A weather forecast tells you about the day. An observer needs to know about four hours of one night, at one set of coordinates, for one kind of target.
Cloud is reported as a single percentage when the layer matters more than the total. The moon is reported as a phase when its altitude matters more than its phase. Darkness is assumed when at some latitudes and seasons it never arrives at all. And nothing tells you when to set up and when to pack down.
So this site computes one 0–100 score per hour from cloud broken out by layer, atmospheric transparency, estimated seeing, moonlight and true astronomical darkness — and then shows its working, because a number you cannot check is a number you should not trust.
It is an independent project, not affiliated with any of the data providers. Forecasts are estimates. Always check conditions yourself before travelling to a dark site, and remember that a sky forecast knows nothing about road access, fire closures or whether the gate is locked.