Would a question about the future of numerical weather prediction with quantum computing be off-topic? This is most likely going to create speculative answers, so I'd imagine it would be, but I wanted to make sure.
It really depends on exactly what the question is asking.
For a question like this, it's going to ask for opinions, as you say. I don't think this would be a good fit for Stack Exchange, as it's asking too much for speculation. However, for some questions that are kind of subjective, but have enough data to back up an idea, that's okay.
I'd recommend reading this blog post: Good Subjective, Bad Subjective. Here's what it all boils down to:
Thus, questions that are not answerable — discussions, debates, opinions — should be closed as subjective. It seems simple enough: Fact good; opinion and discussion bad.
In general, SE sites frown on "future predictions" regarding weather prediction, or any other topic. That's because they are too "speculative," and will generate more opinions than fact (more noise than signal).
That said, there is probably a valid exception if some recognized scientific expert made a future prediction. Then a valid question is, what did [recognized expert] say about...?" The content is regarding the future, but the statement was actually made in the past, which is to say that "such-and-such was said by so-and-so" is a fact.
Another valid question might be, "what methods are currently used to predict the future?" The future is the future, but the methods being used to predict it are in the PRESENT.