The wind blows in Los Cerritos community in La Paz, Baja California State, Mexico, during the passage of Hurricane Hilary on Saturday.

Alfredo Estrella /AFP via Getty Images

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Alfredo Estrella /AFP via Getty Images

The wind blows in Los Cerritos community in La Paz, Baja California State, Mexico, during the passage of Hurricane Hilary on Saturday.

Alfredo Estrella /AFP via Getty Images

Tropical Storm Hilary is approaching Southern California and parts of the Southwest, bringing fierce winds and historic rainfall to areas that have not seen tropical storm conditions in more than 80 years.

Hilary, which was downgraded from a hurricane on Sunday, made landfall over the northern Baja California peninsula in Mexico. At least one person died of drowning in the Mexican town of Santa Rosalia amid the storm. Mexico’s hurricane watch has ended, but the Baja California coast is still under threat of flash floods.

As as of 11 a.m. local time on Sunday, the storm was about 215 miles south-southeast of San Diego and traveling at 25 miles per hour. It is expected to sweep across Southern California by the afternoon, according to the National Weather Service.

Hilary is expected to remain a tropical storm before it reaches Southern California — meaning wind speeds are expected to be between 39 to 73 miles per hour. Along with those winds, forecasters warn the heavy rain associated with the system will pose serious threat of “catastrophic and life-threatening flooding” to the region.

Portions of Southern California and southern Nevada are expected to average between 3 to 6 inches of rainfall — but could receive up to 10 inches. Some parts will likely accumulate more rain in just a matter of hours than they typically do in an entire year, forecasters said. Winds will also be particularly strong and gusty on elevated terrain.

The heaviest rainfall is expected to pour overnight

The downpour is expected to last in Southern California until early Monday morning.

Areas in eastern San Diego, northern Los Angeles, San Bernardino, Riverside and Death Valley have the greatest risk for flash flooding, according to Michael Brennan, the director of the National Hurricane Center.

“To make it even more dangerous is, the heaviest rainfall and flooding potential is going to largely occur overnight. So you want to be in your safe place certainly no later than this afternoon,” he said during the center’s livestream on Sunday morning.

Brennan warned that flooding conditions like washed out roads are more difficult to see at night. That will be especially dangerous on major highways like I-10, I-8 and I-40, which are expected to be impacted by the intense downpour.

A majority Nevada, southwestern Utah and western Arizona will also be at risk of flooding, Brennan said. There is also a risk of tornadoes in parts of the Mojave Desert, Lower Colorado River Valley and southeastern California.

The last time Southern California was hit by a storm this strong was 1939

Much of Southern California is under its first-ever tropical storm warning, given that the region is most frequented by disasters like wildfires and earthquakes. Meteorologists say the last time a storm of this strength hit Southern California was back in 1939.

On Saturday night, California Gov. Gavin Newsom issued a state of emergency for several counties, including Fresno, Imperial, Inyo, Kern, Los Angeles, Riverside, San Diego, San Bernardino, Tulare, Orange and Ventura. Some of those communities, like parts of San Bernardino County, have already received evacuation orders.

The Flood Operations Center, Cal Fire and the California National Guard are on standby with water vehicles and water rescue teams amid flood threats. State officials also urged residents to sign up for flood and evacuation alerts from their counties, as well as prepare their pets and family in case they need to evacuate.

NPR’s Julia Simon contributed reporting.