I really like this and am probably going to use this, but my goal was to give players a chance to prevent being ambushed while simultaneously avoid laying out the ambush right out of the gate.
Sure, I get that. I do wonder whether giving them a chance to avoid the ambush is a worthwhile goal.
That's another cryptic statement. So, perhaps I should unpack my thoughts. The discussion started with your conflicting desires to preserve surprise while giving the players a chance to avoid the ambush, which seemed to be at odds. I think the issue of surprise was discussed well already, but the problem I see with both of these goals is they're making you consider passive GM moves and the game is bucking against it. There are no passive GM moves. Announce Future Badness has an active verb. It's an announcement. A proclamation.
While you can kind of twist announce future badness to fit the situation, I don't think it's a great fit. Putting the players somewhere where they know that you know that they know the caravan is going to be ambushed and getting them to roll Read a Sitch or something isn't, to my admittedly idiosyncratic way of thinking, announcing future badness. It's more like implying future badness. The badness is vague and unformed. You've clearly established all manner of potential harm but the players are still looking around for what the badness will be. That's pretty much the default state in AW.
In my example, I didn't really spring the ambush on the players. Not really. There was absolutely no chance that any of the players were going to get hurt by that explosion. None of their stuff was damaged or taken away. I blew up only my own stuff. My NPCs, my vehicles. My stuff is always in the crosshairs. You used your stuff to announce future badness by describing a canyon. JustusGS used his stuff to describe hard packed dirt. I decided to make a definitive statement with my stuff. Here be badness! Then I followed up with another GM move--Put someone on the spot. These are not passive moves, but the fiction is immediately flowing, I'm firmly guided by the GM principles. If they trigger a move like Read a Sitch, I can quickly respond to a miss as easily as a 12+. There lives are not boring, im putting my bloody fingerprints on everything.
And, I think I probably surprised the hell out of em to boot.