A historic atmospheric river drenched central and northern California Sunday with record-setting rains. The high-impact event dented the region’s drought and quelled the fire season but triggered flooding and mudslides.

Up to a half-foot of rain fell at low elevations and over a foot in the mountains. Both San Francisco and Sacramento established new rainfall records for October, just after enduring a historic shortage of precipitation.

At the highest elevations of the northern Sierra Nevada, multiple feet of snow fell, a crucial addition to water resources in the drought-plagued region.

Atmospheric rivers are long, narrow swaths of exceptionally moist air, sometimes sourced from the tropics, that can produce excessive amounts of precipitation. This river was rated a level 5 out of 5 in the San Francisco Bay area by the Center for Western Weather and Water Extremes in La Jolla, Calif.

The parent “bomb cyclone,” the rapidly intensifying ocean storm that drove the atmospheric river into the West Coast, proved the most intense on record offshore the Pacific Northwest. It had a minimum air pressure reminiscent of Superstorm Sandy in 2012, bringing hurricane-force winds over the open ocean waters and 50 to 80 mph gusts along the coast from Seattle to San Francisco.

Two people were killed near Seattle when a falling tree crushed their car. The combination of wind and rain left up to 170,000 customers without power in California on Sunday; that number had diminished to around 115,000 on Monday morning. More than 150,000 customers lost power around Seattle.

The atmospheric river was winding down in intensity Monday while sinking south toward Southern California, but a look at the long-range pattern suggests the likelihood of continued atmospheric river events in the coming weeks.

Record precipitation and moisture

The atmospheric river drenching the West Coast unleashed record rainfall and moisture that brought deluges and flooding to the rain-starved region. Sacramento ended a record 212-day-long streak last Monday, reversing course and suddenly experiencing its wettest day on record on Sunday — a whopping 5.44 inches of rain fell in 24 hours, equating to what would ordinarily fall in two and a half months.

In downtown San Francisco, 4.02 inches of rain fell Sunday, its wettest October day on record and fourth wettest day of any month in records dating to the 1849 during the Gold Rush.

San Francisco International Airport also recorded 4.02 inches of rain on Sunday, bringing its monthly total to 5.5 inches, or roughly 10 times the average for the month. No measurable rain fell there between April and September.

Just north of San Francisco, in Marin County, several locations in the coastal mountain range saw over a foot of rain. Mt. Tamalpais registered 16.55 inches since Saturday and nearly 27 inches since the middle of last week. The Marin County fire department tweeted that it responded to 650 weather-related calls between Sunday and Monday, including three water rescues, 20 vehicle accidents, 163 downed trees and 185 public assists for flooding issues.

In Nevada, Reno received 2.82 inches of rain between Sunday and Monday, an October record.

Here are some other select totals from California:

  • Paradise in Butte County, devastated by a wildfire in 2018, measured 7.57 inches Sunday.
  • White Cloud in Nevada County northwest of Lake Tahoe saw 8.29 inches. Blue Canyon to the south received 10.4 inches, setting an all-time 24 hour record.
  • Redding set an Oct. 24 record of 2.99 inches.
  • Oroville set an Oct. 24 record of 4.57 inches.

Atmospheric rivers transport their moisture most effectively at the mid-levels of the atmosphere, which is why higher elevations wound up with the jackpot totals.

The atmospheric river itself was transporting about a ton and a half of moisture per second over every horizontal meter across the core of the moisture stream. That lead to a record October PWAT, or precipitable water index, at Oakland. PWATs describe how much water is present in a column of atmosphere, the 1.62 inches measured in a weather balloon Sunday night was off the charts for October.

Snow totals measured in feet occurred in the highest terrain. Heavy snow forced the closure of Interstate 80 for a time. Extreme winds ripped across mountain peaks, with gusts topping 100 mph in some locations. Mammoth Mountain clocked a peak gust of 159 mph.

