Interactive Eclipse Weather Map - NOAA's National Centers for Environmental Information has provided data that the North Carolina Institute for Climate Studies incorporated into an interactive map that allows users to click on a location to see how often skies at the location are Clear, Few, Scattered, Broken and Overcast. Read about the interactive map in an article, Ready, Set, Eclipse, available from NCEI.
Astronomy Picture of the Day, January 31, 2017 - This map from NASA shows the percent of cloud cover in the United States in August.
Eclipse.com August Cloud Cover Map - map showing average August cloud cover along the eclipse track. This website also has a chart and table that shows percent probability for various types of cloud cover.