It's the rain that just won't stop -- day after day. It's almost as continuous as the flow of a river, tropical moisture funneled into California by what scientists have come to call an "atmospheric river."
The term was coined only within the last generation of satellite imaging that can actually show the band of moisture.
But there's nothing new about the phenomenon. What scientists now realize was an atmospheric river in 1861-62 brought California 45 straight days of rain and caused flooding of Biblical proportions, evocative of Noah and his ark.. It bankrupted the state.
"The atmospheric river brings in the moisture. How much rain gets dropped out of it has a distribution, just like earthquakes," said Lucy Jones of the U-S Geological Survey office in Pasadena.
Jones is best known for her earthquake expertise. But her team at the USGS Multi-Hazards Demonstration Project develops computer models for a variety of natural disasters, and has just finished a scenario for a modern day "Ark Storm," akin to the 1861-62 storm.
"We actually made a model of this type of storm, just like we made a model of a Southern San Andreas Earthquake," Jones explained. "We used the same techniques to try to assess what the damage would be. And our conclusion is that the storm would cost about four times as much as the shakeout earthquake."
Fortunately, such extreme storms are relatively rare, though lesser atmospheric rivers have also caused severe damage storms, notably in 1969 and 1986. Jones said more moderate atmospheric rivers occur on an annual basis. The one this December recurs perhaps once a decade--but that be premature, given that it's still going.
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"This is like Texas Hold 'Em after the first four cards," quipped JPL scientist Bill Patzert. "We're still waiting for the final cards to be dealt."
Patzert is renowned for his research into the influence of oceanic conditions on weather. This season there is a "La Nina" condition of cooler than usual oceanic waters near the equator, which correlates with Southern California having a dryer than typical winter
Not this December.
Patzert said this current atmospheric river is more typical of the opposite "El Nino" condition. "Clearly there are other factors,' Patzert said. He also noted that sometimes the La Nina condition does not kick in and bring dry conditions before February.
Jones is struck by the coincidence that California's last major Ark Storm occurred so close in time to the last Southern San Andreas Big One in 1857. It appears both recur with a frequency of a few hundred years.
Which raises the question: Which one will we get next?
