You start with a late spring day after fields have been plowed for a number of crops.
You add a relatively dry spring, with rainfall less than 50% of normal over most of the region.
And you throw in strong winds, gusting to 40-70 mph in some places.
The result: blowing dust, obscured roads, and even some visibility-related accidents.
There is dust blowing right now around the Tri-Cities and around I90 from roughly Moses Lake to Spokane. Check out this video from WA State Trooper C. Thorson (click on the link) taken in Richard (I82 and Dallas Rd). Make sure you have the sound on!
Or check out this WSDOT cam in Davenport–pretty hazy.