Hilary, the first tropical storm to move into Southern California since 1997, marked the first time ever that tropical storm watches were issued in the region.

Sunday set a new record for the wettest August day ever in Palm Springs, San Diego and downtown Los Angeles.

Hilary's highest rain total was in Upper Mission Creek in San Bernardino County, where 13 inches of rainfall was recorded.

-ABC News' Melissa Griffin

In Forest Falls, California, about 700 people are sheltering in place after mud and debris flow cut a road off from the rest of the community, according to the San Bernardino County Fire Department. No injuries were reported and crews are working to restore road access, the fire department said.

In nearby Seven Oaks, another 30 people are sheltering in place after flash floods struck several cabins in the area, the fire department said. Crews are working to access the residents and evacuate them, the department said.

All tropical storm warnings have been canceled across Southern California as the remnants of Hilary, which no longer meet the threshold of a tropical cyclone, track north, according to Ariel Cohen, meteorologist in charge at the National Weather Service Los Angeles.

Residual mudslides and rockslides are still a threat in California from the weekend rain.

The moisture from Hilary is now moving through the Rockies. On Monday, the flooding threat will be from the Sierra Nevada mountains into the northern Rockies in Idaho and eastern Oregon.

Los Angeles "avoided a potentially catastrophic set of conditions" from Hilary, LAUSD Superintendent Alberto Carvalho said Monday.

Carvalho defended the decision to close LA's public schools on Monday citing reasons including that many students walk to school and many employees live outside of LA.

"It would have been reckless for us to make a different decision" on Sunday, he said.

Schools appear to be in good condition, he said, noting that about 24 schools don't have phone or internet access and one elementary school that serves students with disabilities appears to have been impacted by a mudslide.

Carvalho said students should expect to resume their regular school day on Tuesday.

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