Cutting Through has a good post on scenario planning:

The idea is to create a range of plausible, internally consistent futures, covering the range of circumstances that the project might have to operate within. The futures can then be used to evaluate how well potential strategies would operate - they are not forecasts as such, but “rich pictures” of possible futures.

A plausible scenario means that there is a coherent set of events leading from the present to the future, rather than the “with one mighty bound Jack was free” leaps of imagination.

An internally consistent scenario doesn’t contain any mutually exclusive assumptions - for example, increased volume of services together with reduced funding might be inconsistent.