The National Weather Service has issued a Dense Fog Advisory which is in effect until 4am tomorrow morning for the following counties, Black Hawk, Bremer, Butler, Cerro Gordo, Franklin, Grundy, Hamilton, Hancock, Hardin, Winnebago, Worth, Wright, Mitchell, Howard, Winneshiek, Allamakee, Floyd, Chickasaw, Fayette and Clayton. Visibilities will drop to under 1/4 mile at times across northern and northeast Iowa tonight into early tomorrow morning. Remember to use low beams if driving tonight and early tomorrow morning.