Crews worked to dig roads, buildings and care home residents out of the mud across a wide swath of the southwestern United States desert, as the first tropical storm to hit Southern California in 84 years headed north, prompting flood watches and warnings in half a dozen states.

The National Hurricane Center in Miami said on Monday that Tropical Storm Hilary had lost much of its force as it headed to the Rocky Mountains, but warned that “continued life-threatening and locally catastrophic flooding” was expected in parts of the region.

Forecasters said the threat for flooding in states farther north on Monday was highest across much of southeastern Oregon into the west-central mountains of Idaho, with potential thunderstorms and localised torrential rains on Tuesday.

As Hilary moved east into the neighbouring state of Nevada, flooding was reported, power was out and a boil-water order was issued for about 400 households in the Mount Charleston area, where the only road in and out was washed out. The area is about 40 miles (64 kilometres) west of Las Vegas.

Hilary first slammed into Mexico’s arid Baja California peninsula as a hurricane, causing one death and widespread flooding before becoming a tropical storm. So far, no deaths, serious injuries or extreme damage have been reported in California, though officials warned that risks remain, especially in the mountainous regions where the wet hillsides could unleash mudslides.

Hilary is the latest potentially climate-related disaster to wreak havoc across the US, Canada and Mexico. Hawaii’s island of Maui is still reeling from a blaze that killed more than 100 people, making it the deadliest US wildfire in more than a century. Firefighters in Canada are battling that nation’s worst fire season on record.

Hilary shattered daily rain records in San Diego and likely dumped the equivalent of a full year’s worth on Death Valley National Park, forcing the park to be closed indefinitely and leaving about 400 people sheltering at Furnace Creek, Stovepipe Wells and Panamint Springs until roads could be made passable, park officials said.

A tropical storm last roared into California in September 1939, ripping apart train tracks, tearing houses from their foundations and capsizing many boats. Nearly 100 people were killed on land and at sea.

Southern Texas was also preparing for the arrival of a separate tropical system that was expected to bring badly needed rain but also possible flooding. The National Hurricane Center said tropical storm conditions could arrive in coastal areas by early Tuesday, including near the US-Mexico border, where some residents grabbed sandbags in preparation.

In the Caribbean, meanwhile, Tropical Storm Franklin churned on Monday near Haiti and the Dominican Republic.