The National Weather Service issued a dense fog advisory at 10:27 p.m. on Wednesday in effect until Thursday at 10 a.m. for Northern Clinton, Northern Centre, Southern Centre, Northern Lycoming, Southern Clinton and Southern Lycoming as well as Potter, Elk, Cameron, Clearfield, Cambria, Blair, Huntingdon, Somerset, Bedford, Fulton, Franklin, Tioga, Union, Snyder, Perry and Cumberland counties.