Thanks for the hint, though the forecasts are not part of the entity’s attributes, neither is a timestamp of the last update to the cache, so this would only trigger if there is change to the current weather between two updates.
Of course the current weather will change regularly, as temperature, humidity, … change over time, but it would be at least theoretically/potentially error prone.
Maybe a combination of state trigger plus time_pattern trigger at a longer interval is the way to go.