California has more risk of catastrophic storms than any other region in the country – even the Southern hurricane states, according to a new study released Thursday.

The two-year study by the U.S. Geological Survey is the most thorough effort yet to assess the potential effects of a "worst-case" storm in California.

It builds on a new understanding of so-called atmospheric rivers, a focusing of high-powered winds that drag a fire hose of tropical moisture across the Pacific Ocean, pointed directly at California for days on end. The state got a relatively tame taste of the phenomenon in December.

The team of experts that developed the scenario can't say when it will happen. But they do say it has happened in the past and is virtually certain to strike again.

"This storm, with essentially the same probability as a major earthquake, is potentially four to five times more damaging," said Lucy Jones, USGS chief scientist on the study. "That's not something that is in the public consciousness."

The study aims to fix that.

A conference on the subject, ending today at California State University, Sacramento, brings together hundreds of emergency planners to discuss the worst-case storm and how to prepare for it.

The USGS is assessing a variety of natural hazards across the country. California was chosen for the latest project, called ArkStorm, because the state "has the potential for the biggest rainfall events in the country," Jones said.

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