Sunsets on the frozen tundra of Eriador, the wind howls and the wargs are closing in.

At the end of the turn, the Forced on Nighttime reveals a new encounter card, which is Predatory Wolves . If we choose the Blood-thirsty Warg , are we still resolving the original Nighttime flip?

The Blood-thirsty Warg wasn't in play when it became Night, so you're safe. I recall seeing a ruling about that recently -- Forced effect not triggering even if you're still "in that moment" because the card was not in play when the trigger went off.... different from player card Responses (you can trigger a Foe-hammer that you just drew by playing another Foe-hammer(

Can't seem to recall the context.

I asked a similar question a while ago. I received the answer:

Forced effects do not trigger unless they are in play at the time that they would trigger. This is true for Response effects as well.

The distinction GrandSpleen is making is based on player cards Responses that are played from your hand---since they are in your hand rather than in play, they are not bound by this ruling.