How do you manage the unknown?
For starters, make it part of your risk work. Risk is the potential for events with impact on your goals. Focus on low-likelihood events with high impact on availability. This is Continuity Risk - your continuity compass.
Have it integrated with your Risk Management. Identify, describe and quantify. Use your history, known events which could have a higher impact if repeated. How bad could it get on a rainy day? Analyse and treat. You know the drill. Expect the unexpected.