Dense Fog Developing Across The Treasure Coast and Lake Okeechobee Regions, Including Southern Osceola County... At 335 AM, surface observations, satellite imagery, and traffic cameras showed dense fog had developed across Okeechobee, Saint Lucie, Indian River, and southern Osceola Counties. The fog was reducing visibilities to between one quarter and one half mile in some areas. Through sunrise, the fog could expand into Martin and southern Brevard Counties. If dense fog reducing visibility to one quarter mile or less becomes widespread, then a Dense Fog Advisory will be needed. The fog is expected to dissipate between 8 and 9 AM. Persons out driving this morning should prepare to encounter rapid lowering of visibilities in fog. When driving in dense fog, reduce your speed, use only your low beam headlights, and leave extra following distance between your vehicle and the one in front of you.