USGS: Beware the ARkStorm

California has experienced biblical-scale storms and flood events in its history and needs to be ready for the next one, according to a team of researchers led by the U.S. Geological Survey.

The team described a scenario befitting of Noah that it dubbed “ARkStorm.” In the scenario, a long-duration storm hits California, fed by a steady flow of moist warm air known as an atmospheric river.  It dumps an amount of water on the state comparable to what large landfalling hurricanes deliver in the southeastern states. The precipitation from atmospheric rivers often triggers widespread flooding that inundates the state, causing severe damage to cities, agricultural regions and other parts of California’s infrastructure.

The researchers led by Mike Dettinger, a USGS hydrologist based at Scripps Institution of Oceanography, UC San Diego, said they created the scenario to help disaster response officials plan for an event that geologic evidence shows has happened about every 200 years. One of the most famous instances took place in 1862 when 45 days straight of rain caused flooding extensive enough to force Governor Leland Stanford to take a rowboat to his own inauguration in Sacramento and then forced the legislature to temporarily relocate to San Francisco.

The simulated ARkStorm, said Dettinger, does not depict the worst California has ever seen but “it’s by design worse than anything in the last 50 years or even in the 20^th century,” he said.

Dettinger co-directed researchers from Scripps, NOAA, the USGS, and other institutions as they developed and filled in the details of the ARkStorm event and as they converted it into flood maps. He said the USGS-led team hopes the scenario will lead to preparedness efforts comparable to what the state undertakes to ready itself for large earthquakes.

The name ARkStorm reflects a storm caused by atmospheric rivers (ARs), the most commonly known of which is the so-called “Pineapple Express” in which moisture travels over the Pacific Ocean from Hawaii and other subtropical regions. The “k” in the name assigns a strength to the hypothetical ARkStorm atmospheric river of 1,000 on a scale to be created by atmospheric scientists. The scale will be used to rate and communicate the magnitude of future California storms.

USGS officials said that in upcoming phases of the Multi Hazards Demonstration Project, in which ARkStorm was created, they will explore how to maximize the benefit of storm forecasts to disaster planners and devise ways to customize scenario data to various end users.   

Dettinger also pointed out that the research team made a point of steering clear of linking such a megastorm to human-caused global warming.

“You don’t have to go there; this happens naturally,” said Dettinger.  “A storm like ARkStorm doesn’t need climate change to happen although climate change makes it more likely.”

• Robert Monroe