Special Weather Statement issued December 1 at 11:59PM PST by NWS Portland OR

Areas of dense fog will impact mainly parts of Clackamas County
in the Portland metro area overnight into early Monday morning.
This includes Milwaukie, Lake Oswego, Oregon City, and Happy
Valley. Visibility will be reduced to one quarter mile or less at
times, and low visibility could make driving conditions hazardous
in these locations. Locations within the City of Portland and
Multnomah County will mostly remain clear due to east winds out of
the Columbia River Gorge. If you encounter dense fog while
driving, slow down and leave plenty of distance ahead of you.
Pedestrians and cyclists should wear bright or reflective clothing
to increase the chances of being seen by others in poor
visibility conditions.