Visibility levels in Southeast Pennsylvania and Northern Delaware are pretty awful this morning -- generally in the quarter to half mile range or worse depending on location. A warm front generally resides along the New Jersey Turnpike across South Jersey, separating 50's across coastal sections of the Garden State from 40's along the river and northwest. This warm front should lift slowly north and northwest through the course of the morning, allowing visibility levels to improve gradually later on. Essentially those areas in the 40's in Pennsylvania early this morning and closest to the warm front boundary are dealing with the densest fog.
It may take until late morning for Pottstown, Quakertown, and points north to see improved visibility levels but as the warm front pushes northward, the warm surge should help push that fog bank out of your area.
Until then, expect areas of locally dense fog across Eastern and Southeastern Pennsylvania, with visibility levels ranging from a few hundred feet to as "good" as a mile in spots.