Hurricane Hilary, once a Category 4 hurricane, was downgraded to a tropical storm Sunday as it approaches the West Coast.

The center of the storm system is currently located about 220 miles south-southeast of San Diego. It is moving about 25 mph to the north-northwest.

About a quarter of an inch of rain has already fallen in Palm Springs on Sunday morning, while San Bernardino has seen about .8 of an inch, according to the National Weather Service.

Tropical storm force winds will start moving into Southern California over the next few hours.

Catastrophic and life-threatening flooding is likely over portions of the Southwest through Monday, forecasts show.

-ABC News' Kenton Gewecke

Hilary continues to weaken as it moves north-northwest, now a category 1 hurricane with maximum sustained winds of 90 mph.

The San Bernardino County Sheriff's Department issued a mandatory evacuation order for several regions at 7:15 p.m. PT. The regions include Oak Glen, Forest Falls, Mountain Home Village, Angelus Oaks, and northeast Yucaipa.

Earlier Saturday, San Bernardino County Sheriff's issued an evacuation warning to its residents. At the time, San Bernardino County spokesperson David Wert told ABC News, "The purpose is to warn the population that there is an impending danger. They should start making arrangements to evacuate."

The Sheriff’s office released an evacuation map to help residents know if they’re in the affected area.

California Governor Gavin Newsom declared a state of emergency for much of Southern California on Saturday ahead of Hurricane Hilary making landfall.

"California has thousands of people on the ground working hand-in-hand with federal and local personnel to support communities in Hurricane Hilary's path with resources, equipment and expertise," the governor said in a statement.

There are currently “more than 7,500 boots on the ground deployed” to help local communities the governor’s statement continued.

The impacts of the storm are expected to begin Saturday evening lasting through Monday, according to the statement.

The National Weather Service has issued a flash flood warning for parts of Southern California as the region begins to experience the effects of Tropical Storm Hilary.

Los Angeles County, including downtown Los Angeles, and Ventura County were included in the advisory.

Heavy rain is now becoming more widespread across portions of the region. The heaviest rain is focused over much of the Interstate 5 corridor, north of San Diego into Los Angeles.

Rainfall rates up to 1 inch per hour are possible in this area, and flash flooding will likely develop, forecasts show.

Rounds of heavy rain are expected to continue to sweep across the greater Los Angeles metro area through the next several hours.

Hilary will then move northward over the next 24 hours. As the system moves over land, it will weaken and become disorganized but could remain a tropical storm as it tracks across Southern California.

A tropical storm warning remains in effect for San Diego, Palm Springs, Riverside and Los Angeles.

Flood watches remain across a large swath of the Southwest, including cities like San Diego, Palm Springs, and Las Vegas. These watches have also been extended northward up to Idaho for the potential for heavy rain and flash flooding early in the week.

The National Oceanic and Atmospheric Administration's Weather Prediction Center has issued a high risk alert for excessive rainfall across Southern, California, from Palm Springs to San Bernardino and up to Death Valley. "High Risk" days are rare and typically dangerous -- 39% of all flood-related fatalities and 83% of all flood-related damages in the U.S. occur on "High Risk" days, records show.

A moderate risk alert for extreme rainfall is in effect from Las Vegas to San Diego and Los Angeles.

The biggest rainfall totals will be across the mountains east of San Diego, north and east of Los Angeles. Through Monday, a widespread 1 to 3 inches of rain is forecast across much of Southern California, the desert Southwest and across southern Nevada.

Three to 6 inches or more of rain is forecast across the mountains areas in Southern California. Several inches could still fall in some desert locations, with the greatest chance closer to Death Valley. Isolated amounts of 6 to 10 inches are still possible in some regions.

One to 2 inches of rainfall quickly falling in a desert location can cause major impacts due to the oversaturated soil.

-ABC News' Daniel Peck