yr.no is the only one that I've found that really does cloud coverage well.

https://www.yr.no/en/details/graph/2-4887398/United%20States...

https://www.yr.no/en/details/table/2-4887398/United%20States...

You'll note not only cloud cover %, but fog, low, middle, and high level cloud amounts.

The API is documented https://developer.yr.no

https://api.met.no/weatherapi/locationforecast/2.0/compact?l...