  The modulational instability of waves in a medium under the action of an
external monochromatic force and dissipation is considered. The model which
describes the nonlinear stage of the modulation instability was constructed
with using methods of so-called $S$-theory. Following this theory, we take into
account only interactions between modes with slowly varying relative phases.
Within framework of this model, it is shown that the waves satisfying the
criterion of abnormality appears as a rule at the initial stage of the
instability. With further development of the process, the abnormal waves can be
also observed but their amplitudes decrease gradually with decreasing of the
average wave height.
