Tropical Storm Hilary made its historic arrival in California on Sunday, with forecasters warning of possibly catastrophic flooding after the system pummeled Mexico's Baja California peninsula with heavy rains.

One person died in Mexico amid reports of flash flooding in the peninsula, where some roads were swept away and images on social media showed raging torrents gushing down city streets turned into rivers.

California Gov. Gavin Newsom declared a state of emergency for much of Southern California, with flash flood warnings in effect throughout a region more accustomed to drought.