Tropical storm Hilary drenched much of Southern California before its remnants moved on to douse several Western states. While some communities suffered severe flooding and mudslides, most got a beneficial soaking. But experts say that given the overall setup, the aftermath could have been much worse — and both luck and preparation played a role in avoiding a more dire outcome.
“The fact is that this was an incredibly unusual event for Southern California,” said Greg Carbin, chief of the forecast operations branch for the National Weather Service’s Weather Prediction Center.
Thanks to California’s steep terrain, dense population and vast area burned by wildfires over the past several years, it probably takes less rain to cause serious flooding in the state than in other locations typically hit by hurricanes and tropical storms.
Advertisement
“Forecasting extreme rainfall is really, really tough,” Carbin said. “One thing we do know, though, is that tropical weather is usually associated with extreme rainfall — it was in this case, and it will be in the next case.”
Forecast rain amounts closely matched what the region actually received, according to Paul Iñiguez, a meteorologist at the Center for Western Weather and Water Extremes at the Scripps Institute of Oceanography at the University of California at San Diego.
Desert towns were hit by unusually heavy rain, while the Southern California mountains racked up totals of 7 to 10 inches, with isolated reports of nearly 12 inches, along with treacherous debris flows.
This is footage from last night in Forest Falls (8/20/23) near #SBCoFD Station 99. It shows the power of debris flows. They happen fast, and they're unpredictable. When evacuation orders are issued, leave immediately. Property can be replaced. You & your loved ones lives cannot. pic.twitter.com/1gbRtWNkYq
— San Bernardino County Fire (@SBCOUNTYFIRE) August 21, 2023“I think we’re all in agreement that the deserts and up into the mountains saw that damaging and catastrophic flooding that was being messaged and forecast,” said Casey Oswant, a meteorologist with the National Weather Service in San Diego.
Advertisement
Most of the major population centers in Southern California set summer rainfall records. These include San Diego (1.82 inches), Downtown Los Angeles (2.48 inches), Burbank (3.28 inches), Long Beach (2.27 inches) and Palm Springs (3.18 inches). Palmdale, a town about 35 miles north of Los Angeles, set a daily record of 3.93 inches.
The heavy rains were partly due to the abundant water available in the atmosphere, which on Sunday tied for the second-highest ever measured in San Diego, according to Iñiguez. Because warmer air can hold more water, precipitation extremes have increased and will continue to rise with climate change.
Death Valley National Park experienced its wettest day on record at 2.20 inches — an entire year’s worth of rain — breaking the previous record of 1.70 inches set in August 2022. The park remains closed, as flash flooding and debris flows have washed out several roads, stranding visitors.
Advertisement
As Hilary’s remnants swept north, four states — Nevada, Oregon, Idaho and Montana — set new tropical cyclone rain records, meaning that this storm delivered the highest single rain observation from a tropical system in each state.
The tropical cyclone (and their remnants) state rainfall maxima map has been updated due to Hilary. NV, OR, ID, & MT have new records. Data is preliminary and could fluctuate over the coming days and weeks.
Link to our tropical cyclone rainfall pages: https://t.co/sZfxYTgU3B pic.twitter.com/4u2zYEdqgA
— NWS Weather Prediction Center (@NWSWPC) August 22, 2023The soaking will dampen fire risk in parts of the West — possibly for weeks on end in Southern California. And despite damage from fast-moving floods and mudslides, the state has reported no fatalities from the storm.
“I wouldn’t want to downplay the level of impacts,” Iñiguez of Scripps said in an email. “That said, surely impacts could have been more severe, and perhaps (hopefully!) the excellent forecasts ahead of this system and the unified message from the weather enterprise … persuaded people to take precautions and not put themselves in dangerous situations.”
Advertisement
UCLA climate scientist Daniel Swain said in a briefing Monday that rainfall intensities may not have reached the expected high levels forecast, preventing more widespread serious flooding.
Another stroke of luck is that Hilary moved fairly quickly through the state. Slower storms drop more rain and are more dangerous in terms of flooding, Carbin of the Weather Prediction Center said.
In 2017, Hurricane Harvey dumped more than 60 inches of rain in southeastern Texas, causing at least 68 deaths. And last year, more than 150 people were killed in Florida after Hurricane Ian ravaged the state with catastrophic storm surge, damaging winds and flooding.
“Every tropical system has the potential to be that bad,” Carbin said. “Thankfully, we dodged maybe a bit of a bullet on this one, but it was still a very remarkable, unusual event that needed to be messaged in a way that people took appropriate action.”
ncG1vNJzZmivp6x7uK3SoaCnn6Sku7G70q1lnKedZMSmrdOhnKtnYmV%2FdHuPcWZra1%2Bdtq2t0bJkrKyfp7puss6rnJyZo6l6pa3Mmp6eZw%3D%3D