Dense Fog in Massac, Illinois

Surface high pressure was draped across the southern Illinois from the evening of the 5th through the morning of the 6th. As a result, skies were clear and winds were calm, which allowed dense fog to develop generally along and east of the Interstate 57 corridor overnight through sunrise. The fog first developed near Mount Vernon in the late evening of the 5th, and gradually spread southward through the pre-dawn hours on the 6th.