Aldarc
Legend
IMHO, the problem is that you appear to be thinking about this in terms of D&D prep where you pre-plan in advance an ambush or surprise encounter. That's not fundamentally how PbtA games work.So you've answered my question (thanks for that!) but sadly, it's the answer I kind of expected but didn't want to see: I can't ever truly ambush or even surprise the PCs - they always get a warning.
But, I'd guess the PCs can take steps to ambush their NPC foes and, if lucky, drop those foes before they even knew trouble was upon them. So how is that fair, in terms of in-fiction consistency?
The GM frames a situation and then asks the players what they would do in response. If their PC's actions trigger a Move, then the player roll. Based on the roll, the GM adjudicates the negative consequences (i.e., soft and hard moves) as appropriate to the fiction to set up a new situation. What you want to do here is outside of your job description as a GM in PbtA.
This doesn't mean that the PCs can't be ambushed. For example, the PCs may plan an ambush for some goblins at a campfire. The player rolls a 6- on whatever move they triggered while trying to stealthily get into position (e.g., Defy Danger). The GM decides that instead of the PCs ambushing the goblins, the goblins have actually ambushed them! It turns out that those goblins were the bait! (GM Move: Turn their move back on them.)
Alternatively, the GM may set up a situation that points to a potential danger of goblins in the area: reveal an unwelcome truth or show signs of an approaching danger. This can pave the way for a goblin ambush but you aren't supposed to plan one in advance.