To answer your initial question first: an argument can be valid if its premises are merely opinions, or even if they are false. An argument is conventionally said to be sound if it is valid and its premises are true, so an argument with premises that are merely a matter of opinion has a soundness that is merely a matter of opinion. "Chocolate always tastes better than vanilla, therefore this chocolate ice cream will taste better than that vanilla one" is valid, but its soundness will depend on whether you share the opinion expressed by the premise.
As to the argument you give, there are at least two problems with it. The first premise is highly contentious. It relates to the issue called future contingents, and philosophers have been arguing about this at least since Aristotle. There is an article about it in the Stanfard Encyclopedia of Philosophy. It is not uncommon to hold that statements about the future are neither true nor false until they actually happen, in which case one would say that premise 1 is false.
The other problem is that the argument makes a modal jump from premises that are expressed indicatively as statements of fact, to a conclusion about what we 'must' do. To move from what is true to what must be true is also highly contentious and is arguably simply invalid without additional assumptions. In general, making decisions like whether to get into an air raid shelter may depend on many different considerations, including the probability and impact of the various possible outcomes, and also what alternative actions are open to you and whether these have a priority claim over avoiding getting hurt.
