The mystery surrounding a white, milky rain that fell across Eastern Washington and parts of Oregon and Idaho Friday has a new theory, although I’d call it more of a tweak of the previous theory.

The event coated vehicles and windows in more than 15 cities, including Spokane, the Tri-Cities, and Hermiston, Oregon. Initial thoughts of the source originating as volcanic ash from a distant eruption or debris blown from summer wildfire-scarred terrain were quickly disproven.

Then on Monday, meteorologists with the the Spokane National Weather Service said their research indicated the source was from a dust storm in northern Nevada that carried some of the light-colored sands from dry lake beds into our region.

Well, now there’s updated research that tweaks the theory — the dust storm what whipped up light-colored sands is still the likely cause, only the source has now been better traced to a dust storm that occurred around Summer Lake in south-central Oregon.

Here is the full story from AP writer Nicholas K. Geranios

SPOKANE, Wash. (AP) – The mysterious milky rain that hit parts of the Pacific Northwest last Friday was the result of a rare weather phenomenon that began nearly 500 miles away near an Oregon lake, a Washington State University meteorologist said Tuesday.

A meshing of weather systems that appears to have started in southern Oregon ultimately caused dirty-white-colored raindrops to fall in eastern Washington and northeast Oregon, researcher Nic Loyd said.

The National Weather Service received reports Friday of ashy debris coating vehicles and windows in more than 15 cities, including Spokane, the Tri-Cities, and Hermiston, Oregon.

While the ash-like substance has not yet been scientifically identified, the Weather Service says it’s believed to be from a dust storm that struck Oregon’s Summer Lake on Thursday night.

Originally, a large storm that hit northwest Nevada was blamed for the unusual rainfall. “But the trajectory just didn’t add up,” said meteorologist Mary Wister of the Weather Service’s Pendleton, Oregon, office.

“The wind direction would have carried the dust into western Montana,” she said.

Instead, Loyd believes very high winds whipped across the Summer Lake region in south central Oregon, “lofting dry, light-colored sands and soils into the air.”

Next, southerly winds carried the sand and soil particles northward.

“Had the winds not been so strong or constant, the dust plume would have dispersed before it got here,” Loyd said. “As it was, it was able to travel a large distance in less than 12 hours.”

Finally, when the plume arrived, a rainstorm passing through pockets of Washington and Oregon drove the debris downward in the form of milky raindrops, he said.

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