In previous studies, we have developed and tested observable-based parameterization (OBP) of time-harmonic wavefield scattering by periodic or aperiodic finite arrays of planar strip and filament scatterers. The resulting algorithm is based on truncated Floquet modes and Floquet-modulated edge diffractions due to the truncations of the array. The corresponding robust wave processes link features (observables) in scattering data with geometrical features in the model configuration in such a manner as to be useful for subsequent application to target classification and identification. The present study extends these investigations to a finite array of filamentary scatterers located on the surface of an infinitely extended dielectric slab, thereby parameterizing (Floquet mode)-(leaky mode) interaction as a classifier of the more complicated phenomenology in this composite configuration. The outcome is an OBP with two separate constituents that can be interpreted, respectively, as slab-modified Floquet scattering by the truncated array and as truncated-Floquet-induced excitation of slab-guided leaky waves. This new OBP for the composite problem is validated by comparison with reference solutions generated numerically. Its relevance to wave-oriented data processing is demonstrated in the companion paper. © 1996 IEEE.