The powerful storm is threatening parts of Mexico and the southwestern US with “significant flooding” and prompted the country’s National Hurricane Center (NHC) to issue its first-ever tropical storm watch for California.

NHC’s deputy director Jamie Rhome said although cold waters off California’s coastline usually weaken hurricanes and tropical storms, “this system is expected to hold onto to its strength because it will be moving fast.”

Hilary was moving west-northwest at nearly 17kph, packing maximum sustained winds of nearly 230kph, after being upgraded to a Category 4 overnight, the NHC said in its latest advisory.

Hilary is expected to approach the west coast of Mexico’s Baja California as a hurricane this weekend but weaken to a tropical storm before hitting the US state on Sunday afternoon.

Rhome said California and southern Nevada faced risks from severe flooding caused by up to 10 inches of rainfall.

A storm surge could cause coastal flooding and destructive waves along the Baja California peninsula, the NHC said.

Hilary’s heavy rainfall is expected to hit California, Nevada, and neighbouring Arizona following a record-breaking summer heatwave.

Phoenix, Arizona, endured a month-long stretch of temperatures exceeding 43 °Celsius throughout July, according to the National Weather Service, trapped under a “heat dome” of stagnant air.

In California’s Death Valley desert, temperatures hit 53°C in mid-July, among the highest temperatures recorded on Earth in the past 90 years, while tens of millions of Americans were put under heat alerts.

Before the heatwave, about a dozen atmospheric rivers had battered southern California, its central coast, and agricultural heartland, causing flooding, landslides, and road closures as authorities ordered thousands to evacuate.