Milky Rain in Eastern Washington, a donation from Oregon!

Sample of the 'milky rain' that hit Eastern Washington
from KOMONEWS.com here
Sample taken in the rain gauge at the NWS office, Spokane

Last Friday a "white, milky rain" fell across parts of Eastern Washington, Oregon and Idaho, falling on 15 cities and grabbing headlines even in USA Today. Initial speculation about the cause even included the speculation that it was volcanic ash from a distant volcanic eruption. The Walla Walla County emergency management staff attributed the ash to a January eruption of Shiveluch volcano in Kamchatka, 3000 miles away. CNN meteorologist Derek Van Dam pointed out an eruption two days earlier near Colima, Mexico, 2000 miles away.

Summer Lake during an autumn storm
Image from Wiki here, no attribution given

According to the story referenced in the figure caption, by writers Scott Sistek and Nicholas K. Geranios, the explanation is more close by.
 Meteorologists traced the dust backwards to a dust storm over Summer Lake in Oregon on Thursday night. Summer Lake is a large shallow (1 foot-2 feet max depth) alkali lake in Lake County, Oregon. It is surrounded by arid lands, the remnants of an enormous Lake Chewaucan that formed in the late Pleistocene. The last high water was about 13,000 years ago as the Ice Ages ended. As the lake dried up, salts and alkali minerals were concentrated in the remaining waters and in sands and soils around the remaining water. Prevailing westerly winds formed sand dunes that lie on the east side of Summer Lake.
         Last week, high winds lofted these dry alkali sands and soils from the desiccated lake bed according to the NWS Mary Wister. Southerly winds then carried the particles northward, carrying the dust a large distance in less than 12 hours. When the dust got into Washington and Oregon, it ran into rainstorms which dragged it down as dusty milky raindrops.