At continuous casting plants the liquid steel level in the mold has to be stabilized by the mold level controller. Many plants are however perturbed by an effect called dynamic bulging, which is an almost periodic disturbance consisting of the fundamental and harmonics up to the 6th order. Its frequency can be assumed to be approximately known, but can change with a known parameter, given by the casting speed. A measurement, which can be used to estimate the periodic disturbance is available, hence the periodic components of this signal can be observed and predicted with an internal model observer and be applied as disturbance feedforward to the control input. The prediction of the signal is necessary because the plant has a substantial input delay. Since the internal model depends on the frequency, which is available as parameter, LPV techniques are used to show convergence of the observer.