Why the forecast changed since you last checked

Weather models rerun several times a day on new observations. A night three days out routinely moves by twenty points between runs. A night tonight rarely moves much at all.

The underlying data comes from MET Norway, whose model assimilates fresh observations and reruns on a fixed cycle. Each rerun is a new best estimate, not a correction of an error. Cloud is the least predictable field in the whole forecast, and it is also the one that gates the score, so score volatility is dominated by cloud volatility.

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.

REVIEWED 2026-08-12 · SOURCES: MET Norway, sun and moon positions computed on our own servers