Tropical Storm Hilary has made landfall in Southern California, where at least nine million people are under flash-flood warnings.

“Preparations for flooding impacts across the southwestern US should be completed with the peak intensity of heavy rainfall expected later today through early Monday morning,” the US National Hurricane Center said in an advisory on Sunday.

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It is the state’s first tropical storm in 84 years and has already been described as an “unprecedented weather event” by Los Angeles Mayor Karen Bass.

Hilary has so far brought strong winds of 70mph (119km/h) to Mexico’s Baja California peninsula where one man died while trying to cross a stream in his car, the BBC reported.

Despite earlier stripping the storm of “hurricane” status, US meteorologists have warned of the possibility of “life-threatening” flooding.

“The potentially historic amount of rainfall is expected to cause life-threatening to locally catastrophic flash, urban, and arroyo flooding including landslides, mudslides, and debris flows through early Monday morning. Localised flooding impacts, some significant, are also expected across northern portions of the Intermountain West,” the agency said in the advisory.

“Large swells generated by Hilary will affect portions of the Baja California Peninsula and southern California over the next day or so. These swells are likely to cause life-threatening surf and rip current conditions,” it added.

Recent abnormal weather events that have plagued the US – and other countries – have been influenced by human-caused climate change, the British news broadcaster reported, citing experts.