The milky rain that fell through central and eastern Oregon and into Spokane most likely came from Nevada's desert plains.

Scientists with the National Weather Service believe they've cracked the case of the cloudy water that fell from the sky throughout inland Oregon and Washington on Friday, prompting a flurry of calls to their office and boosting business at area carwashes.

The agency heard from residents in Coeur d'Alene, Idaho, the Tri-Cities in Washington, and Hermiston, Pendleton and several other cities in Oregon. Callers complained of chalky residue on their windshields and raincoats. Most reports were concentrated east of the Cascades.

Meteorologists had doubts about initial reports that the whitish residue could have been ash from a volcano in Russia or Mexico. Particles that had traveled that far likely would have been highly diluted, but the stuff that fell Friday was dense with particles. And if the dust cloud crossed the Pacific Ocean to get here, why hadn't locations toward the coast been affected?

It had to be something closer to home, and more recent.

Meteorologists knew high winds had whipped across northwest Nevada on Thursday night, downing power lines, causing traffic accidents and clouding the air with thick chalky dust similar in shade to the stuff that fell in the Northwest a day later.

Suspecting they had found the culprit, they ran some stats to be sure.

The incident coincided with a 15-hour period of south winds averaging 46 miles per hour in the eastern half of Oregon and Washington -- strong enough to carry dust hundreds of miles northward.

Traveling at an average of 40 miles per hour, it would have taken the dust cloud about 12 hours to reach Spokane. That times Friday's first milky rain reports perfectly with the Nevada storms of Thursday night.

Meteorologists can't be 100 percent sure their theory is correct without conducting mineral tests, but "putting the pieces together, it seemed to be the most reasonable answer," said Steve Bodnar, a meteorologist with the National Weather Service in Spokane.

"The fact that there was a big dust storm in a desert area with a lot of white soils kind of narrows it down," he said.

Since then, the rain pouring down across the Northwest hasn't let up, but the droplets are coming down clear again.

-- Kelly House and Lynne Palombo

khouse@oregonian.com

503-221-8178

@Kelly_M_House