Why the forecast changed since you last checked

Weather forecasts update as new observations arrive. Cloud timing can change even on the day you plan to observe, so use the outlook to plan and check again before leaving.

The forecast comes from MET Norway and is refreshed as its underlying weather models update. New observations change the estimated starting conditions, and model uncertainty grows as a forecast looks further ahead. Cloud changes can strongly affect the GoodSeeing score because cloud cover gates the observing window; no fixed number of points describes every update.

Practically: treat anything beyond about three days as a planning signal rather than a commitment. Use it to decide which nights are worth keeping free. Re-check the evening before, and again a couple of hours before you load the car.

For the parts that genuinely do not change — when astronomical darkness runs, where the moon is, how much of it is lit — use the moonless night planner. Those are orbital mechanics, not meteorology, and they are as reliable a month out as they are tonight.