Use 'unawares' to describe being surprised by something happening quickly and without warning. The sudden downpour caught me unawares.
Pro Tip 2/3
Impacts Reactions
Usually implies a reaction is needed when something happens unawares, like dodging a ball. I had to jump back unawares when the ball headed my direction.
Pro Tip 3/3
No Time to Prepare
Emphasizes an absence of time to prepare for an event or action. The quiz took the class unawares.