This paper presents a RWA strategy based on the stochastic estimation of the Effective Number of Available Wavelengths (ENAW) along interdomain paths. We propose an approximate model to roughly estimate the ENAW on the paths across multiple domains, and then refine this estimation by means of observations and an adaptive prediction-correction Kalman filtering process. Unlike traditional Kalman filters, which use noisy measurements as their observations, we use the information contained in the routing updates as noisy measurements of the existent wavelength occupancy along the paths. Based on these observations, we estimate the ENAW on the candidate paths, and use this information to influence the RWA decision when the number of available wavelengths makes a lightpath request prone to be blocked. The approximate model we are proposing in this paper is based on a noisy extension of a simplified model derived for two domains. Our results validate the usefulness of the model, and confirm that by estimating the wavelength occupancy prior to the RWA decision the blocking ratio can be considerably reduced.