“If you look outside, you might think it’s just clouds,” said Logan Johnson, a meteorologist with the National Weather Service’s Seattle office. The downpour of salt-and-pepper ash, he added, is like “nothing we’ve observed in quite some time.”

Meteorologists say it has been an unusually dry summer in a region known for rain. It has not rained significantly in Seattle since June, Mr. Johnson said, and meteorologists say it has been more than 50 days since measurable precipitation fell in Portland, Ore.

Instead, a strong ridge of high pressure has settled over much of the Pacific Northwest, heating the air and blocking storms from entering the area. As a result, trees, grass and other foliage have dried out, creating fuel that officials say is ripe for ignition if lightning strikes or sparks fly.