As of 11 p.m. ET/8 p.m. PT, Hilary remains a tropical storm with maximum sustained winds now down to 45 mph. The center of Hilary is located about 105 miles northwest of San Diego. It is moving at 28 mph.

Widespread flash flood warnings remain in effect across Southern California as areas of heavy rain continue to sweep through the region. The flash flood warnings in Los Angeles and Ventura Counties have been extended until 3 a.m. PT Monday. Flash flooding is ongoing across the region. There are also warnings in many desert areas, including Death Valley.

The widespread heavy rain in Southern California will begin to ease up soon, first in the southeastern part of the state. After 10 p.m. PT Sunday, there is a notable improvement regionwide, with the worst of the heavy rain over and the rain becoming less intense. There is lingering rain overnight in some areas but not as heavy as earlier.

Even though the rain ends, there could still be residual impacts, like landslides or rockslides.

- ABC News Meteorologist Daniel Peck

Hilary is continuing as a tropical storm with maximum sustained winds now down to 50 mph as of 5 p.m. PT Sunday. The center of Hilary has now officially moved into Southern California and is located about 25 miles south, southwest of Palm Springs. It is currently moving north at 23 mph.

- ABC News Meteorologist Daniel Peck

Schools in the Los Angeles Unified School District (LAUSD), the second largest school district in the country, will be closed on Monday as Tropical Storm Hilary makes its way to the area, the district's superintendent said at a press conference on Sunday.

LAUSD serves 565,479 students. Superintendent Alberto M. Carvalho expects schools to be open on Tuesday.

The San Diego Unified School District also announced school closures for Monday.

As the Southland battles heavy rain and continues preparations for the heart of the storm to make its way up through California, there was a 5.1 earthquake, per the U.S. Geological Society. The earthquake was centered near Ojai, and was followed by several smaller aftershocks.

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