could ground flights, with takeoffs not recommended when standing water on the runway exceeds 0.5 inches.

"It doesn't mean it's certainly going to happen because it's all about timing," Fraser Shilling, director of the University of California-Davis Road Ecology Center, told KGO on Wednesday .

But with this week’s record tides seen up and down the California coast, experts fear much worse flooding is on the way.

"This was just like a preview. This is a trailer," Shilling said.

The holiday travel season could see flooding at California airports, a new report warns.…