Storms packing torrential rain and high winds have battered California this winter. Historic levels of flooding caused numerous deaths and left tens of thousands of people without power. Worryingly, they could become a more frequent and severe occurrence.
An atmospheric river, a type of weather event first identified by scientists in 1998, is largely to blame for the recent extreme weather on the U.S. west coast. These long, narrow conveyor belts of storms can strike the west coasts of most continents and landmasses, but California seems to be a prime target.
Typically, an atmospheric river flows about 1.5 kilometers above the ocean surface and can extend thousands of miles out to sea, carrying as much water as 15 Mississippi Rivers. It strikes a coast as a series of storms that last for days or even weeks on end. Each storm can dump many inches of rain and/or meters of snow.
As many as nine small atmospheric rivers reach California every year, each lasting two to three days, including the famous “Pineapple Express” storms that come straight from the Hawaii region of the Pacific Ocean.
This year’s extended storm was made even worse by the presence of a bomb cyclone – a rapidly intensifying area of low pressure that occurs when atmospheric pressure drops 24 millibars or more in 24 hours.
Wild West Weather
Over the years, atmospheric river storms have been responsible for most of the deadly historical floods in many western states.
The largest one to hit the American West in relatively recent history occurred during the winter of 1861–1862. California bore the brunt of the damage although flooding also occurred in parts of northern Mexico, Oregon, Washington, and British Columbia, as well as reaching inland into Nevada, Utah and Arizona. Rain continued for 43 days. Sixty-six inches fell on Los Angeles, and the Sierra Nevada received 10‑15 feet of snow followed by warmer weather. The resulting flooding formed big lakes in the southern California deserts and created a huge inland sea in the Central Valley that was at least 300 miles long and 20 miles wide, with depths up to 30 feet.
The property damage of that winter storm was biblical. One-third of California’s property was destroyed, including one in eight homes. The levees failed around Sacramento, which remained underwater for months and resulted in the city embarking on a long-term project of raising the downtown district by at least 10 feet in the years after the flood. The total costs of the flooding were devastating: one quarter of the California’s economy was destroyed, forcing the state into bankruptcy.
The extraordinary impacts resulting from “GF1862,” the official name assigned to the 1861–1862 storm, were used by the U.S. Geological Survey Multi Hazards Demonstration Project to produce a California state-wide realistic disaster scenario, known as ARkStorm, launched in 2010.
They examined the resulting physical damage and social and economic consequences of a hypothetical storm striking the U.S. West Coast and, similar to the winter storms of 1861–1862, leaving the Central Valley impassible because of precipitation experienced on average only once every 500 to 1,000 years.
Just as in an actual event, the hypothetical storm causes serious flooding in the Central Valley, Orange and Los Angeles Counties, San Diego, the Bay Area, and other coastal areas, overwhelming flood control measures that are designed for 100- to 200‑year runoffs.
Winds across wide areas reach 60 miles per hour, with gusts of 125 miles per hour in some places. Flooding and landslides damage structures and highways. Utilities take weeks or months to be restored.
Subsequent analysis of the ARkStorm scenario1 suggests that such an event would likely produce widespread, catastrophic flooding and subsequently lead to the displacement of millions of people, the long-term closure of critical transportation corridors and ultimately to nearly $1 trillion in overall economic losses (2022 dollars) – some of which would translate into insurance claims.
Scientists say that climate change has already increased the risk of a GF1862-like megaflood scenario in California and that future climate warming will likely bring about even sharper risk increases.2 Climatologists further find that runoff in a future extreme storm scenario is 200% to 400% greater than historical values in the Sierra Nevada because of increased precipitation rates and decreased snow fraction.
Their conclusions have major implications for flood and emergency management as well as for hazard mitigation and climate adaptation preparation in California. With the risk profile of the country’s most populous state changing so dramatically, the Property/Casualty industry needs to keep its underwriting models under regular review.
- Xingying Huang and Daniel L Swain, “Climate change is increasing the risk of a California megaflood,” Science, August 12, 2022, https://www.science.org/doi/10.1126/sciadv.abq0995