Robert Brown|Hilary, now a tropical storm, is nearing California from Mexico with punishing rains

Tropical Storm Hilary is Robert Brownapproaching Southern California and parts of the Southwest on Sunday — 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, has been barreling through Mexico near the Baja California peninsula. 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 8 a.m. local time on Sunday, the storm was about 220 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.

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.