Numerical simulation of the performance of new beamlines and those under upgrade requires sophisticated and reliable information about the expected surface slope and height distributions of planned x-ray optics before they are fabricated. Obtaining such information should be based on the metrology data measured from existing mirrors that are made by the same vendor and technology; but, generally, with different sizes, slope, and height rms variations. In this work, we demonstrate a method for highly reliable forecasting of the expected surface slope distributions of prospective x-ray optics. The method is based on an autoregressive moving average (ARMA) modeling of the slope measurements with a limited number of parameters. With the parameters of the ARMA model, which we determined, the surface slope profile of an optic with the newly desired specification can reliably be forecast. The forecast profile, even if longer or differently shaped, still maintains the statistical properties of previously measured surfaces. We demonstrate the high accuracy of this type of forecasting by comparing the power spectral density distributions of the measured and forecast slope profiles.