California and the rest of the western US face a billion-dollar-a-year risk in flood damages from so-called "atmospheric rivers", the huge plumes of moisture that periodically surge ashore from the sub-tropics, according to a study published Wednesday by UC San Diego.
Such storms pose a particular threat in San Diego County, where the columns of moisture flow up and along the side of the region's mountains, condense, then fall as heavy rain.
The storm that blew into San Diego early Wednesday is a weak atmospheric river, scientists say.
The new damage estimate is based on 40 years of weather, climate and insurance data, and represents an ambitious but incomplete attempt to understand the economic impact of one of the wildest phenomenons in nature.
Atmospheric rivers "have been identified as the primary source of hydrologic flooding in the western United States, yet their costs remain largely unquantified," says the research paper, which UCSD's Scripps Institution of Oceanography published in the journal Science Advances, in collaboration with the US Army Corps of Engineers.
The phenomenon isn't new. But it has only recently started to become widely-known to the general public, partly because Scripps began to categorize "ARs" like hurricanes, on a one to five scale, with five being the strongest.
"ARs that rank one and two are relatively low in intensity and duration and are primarily beneficial because they help replenish the water supply," said Tom Corringham, a Scripps researcher and a lead author on the new paper.
"But the longer and larger ones can cause damage."
About 40 ARs per year make landfall on the West Coast, between Baja California and British Columbia. Most of them are modest in size. They are different from other widely known systems like low pressure storms that arrive from the Gulf of Alaska on the Pacific Northwest.
"The storm that occurred in mid-November was an AR and it was unusual because it delivered about 20 percent of average annual rainfall at some locations around San Diego County, from the desert to the mountains and coast," said Marty Ralph, director of Scripps Center for Western Weather and Water Extremes."
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