Weapon Declarations in Conflicts

When (and how) should the GM declare weapons for monsters? Is the intention that the players should know the weapons in advance before picking their actions or is it a surprise when revealing actions? And is it better to just say the name of the weapons the monsters or using or just try to describe them narratively instead?

Yup. During the weapon declaration phase in between rounds.

Per the rules, all you need to do is announce the weapon name.

In my subjective experience, the game is greatly served by the GM describing weapons in as much thrilling detail as possible. We even go as far as to describe the nature of the action, subtly, without referring to the actual action type. We call this “Chewing the Scenery” and it helps the players role-play their own actions well without sacrificing the capacity for strategy. Of course, the GM is permitted to openly mislead the players with his description in the case of a Feint, and that keeps things unpredictable enough.