Hilary could drench Calif. with flash floods as first tropical storm in 84 years

Even a downgrade from hurricane status left little room for relief, as the storm dropped dangerous rains and caused extensive flooding, as well as mudslides in mountainous regions. It’s the first tropical storm to officially hit Southern California in 84 years.

The National Weather Service reported “very heavy rain” in Los Angeles and Ventura counties, with more than 6 inches of rain in some places by Sunday evening. Drivers were advised to stay off the roads.

Flooding inundated roads around San Diego and Los Angeles, while high winds toppled trees and knocked out power. Palm Springs, Calif., recorded its heaviest hourly rainfall on record, with flash flooding that was expected to worsen overnight Sunday.

Also see: Hurriquake! Moderate earthquake rattles nerves in L.A. as Tropical Storm Hilary hits

Hurricane Center Director Michael Brennan said it’s the water, not the wind, that people should watch out for most.

“Rainfall flooding has been the biggest killer in tropical storms and hurricanes in the United States in the past 10 years and you don’t want to become a statistic,” Brennan said in an online briefing from Miami, according to the Associated Press.

While a drought-plagued California can still use more rain to replenish reservoirs, sudden rainfall to this extent can be dangerous.

The U.S. National Hurricane Center said a tropical storm watch has been issued for Southern California, the first time such an alert has been given.

“Heavy rainfall in association with Hilary is expected to impact the Southwestern United States through next Wednesday, peaking on Sunday and Monday,” the hurricane center said. It said there was a substantial danger of flash flooding in an area stretching from San Diego to Las Vegas.

“Rainfall amounts of 3 to 6 inches, with isolated amounts of 10 inches, are expected across portions of southern California and southern Nevada, which would lead to significant and rare impacts. Elsewhere across portions of the Western United States, rainfall totals of 1 to 3 inches are expected.”

Read: Atlantic hurricane season ready to rile up storm clouds for oil and gas

California Gov. Gavin Newsom declared a state of emergency, and President Joe Biden on Friday said his administration is “closely monitoring Hurricane Hilary, which has the potential to bring significant rain and flooding to Southern California.” 

With Hilary, “multiple years’ worth of precipitation could potentially fall in some of the driest parts of California,” Daniel Swain, a climate scientist at the University of California at Los Angeles, said Wednesday in a YouTube presentation.

Among those spots is Death Valley, Calif., the hottest place on Earth. Death Valley typically receives about 2 inches of rain across an entire year, according to NWS data. Moisture from Hilary could dump enough rain to give Death Valley one to two years worth of rainfall in a single day. And Las Vegas could get 2 to 4 inches of rain; it typically averages 3.75 inches of rain a year.

Prolonged rain may oversaturate the ground and overwhelm waterways, potentially worsening the flood threat.

At least 9 million people were under flash-flood warnings Sunday. The National Weather Service in Los Angeles has also warned of the potential for dangerously high surf, rip currents and coastal flooding.

“Hilary is a large and very intense hurricane that is embedded in a very moist air mass, with midlevel relative humidity around 80%. The storm is towing an impressive amount of moisture northward with it, and the amount of water vapor in the atmosphere may approach all-time records in some locations along or near Hilary’s path,” said researchers at Yale University’s Climate Connections.

“The storm’s impacts are likely to be severe and highly damaging to the Southwest U.S., with damages likely running at least into the hundreds of millions of dollars,” they added. 

Why are hurricanes rare in California?

No tropical storm has made landfall in Southern California since Sept. 25, 1939, according to the National Weather Service.

Thanks to cooler ocean water temperatures and a regional high-pressure system, storms rarely gain or hold enough strength to make landfall as a tropical storm or a hurricane in California. But there have been a few occasions when they have; the 1939 Long Beach Tropical Storm for one, while in 1978, Norman, brought some chaos as a tropical depression.

Read: Homeowners can check a property’s flood, heat and wildfire risk for free with this high-powered app

Flooding, like most disasters, involves a number of competing factors that may impact frequency and intensity, sometimes in opposing ways. Climate change, which is worsening extreme rainfall in many storms, is an increasingly important part of the mix, scientists say.

How does a flash flood differ?

According to the National Weather Service, a flash flood is a flood caused by heavy or excessive rainfall in a short period, generally less than six hours. Flash floods are usually characterized by raging torrents after heavy rains that rip through river beds, urban streets or mountain canyons, sweeping along everything in their path. They can occur within minutes or a few hours of excessive rainfall.

One of the biggest differences between flash floods and generalized flooding is that flash floods can be more dangerous than people think. Even a few inches of water can have a current that knocks people down and just a few feet of water can send a car floating. Flash flooding at night is particularly dangerous, because unknown depths are disguised, looking simply like wet surfaces.

Flash floods can also occur even if no rain has fallen, such as after a levee or dam has failed, or after a sudden release of water by a debris or ice jam. What’s more, the inability of some municipal storm sewers to keep up with the water removal can aggravate the situation, and that’s been especially true as the nation’s infrastructure has aged.

Flash flooding isn’t unheard of in the Southwest U.S. during monsoon season, when a seasonal change in winds draws moisture into the area and can lead to more showers and thunderstorms. But exceptionally dry years mean that the bare ground can also be as hard as concrete in some places, allowing for restricted absorption.

Related: Flash floods, like in Las Vegas, are deadlier than hurricanes, tornadoes or lightning

Sometimes both flash flooding and ongoing rising waters from rivers and creeks can devastate an area at the same time. In 2017, after the very dangerous and expensive Hurricane Harvey swamped coastal areas, it stalled around southern Texas for days as a weakening hurricane that brought catastrophic flash and river flooding, the NWS archives say.

And during 2021’s Hurricane Ida, the NWS office in New York declared its first-ever set of flash flood emergencies in the region, an alert level that is reserved for “exceedingly rare situations when a severe threat to human life and catastrophic damage from a flash flood is happening or will happen soon,” officials said at the time.

Climate change: Flash flooding expected to increase

Most worrying, perhaps, is that flash flooding is expected to increase as there are more extreme precipitation events brought on by climate change and man-made atmospheric warming, meteorologists and climate scientists say. This is because warmer temperatures increase evaporation, which puts more moisture into the atmosphere that then gets released as rain or snowfall.

As global warming continues to exacerbate sea level rise and extreme weather, our nation’s floodplains are expected to grow by approximately 45% by century’s end.

Already, inland U.S. flooding risk increasingly factors into interactive warning maps and real estate and insurance decisions, as homeowners, business operators and politicians grapple with all the ways climate change should change how we talk about and prepare for weather extremes, including the frequency of events and extended seasons. Because flood insurance is handled through the federal government’s Federal Emergency Management Agency, and not individual private policies, there’s often more to understand about paying for damages when you are impacted. 

Planning and engineering trade groups say cities must dedicate more of their infrastructure budget to fortifying storm sewers. And cities might think of leaning on nature to fight nature.

The Natural Resources Defense Council argues that populated areas, especially, must think of permeable pavement and rainwater harvesting (instead of simply allowing for too-slow runoff), green roofs, rain gardens and additional tree planting. 

Here’s more from the NRDC.

For now, the Red Cross has some tips for flood and flash flood safety, such as getting out of your vehicle sooner versus later, and moving to higher ground, if you get caught on a flooded road.

Read: ‘I started just connecting the dots’: Iowa TV meteorologist hounded off the air over climate-change reporting

See additional coverage on the MarketWatch Living With Climate Change page.

The Associated Press contributed to this report.