The National Weather Service has issued a dense fog advisory for five New Jersey counties until Saturday morning, with visibility expected to drop to a quarter-of-a-half. The advisory is in effect until 8 a.m. for Sussex, Warren, Morris, Hunterdon and Somerset counties. The weather service advises motorists to slow down and use low-beam headlights, leave plenty of distance ahead of them, and use high-beam lights. These conditions can make driving difficult and potentially cause accidents.