This week's fog was shared with 25 other states

This week's fog across western Kentucky and southern Illinois turned out to be much bigger than just our region.

Since Tuesday, record amounts of fog have blanketed all or parts of 27 states from the Gulf Coast to Canada, and from the Great Plains to the Appalachians. More than 100 million Americans had to feel their way through at least three days of the foggiest phenomenon in at least twenty years.

On Thursday morning, dense fog advisories still covered the entirety of Iowa, Missouri, Louisiana, Mississippi, Illinois, Indiana and Tennessee and parts of states from Texas to New York.

A mashup of webcams from AccuWeather showed fog enveloping cities including Kansas City, St. Louis, Cincinnati, Nashville, Atlanta, Pittsburgh, Dallas, Oklahoma City, New York City, Washington and Chicago.

Tuesday, Wednesday and Thursday mornings set consecutive daily records for the number of dense fog advisories nationally since 2005.

On Tuesday, 1100 National Weather Service forecast zones were under a fog advisory, indicating visibility under a quarter mile. By Wednesday, the number increased to over 1300, and on Thursday more than 1,500 forecast zones were still under a fog advisory.

The National Weather Service said it's an extreme case of what's termed advection fog. It forms when warm, moist air is transported over the top of a layer of cold air near our recently frozen ground.

That happened on a continental scale as warm and moist air from the Gulf of Mexico was pumped northward over what was a record snowpack last week across the Lower 48.

We could still see those conditions spur more local fog into Friday morning, but it will diminish as a storm system sweeps from the Mississippi Valley to the East Coast by the weekend.



Thursday morning's national map was dominated by dense fog advisories over most of the Central and Eastern U.S. (National Weather Service chart)