Tropical Storm Hilary slams Southern California with historic rainfall

• Nevada's governor declared a state of emergency due to the threat of potential "flash flooding, rockslides and mudslides" amid forecasts that the south of the state would exceed its annual rainfall total in a short period of time due to the storm.
• In Arizona, the Mohave County Sheriff's Office issued evacuation orders for parts of Lake Mead National Recreation Area "due to flooding risks from Tropical Storm Hilary."

Threat level: The storm brought two years or more worth of rain in California's desert areas. Now a post-tropical storm swirling through Nevada, it produced tropical rainfall rates, which can be torrential, to areas that have rarely, if ever, experienced them.

• California is under a state of emergency, and evacuations away from the most vulnerable areas to mudslides, debris flows and other hazards were expected in Southern California especially.
• The manager of Palm Springs County in Southern California declared a local emergency Sunday "due to the critically dangerous impacts" including "unprecedented rainfall & flooding of local roadways and at least one swift water rescue," officials said.

Storm impact: Tropical Storm Hilary caused road closures and flood damage across Southern California and led to over 4,500 flights being delayed or canceled across the U.S. Sunday, per FlightAware.

• There were multiple reports of water rescues and people trapped in cars due to the inundation across the state on Sunday.
• The Los Angeles Unified School District, the nation's second largest, announced all schools, campuses and after-school programs would close Monday due to the storm.

Of note: The Weather Prediction Center had painted a large zone in southern California and Southwestern Nevada as facing a "high risk" of excessive rainfall and flash flooding Sunday — including San Bernardino, Death Valley, Barstow, Indio, and Palmdale, Calif.

• High risk-designated days are associated with about 40% of all flash flood-related fatalities and 83% of all flash flood-related damages in the U.S.

By the numbers: Precipitation totals from the storm have ranged from 11.74 inches at Mount San Jacinto, Calif., to 6.61 inches in Whitewater, Calif. and 2.5 inches in Carlsbad.

• Some locations in the San Bernardino Mountains picked up 2 inches of rain in an hour.
• The top amounts in L.A. County through Monday morning were between 5 and 7 inches, with most areas setting daily and monthly records. Downtown LA picked up 2.38 inches.
• The strongest wind gust was 84 mph at Big Black Mountain in San Diego County, per NWS.

• 42.6 million people were under tropical storm warnings Sunday, from the LA metro area south to the U.S.-Mexico border. Gusty winds are expected to be strongest along the coast and in areas of higher terrain.
• 28 million people in half a dozen states were under flash flood watches, which extend all the way north into eastern Oregon and Idaho.

Context: Human-caused climate change is leading to more frequent and intense extreme precipitation events. It is also causing tropical storms and hurricanes to produce more rainfall, as a warmer atmosphere and oceans allow these storms to carry more moisture.

• The presence of an intensifying El Niño event, with above-average sea surface temperatures in the equatorial tropical Pacific Ocean, may be enhancing the moisture funneling into this storm.

Go deeper: In photos: Tropical Storm Hilary's heavy rains flood California streets

Editor's note: This article has been updated with new details throughout.