While snow levels were initially quite high, at over 8,000 feet, they were dropping on Monday. As of Monday morning, 29 inches had fallen at Donner Pass in the northern Sierra Nevada, which is at an elevation of around 7,000 feet, and it was still snowing.

Flooding, mudslides and debris flows

The National Weather Service Weather Prediction Center hoisted a rare level 4 out of 4 “high risk” of excessive rainfall and flooding in the northern Sierra Nevada. In San Francisco, residents could be seen working to clear storm drains at the intersection of 31st and California. The Santa Rosa Fire Department tweeted footage depicting roaring rivers as water gushed down streets and threatened structures.

The high rainfall rates, which topped an inch per hour, were particularly problematic within the burn scars of wildfires that torched parts of central and northern California in the past several years. The Weather Service issued flash flood and debris flow warnings for the Dixie, Caldor, River and Creek fire burn scar areas.

“Excessive rainfall over the warning area will cause mudslides near steep terrain,” it wrote. “The mudslide can consist of rock, mud, vegetation and other loose materials.”

On Sunday, a landslide occurred along State Route 70 near Tobin, Calif., on the Butte-Plumas County line, forcing the closure of the highway. The area was within the burn scar of the Dixie Fire, which burned nearly a million acres this year.

“If you are near a burn scar, it may be too late to evacuate,” tweeted the Weather Service in Sacramento on Sunday. “Do not attempt to cross a debris flow. Take shelter in the highest floor of your home.”

A record-setting ‘double bomb’

Steering the atmospheric river into the Golden State was a swirl of low pressure several hundred miles off the coasts of Washington and Oregon. The low brought wind gusts to around 60 mph in coastal stretches of the Pacific Northwest and wave heights of up to 20 feet, but it was a powerhouse storm over the open ocean with hurricane-force winds. It intensified at an extreme rate — twice the pace of “deepening,” or strengthening, needed to qualify for “bombogenesis.”

That lead to an air pressure similar to that of Hurricane Sandy in 2012, signifying the vacuum-like effect of the storm as it spiraled in air from all directions. It proved the lowest air pressure on record in that part of the ocean.

The storm will rapidly weaken west of British Columbia in the coming 24 hours, lashing Vancouver Island with wind and rain before dissipating. The tandem atmospheric river, meanwhile, will dissipate as it drifts south and fragments.

Looking ahead

More than 86 percent of California was in “extreme” or a top-tier “exceptional” drought last week according to the U.S. Drought Monitor. While the drenching will make a dent in the deficit, the years-long problematic paucity will be far more difficult to erase. Due to the storm’s warm nature, less of the precipitation fell as snow; snow is more useful from a water resources standpoint since it can easily be stored in the Sierra Nevada as it melts more gradually. That creates a more reliable reservoir of water.

Data indicates that climate change is causing the expansion of California’s dry season (and fire season) deeper into October and November as the wet season becomes compressed. This latest storm proved an anomaly and a welcome nail in the coffin for 2021′s fire season in central and northern part of the state.

While some rain is predicted in Southern California on Monday, generally between 1 and 2 inches, it will be insufficient to meaningfully relieve drought conditions there or end its wildfire season.

A glance at the extended pattern indicates that more atmospheric rivers will be on the way in the coming weeks, though none in the forecastable future appears as intense as that which befell the West Coast over the weekend.

The sudden switch from record-dry to record-wet conditions in California is what Daniel Swain, a climate scientist at the University of California at Los Angeles, has referred to as “precipitation whiplash.” A dramatic example of this occurred in 2016 when California had its wettest year on record after a historic drought from 2012 to 2016.

In 2018, Swain and colleagues published a study that concluded human-caused warming of the climate will increase such whiplash in the future, projecting a 25 to 100 percent increase in dry-to-wet precipitation events. “Such hydrological cycle intensification would seriously challenge California’s existing water storage, conveyance and flood control infrastructure,” the study stated.