The National Weather Service has announced a "Dense Fog Alert" as well as an air stagnation warning:

.DENSE FOG AND LOCAL FREEZING FOG WILL REDUCE VISIBILITY TO LESS THAN A HALF MILE OVER MUCH OF THE WESTERN WASHINGTON INTERIOR LOWLANDS THROUGH FRIDAY MORNING. IN ADDITION AN AIR STAGNATION ADVISORY WILL REMAIN IN EFFECT THROUGH EARLY NEXT WEEK AS LIGHT
WINDS AND A LOW LEVEL TEMPERATURE INVERSION TRAP POLLUTANTS NEAR THE SURFACE.

...DENSE FOG ADVISORY IN EFFECT UNTIL NOON PST FRIDAY...
...AIR STAGNATION ADVISORY REMAINS IN EFFECT UNTIL NOON PST
TUESDAY...

THE NATIONAL WEATHER SERVICE IN SEATTLE HAS ISSUED A DENSE FOG ADVISORY...WHICH IS IN EFFECT UNTIL NOON PST FRIDAY.

* TIMING...TONIGHT AND FRIDAY MORNING.

* VISIBILITY...LESS THAN A HALF MILE IN MOST AREAS AND LOCALLY NEAR ZERO.

* IMPACTS...LOW VISIBILITY WILL CREATE DIFFICULT DRIVING CONDITIONS AND FREEZING TEMPERATURES WILL CREATE LOCALLY ICY ROADS. STAGNANT CONDITIONS IN THE LOWER ATMOSPHERE WILL ALLOW POLLUTANTS...SUCH AS WOOD SMOKE AND VEHICLE EMISSIONS...TO INCREASE NEAR THE SURFACE.

* RESTRICTIONS/BURN BANS...CHECK WITH YOUR LOCAL AIR QUALITY AGENCY FOR ANY RESTRICTIONS IN YOUR AREA.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

AN AIR STAGNATION ADVISORY MEANS THAT LIMITED MOVEMENT OF THE AIR
MASS OVER THE ADVISORY AREA WILL ALLOW POLLUTION TO INCREASE TO
DANGEROUS LEVELS. PERSONS WITH RESPIRATORY ILLNESS SHOULD FOLLOW
THEIR PHYSICIANS ADVICE FOR DEALING WITH HIGH LEVELS OF AIR
POLLUTION.

A DENSE FOG ADVISORY MEANS VISIBILITIES WILL FREQUENTLY BE
REDUCED TO LESS THAN ONE QUARTER MILE. IF DRIVING...SLOW DOWN...
USE YOUR HEADLIGHTS...AND LEAVE PLENTY OF DISTANCE AHEAD OF YOU.