Hurricane Hilary, a large and powerful Category 4 storm, was on Friday afternoon barreling toward the Baja California Peninsula and the Southwestern United States, where it may cause “life-threatening and potentially catastrophic flooding,” meteorologists said.

The National Hurricane Center issued its first-ever tropical storm watch for Southern California, including downtown Los Angeles, on Friday.

A watch means that tropical conditions are possible within the area over the next 48 hours. The watch stretches from the California-Mexico border to the Orange County and Los Angeles County line and for Catalina Island, forecasters said.

The system had sustained winds near 130 miles per hour, according to the National Hurricane Center.Tropical disturbances that have sustained winds of 39 m.p.h. earn a name. Once winds reach 74 m.p.h., a storm becomes a hurricane, and, at 111 m.p.h., it becomes a major hurricane.

Hilary will bring up to six inches of rain, with isolated amounts up to 10 inches, across portions of the Baja California Peninsula through Sunday night, with the possibility of flash flooding.

Portions of Southern California and Southern Nevada will record similar rainfall totals through Tuesday morning, which could lead to “dangerous and locally catastrophic flooding,” forecasters said.

flood watch was issued for much of Southern California, including Los Angeles, Riverside, Orange, San Bernardino, San Diego and Ventura Counties. Other areas across the West can expect a few inches of rain.

Forecasters said that strong winds would occur ahead of the storm’s center.

Residents in Southern California raced to prepare sandbags and fill generators ahead of Hilary’s arrival as emergency officials prepared evacuation centers. Some expressed particular concern about the impacts to the mountain and desert regions.

The National Weather Service said runoff could “rage down valleys while increasing susceptibility to rockslides and mudslides.”

The Eastern Pacific hurricane season has been active this summer, but most of these recent storms have tracked west toward Hawaii, including Hurricane Dora, which helped enhance extreme winds that led to the devastating wildfires on Maui.

It is “exceedingly rare” for a tropical storm to come off the ocean and make landfall in California, said Stefanie Sullivan, a forecaster with the National Weather Service in San Diego. The only tropical cyclone to truly make landfall in Southern California was an unnamed storm in 1939 that made landfall in Long Beach, she said.

However, storms have come close or weakened before coming ashore, still causing flooding and dangerous winds, like Kay, a post-tropical cyclone, last year. Sometimes storms even move across the state from Mexico; in 1997, Hurricane Nora made landfall in Baja California before moving inland and reaching Arizona as a tropical storm.

Hurricane season in the Eastern Pacific began on May 15, two weeks before the Atlantic season started. The seasons run until Nov. 30.

Complicating things in the Pacific this year is the development of El Niño, the intermittent, large-scale weather pattern that can have wide-ranging effects on weather around the world.

An average Eastern Pacific hurricane season has 15 named storms, eight hurricanes and four major hurricanes. The Central Pacific typically has four or five named storms that develop or move across the basin annually.

There is solid consensus among scientists that hurricanes are becoming more powerful because of climate change. Although there might not be more named storms overall, the likelihood of major hurricanes is increasing.