Transient dynamics of spiral waves in a two-dimensional Barkley model is shown to be governed by pattern formation processes resulting in regions of synchronized activity separated by moving interfaces. During the transient the number of internally synchronized regions decreases as synchronization fronts move to the boundary of the simulated area. This spatiotemporal transient dynamics in an excitable medium is detected and visualized by means of an analysis of the local periodicity and by evaluation of prediction errors across the spatial domain. During the (long) transient both analyses show patterns that must not be misinterpreted as any information about (spatial) structure of the underlying (completely homogeneous) system.