MEXICO CITY (AP) — Hurricane Hilary grew rapidly to Category 4 strength off Mexico’s Pacific coast on Friday and could reach Southern California as the first tropical storm there in 84 years, causing “significant and rare impacts” including extensive flooding.

The U.S. National Hurricane Center said Hilary had sustained winds near 145 mph (230 kph) at 6 a.m. and was expected to continue its rapid intensification through Friday before starting to weaken. It will nevertheless still be a hurricane when it approaches Mexico’s Baja California peninsula on Saturday night, and will approach Southern California on Sunday as a tropical storm.

No tropical storm has made landfall in Southern California since Sept. 25, 1939, according to the National Weather Service.

Early Friday, Hilary was centered about 400 miles (640 kilometers) south of Los Cabos on the southern tip of the Baja peninsula. It was moving west-northwest at 13 mph (20 kph), but was expected to turn gradually toward the north through Saturday.

The Mexican government extended its hurricane watch and tropical storm warning northward for parts of Baja California Sur state, and also issued a tropical storm watch for parts of mainland Mexico.

“Heavy rainfall in association with Hilary is expected to impact the Southwestern United States through next Wednesday, peaking on Sunday and Monday,” the hurricane center said. It said there was a substantial danger of flash flooding in an area stretching from San Diego to Las Vegas.

“Rainfall amounts of 3 to 6 inches, with isolated amounts of 10 inches, are expected across portions of southern California and southern Nevada, which would lead to significant and rare impacts. Elsewhere across portions of the Western United States, rainfall totals of 1 to 3 inches are expected.”

SpaceX delayed the launch of a satellite-carrying rocket from a base on California’s central coast until at least Monday. The company said conditions in the Pacific could make it difficult for a ship to recover the rocket booster.

The Mexican government said a weakened Hilary might hit the coast Sunday night between the cities of Playas de Rosarito and Ensenada, in Baja California state.

Meanwhile, the city of Yuma was preparing Thursday by providing residents with a self-serve sandbag filling station.

The sandbag station will be stocked with sand and empty bags for self-filling while supplies last. Residents were allowed five sandbags per vehicle.