The National Weather Service in Portland issued a dense fog advisory from 7 p.m. Christmas Eve through noon Christmas day.
Fog briefly lifted Tuesday afternoon but widespread dense fog with visibility one quarter mile or less returned later in the evening and was expected to persist through late Christmas morning.
Affected areas included Hillsboro, Portland, Oregon City, Gresham, Troutdale, Vancouver, Battle Ground, Camas and Washougal.
Motorists were advised to use caution and slow down as visibility may vary considerably over short distances. A few pockets of near freezing temperatures will also bring a threat for icy or slick roads and sidewalks.