A Dense Fog Advisory is in effect until noon today for the central valley including the western, lower portions of Tuolumne and Calaveras counties.
According to the National Weather Service, visibility has been reduced to less than a eighth of a mile. There are reports of visibility near zero.
Locations of dense fog include Stockton, Modesto and Sacramento.
A Dense Fog Advisory means visibilities will frequently be reduced to less than one quarter mile. If driving, slow down, use your low beam headlights and leave plenty of distance ahead of you. If possible, postpone travel until the fog lifts.