The storm was triggering flash flood warnings in Orange County and in burn areas, and leading to accidents on freeways and surface streets.
Magic Mountain said tickets purchased for Tuesday will remain valid through Dec. 31.
Knott's Berry Farm said its California Marketplace dining and shopping area remains open.
The region's two other major theme parks, Universal Studios Hollywood and Disneyland, remained open.
PARK UPDATE: Due to inclement weather, #SixFlagsMagicMountain will be CLOSED today December 14. All tickets purchased for today will be valid through 12/31/21.— Six Flags Magic Mountain (@SFMagicMountain) December 14, 2021