President Joe Biden has received a briefing from senior staff on the latest preparedness plans for Hurricane Hilary and his team is working with state and local agencies ahead of the storm, according to the White House.

FEMA Administrator Deanne Criswell said earlier she has been providing "continuous updates" to the White House ahead of the storm.

-ABC News' Justin Gomez

The County of Los Angeles Parks system said it will close on Sunday and Monday "out of an abundance of caution" due to Hilary.

That includes trails, lakes, beaches, pools, botanical gardens and playgrounds. All programs and classes are also canceled.

"While parks are not fenced in, visitors are encouraged to stay home," LA County Parks said.

-ABC News' Jennifer Watts and Alex Stone

Hilary continues to weaken as it moves north-northwest, now a category 2 hurricane with maximum sustained winds of 110 mph.

FEMA Administrator Deanne Criswell told reporters she is providing "continuous updates" to the White House about preparations ahead of the storm.

Criswell said she has also been in contact with California Gov. Gavin Newsom and Nevada Gov. Joe Lombardo to discuss "what type of support that they may need from FEMA and the federal family, but also to remind them that we are ready to support them as needed for this storm."

Criswell said they will not be moving any resources from Maui to California or Nevada to respond to the storm.

"We are still fully engaged in our efforts to help the state and people of Hawaii recover but it's also equally important that we are making sure communities that are in the path of Hilary also have the resources that they may need, and to what is likely going to be heavy impacts from rain and flooding," she said.

-ABC News' Justin Gomez

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