Hurricane Hilary hurtles toward Mexico and prompts a tropical storm watch in California

Category 4 Hurricane Hilary hurtled towardMexico's Baja California peninsula on Friday, a U.S. government agency said, as it issued a tropical storm watch for California and warned of life-threatening and possibly catastrophic floods.

The U.S. National Hurricane Center (NHC) expects the powerful storm to near Mexico's popular Cabo San Lucas resort city by late on Friday, though it should weaken before hitting the U.S. West Coast this weekend, nevertheless bringing dangerous rains.

"Life-threatening and potentially catastrophic flooding are likely over much of Baja California and Southern California this weekend and early next week," the Miami-based agency said in its latest advisory.

Mexico's Baja California peninsula spans two states.

The northernmost one cancelled non-essential public activities on Friday, including school classes through Monday, and authorities in Mexico's second-largest city, Tijuana, urged people in high-risk zones to move to temporary shelters.

In the peninsula's southern state, authorities postponed a local baseball match and said ports would be closed through late Friday.

"Without being alarmist, we must all take precautions and stock up on water and basic necessities at home, without resorting to panic buying," the state's governor said.

NHC Deputy Director Jamie Rhome warned of flood risks from San Diego to Los Angeles and Las Vegas, with particularly high risks around the Palm Springs area as the storm hits at the end the weekend.

"If you've got weekend plans, it's probably time to start altering those plans," he said.

Although cold waters off California's coastline usually weaken hurricanes and tropical storms, Rhome said "this system is expected to hold onto to its strength because it will be moving fast."

Upgraded overnight

Hilary was moving west-northwest at nearly 19 km/h, packing maximum sustained winds of nearly 215 km/h, after being upgraded to a Category 4 overnight, it said.

Rhome said California and southern Nevada faced risks from severe flooding caused by up to 25.4 centimetres of rainfall, while the agency warned a storm surge could cause coastal flooding and destructive waves along Mexico's Baja California peninsula.

Heavy rainfall is expected to hit California, Nevada and neighbouring Arizona following a record-breaking summer heat wave.

Phoenix endured a month-long stretch of temperatures exceeding 43 Cthroughout July, according to the U.S. National Weather Service, trapped under "heat dome" of stagnant air.

In California's Death Valley desert, temperatures hit 53 Cin 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 heat wave, 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.