The gray areas on the map are under “dense fog advisories.” The hunter green colors in North Dakota and near the Illinois-Wisconsin border are “freezing fog advisories.” Right now, Kansas City International Airport has 45 minute arrival delays due to fog.
The reason the fog has been so persistent is that it is so wet in the central U.S. Here is the latest (a new one out tomorrow) Palmer Drought Index.
Note the correspondence between the green and greenish colors and the dense fog advisories. Because we have not had any strong cold fronts or strong cold fronts move through the region, the stagnant air picks up the moisture from the wet ground and melting snow and becomes saturated. Once the air is saturated near the ground or aloft, a cloud forms. Fog is just a cloud on the ground.
Note: The drought depicted in the Southwest will alleviated over the next ten days.