The storm that soaked Northern California this week was the West Coast equivalent of a hurricane, according to experts working to ready the public for the next one.
Called an "atmospheric river," the storm pulled a geyser of moisture from the tropics near Indonesia all the way across the Pacific Ocean. The narrow channel of intense rain scored a direct hit on California's Central Coast, then gushed like a fire hose all day.
Near Big Sur, more than 20 inches of rain were recorded in a single day. Modesto received 1.32 inches, the wettest October day ever. Nearly 900 city trees or big limbs were knocked down.
Had the storm hit farther south, it could have caused devastating mudslides on recently burned Southern California mountain slopes. Had it occurred later in winter, when the ground was saturated, the storm might have caused dangerous floods.
"We just dodged a bullet," said Lucy Jones, chief scientist at the U.S. Geological Survey Multi-Hazards Demonstration Project, which is researching atmospheric rivers.
Smaller versions, born near Hawaii, are popularly known as "Pineapple Express" storms.
"We've shown they are largely responsible for the big rains and big floods that occur," said Marty Ralph, chief of the water cycle branch at the National Oceanic and Atmospheric Administration in Boulder, Colo. "Those are the ones to watch out for."
The USGS and NOAA are working on computer models and forecasting to help government and the public better prepare for similar storms.
One new tool will be a numerical rating system to describe storm inten- sity, similar to the category number used to size up East Coast hurricanes, said Dale Cox, a USGS project manager in Sacramento.
The team is working with the state Department of Water Resources, California Department of Transportation and others to predict how these storms contribute to flooding, mudslides and road closures.
Severity ranking, for instance, could tell homeowners to prepare for power failures and toppled trees. It could help flood officials know where to position emergency supplies, and when and where to recommend evacuations.
Tuesday's storm dealt a relatively modest blow because the worst of the rain and wind hit remote areas.
Cox, however, felt the punch.
"I lost one of my favorite trees," said Cox, who lives in El Dorado Hills, east of Sacramento.
"If you look at the storm itself," he added, "there's a very strong band of precipitation that is much like the eye wall of a hurricane that is, in a sense, stretched out. They bring wind and rain that are comparable to hurricanes on the East and Gulf coasts."
California doesn't get hurricanes; the coastal Pacific Ocean is too cold. But Cox said atmospheric rivers can be seen as a kind of hurricane. Instead of spinning in a concentrated mass, the rain and wind stretch across thousands of miles, like a conveyor belt.
Because of its size, this phenomenon was not recognized as a unique weather feature until the advent of advanced satellite weather technology.
Ralph said atmospheric rivers are not new. Our understanding of their behavior and significance is.
Such storms occur every year all over the planet, said Ralph, and are not necessarily associated with either El Niño or La Niña weather patterns.
But they are particularly worrisome in California, where a large population and vulnerable infrastructure rest on the volatile edge of the world's largest ocean.
That's why a major effort is under way to better understand these storms. An important goal is being able to predict where they will land — and where most of their rain will fall — in time to warn people.
Ralph's lab in 2005 began installing weather instruments in the canyons of the American River, making it the nation's most closely monitored watershed. The river was chosen for its potential to cause disastrous floods.
That monitoring system helped track Tuesday's storm.
Now, Ralph said, NOAA has joined with the state on a $7 million project to install more monitors across the state.
Another part of the proj- ect will model a worst-case storm striking California. Cox said this would involve back-to-back atmospheric rivers striking the state.
This might have been what caused the devastating flood of 1861-62. The rain started over Northern California on Christmas Eve in 1861 and didn't stop for 45 days.
Flooding was so bad that Leland Stanford had to take a rowboat across Sacramento to attend his inauguration as governor. Much of the Central Valley became an inland sea. The state Capitol was moved to San Francisco temporarily until Sacramento recovered.
'Worst case' data
Researchers believe it may have been the worst storm to strike California in the historical record. But because data from that period are limited, the team is combining data from storms in 1969 and 1986 to help prepare this "worst case" storm sequence.
Both years involved atmospheric rivers and caused some of the worst storm damage the state has ever seen, including many deaths.
"I want people to start identifying with atmospheric rivers," said Cox. "These storms are like our hurricanes."