As predicted, Tropical Storm Hilary brought record-breaking rain to Southern California and caused significant inland flooding.
But it still had some tricks up its sleeve.
Hilary — the first tropical storm to hit the region in decades — hit some areas much harder than others. The mighty mountains of Southern California played a role in his journey.
And because the storm's occurrence was so unusual in California, Hilary behaved in a way that tropical storms on the Gulf Coast would not.
Here is an overview:
Why some areas have been hit harder than others
On Monday it was clear that some areas were more affected than others.
As predicted, Hilary settled in a lot of rain, with the interior, desert, and mountainous areas bearing the brunt. But one coastal area, San Diego, which was also once thought to be at higher risk, may have fared slightly better than expected, as Hilary's trail moved farther east than originally expected.
Some areas of Southern California were definitely affected, most notably the Coachella Valley, resulting in extended Interstate 10 closures in the Palm Springs area and the closure of a 120-mile stretch of US Highway 395, the main road between Mammoth Mountain and the Eastern Sierra metropolitan area Southern California.
People were reportedly stuck on rooftops in Cathedral City, where a thick layer of mud and water continued to submerge the streets on Monday. In Death Valley National Park, 2.2 inches of rain fell in one day — a devastating amount that not only broke the record for wettest days on record, but also surpassed the park's average annual rainfall for an entire year.
But there are other places where there was less devastation. Large amounts of rain fell in parts of the coastal counties of San Diego, Orange and Los Angeles in a single day — especially considering it's August, in the middle of the dry season — but not unprecedented for what may have happened during a strong winter storm could.
Indeed, in some areas, precipitation forecasts have generally been within expectations. Rainfall of about 5 cm has been forecast for San Diego; it received 1.82. It was predicted that there would be about 5 inches of rain in downtown LA; it received 2.99 inches.
Palm Springs was forecast for 4 to 5 inches of rain — as much rain as it averages in a year — and 3.23 inches of rain was expected.
Totals were particularly impressive for Mt. San Jacinto, which reached 11.74 inches, “which is incredible for practically 24 hours of rainfall,” said Brandt Maxwell, a weather forecaster with the National Weather Service's San Diego office.
“And of course the steep cliffs – in fact, it's one of the steepest cliffs in the Lower 48 [states] — you go 10,000 feet in less than 10 miles — the water went straight down the mountain and straight into the Coachella Valley,” Maxwell said.
Rainfall from the San Bernardino Mountains also flowed into the Coachella Valley via the Whitewater River, which reached a height of 46 feet at Rancho Mirage, southeast of Palm Springs. “That's an incredible amount of water going through there,” Maxwell said, contributing to the flooding of a section of Interstate 10.
Proximity to the eye offered some protection
But some densely populated areas of San Diego County, considered a potential concern due to its proximity to Hilary's projected eye, were likely sheltered from stronger winds as the storm moved east as it passed.
The storm's actual path “was more over the foothills, up into the mountains,” Maxwell said.
A gust of 51 mph was once recorded in La Jolla on the San Diego coast – which is significant – but in places farther inland where lots of people live, wind speeds were closer to 20 to 30 mph, with some higher gusts, Maxwell said.
Extremely unusual amounts of rain fell in some areas of LA County. Many precipitation records were broken on every single day in August. And Hilary's precipitation dump even resulted in several weather stations breaking records for the entire month of August.
Of course, there are many areas in Southern California that have not experienced catastrophic damage. But the weather service never made such a forecast.
“Not everywhere, not every square foot of LA County was flooded,” said National Weather Service meteorologist Joe Sirard.
Still, there were a number of areas in LA County that experienced devastating flooding, including a commercial section of Melrose Avenue in Los Angeles. The Los Angeles Fire Department received reports of 22 minor mudflows; Highway 138 in Antelope Valley was closed Sunday due to flowing water and debris flows; and flooding on the 5 Freeway in Sun Valley severely restricted access to the northbound lanes early Monday.
Constant rain helped prevent a major disaster
Daniel Swain, a UCLA climate researcher, said the storm's conditions simply did not result in the more extreme precipitation rates that had been forecast as an outside possibility, which he said helped minimize flood damage.
Motorists struggled with a flooded road and stranded vehicles in Palm Springs on Sunday during the heavy rains from Tropical Storm Hilary.
(David Swanson/AFP via Getty Images)
Instead of 3 to 4 inches per hour, most areas were closer to 1 to 2 inches – more manageable. However, Swain still called the amount of rain that fell unprecedented.
“Numerous locations in Southern California experienced their rainiest August day on record…and many locations set new records for the wettest day of summer,” Swain said.
Another factor that helped is that rainfall fell more evenly, rather than the short bursts of the same amount that would have exacerbated flood concerns.
“Although in many cases the same amount of water fell overall, it didn't fall as fast as feared,” Swain said, avoiding “catastrophic” flood concerns.
He said he expects extreme flooding in some more remote areas, noting that Death Valley has had little reporting as it has been evacuated.
“The hardest-hit areas are often the most inaccessible,” Swain said.
The mountain range helped Hilary find her way
What made Hilary's movements so unusual compared to a Gulf Coast hurricane is that Southern California has many mountain ranges.
Much of the sand at the La Jolla Shores boat dock was washed out to sea during the storm.
(Elisabeth Frausto)
“When Hilary came ashore, it interacted with the mountain barriers,” and a low-pressure area developed along the Ventura County coast, separate from Hilary's main area, Sirard said. As a result, intermittent heavy rain fell late Sunday in an area of eastern Ventura County and western LA County.
“When tropical storms make landfall, especially when they hit mountainous terrain, the circulation gets kind of torn apart and disorganized,” Sirard said.
Times contributors Susanne Rust, Brittny Mejia, Louis Sahagún, Hannah Wiley, and Ruben Vives contributed to this report.
