By Courtney Sherwood
PORTLAND, Ore. (Reuters) - Unusual rain that deposited a mysterious residue across a swath of the U.S. Pacific Northwest appears to be a byproduct of dust storms hundreds of miles (km) away in Nevada, although volcanic ash from Japan cannot be ruled out, the National Weather Service said on Monday.
The so-called "milky rain" left an unusual powdery residue on cars and windows last week across large parts of Oregon and eastern Washington state, leaving residents wondering about its origins.
"We're fairly certain it came from Nevada, but to confirm that we'd need to know exactly what the substance is," Weather Service scientist Mary Wister said. "Nevada had incredibly strong winds Thursday night and Friday morning, and the dirt there is a very alkaline dust."
She added that low-elevation wind gusts have been blowing to the north, and likely carried the dust to rain clouds, which then could have deposited milky drops across a region that spans from remote Fossil, Oregon, to Spokane, Washington, she said.
She said that federal scientists at the mostly decommissioned Hanford nuclear site have suggested another possibility, that higher-altitude winds may have carried ash across the Pacific Ocean from Japan's Sakurajima volcano, which is currently erupting.
But she said that theory appeared less likely than the Nevada wind scenario, and cautioned that without a chemical analysis of the milky rain, it would be impossible to pinpoint its origins.
The National Weather Service is not equipped to perform such an analysis, and has not conducted tests on the substance, she added.
"We're hoping somebody else does tests and shares what they find," she said.
(Editing by Cynthia Johnston and Sandra Maler)