MEXICO CITY • Hurricane Hilary grew rapidly into Category 4 strength off Mexico’s Pacific coast on Friday and could reach Southern California as the first tropical storm there in 84 years, which forecasters warned could cause extreme flooding, mudslides and even tornados.

Hilary had sustained winds near 145 mph early Friday, and was expected to strengthen a bit more before starting to weaken. Nevertheless, it was forecast to still be a hurricane when approaching Mexico’s Baja California peninsula on Saturday night, and a tropical storm when approaching Southern California on Sunday.

The National Hurricane Center on Friday issued its first ever tropical storm watch for much of Southern California, covering a wide swath of the region from the coast to the interior mountains and deserts.

No tropical storm has made landfall in Southern California since Sept. 25, 1939, according to the National Weather Service. The watch warned of numerous potential threats to life and property including extreme flooding, mudslides and tornados.

The Mexican government said a weakened Hilary might skim a sparsely populated area on the western edge of the Baja peninsula early Sunday, and then perhaps hit between the Pacific coast cities of Ensenada, and Playas de Rosarito, a beach community on the edge of Tijuana.

Tijuana Mayor Montserrat Caballero Ramirez said the city was tracking the storm closely, clearing out storm drains.

The sprawling border metropolis of 1.9 million is particularly at risk of landslides and flooding, in part because of its hilly terrain. Shacks are perched on cliffs with little vegetation to hold the land in place. In addition, scores of people have been living under tarps on the streets and in canals in flood zones, including migrants who arrive daily from various parts of the world.

The city was in the process Friday of opening four shelters in high-risk zones and was prepared to increase that if needed, once the storm hits, Caballero Ramirez said. City workers also cleared 34 storm drains of trash and debris, and were going to neighborhoods to warn residents.

“We are a vulnerable city being on one of the most visited borders in the world and because of our landscape,” she said. “We don’t know the magnitude yet of what can happen but that’s why we are coordinating with the military and others so we can move immediately with our emergency response protocols if needed.”

Mexico extended its hurricane watches and warnings northward for parts of Baja California peninsula, and also issued a tropical storm watch for parts of mainland Mexico. Some 18,000 soldiers were put on alert.

Early Friday, Hilary was centered about 360 miles south-southwest of Los Cabos on the southern tip of the Baja peninsula. It was moving northwest at 10 mph, and was expected to turn further toward the north.

In La Paz, the picturesque state capital of Baja California Sur on the Sea of Cortez, a steady rain fell Friday and police patrolled closed beaches to keep swimmers out of the whipped-up surf. Schools were shut down in five municipalities.

Near the tip of the peninsula in Cabo San Lucas, some schools were being prepared to be temporary shelters Friday, said Flora Aguilar, a city official.

Elsewhere in the state, command centers were being set up and heavy machinery put in place in case of landslides.

It was increasingly likely that Hilary would reach southernmost California early Monday while still at tropical storm strength, though widespread rain was expected to begin as early as Saturday, the National Weather Service’s San Diego office said.

Hurricane officials said the storm could bring heavy rainfall to the Southwestern United States that could dump 3 to 6 inches in parts with isolated amounts of up to 10 inches to portions of southern California and southern Nevada, hitting large desert areas unaccustomed to much rain.

“Two to three inches of rainfall in Southern California is unheard of” for this time of year, said University of Albany atmospheric scientist Kristen Corbosiero, who specializes in Pacific hurricanes. “That’s a whole summer and fall amount of rain coming in probably 6 to 12 hours.”