The ninth atmospheric river in a three-week series of major winter storms was churning through California on Monday, leaving mountain driving dangerous and the flooding risk high near swollen rivers even as the sun came out in some areas.

Heavy snow fell across the Sierra Nevada and the National Weather Service discouraged travel. Interstate 80, a key highway from the San Francisco Bay Area to Lake Tahoe ski resorts, reopened with chain requirements after periodic weekend closures because of whiteout conditions.