The Met. Office currently operates a high resolution mesoscale model that provides guidance on the likely formation of radiation fog on the spatial scale of 30-40km. A project has been under way to build a Site-Specific Forecast Model (SSFM) using a one-dimensional version of the Met. Office Unified Model as a basis, with suitable modifications to its surface treatment and, driven by output from three-dimensional NWP models. The SSFM has been applied to the forecasting of radiation fog as part of the European Commission sponsored project 4MIDaBLE (4Dimensional Met. Information DataBase Linked across Europe). This paper describes the influence of site observations on the model evolution for two real fog cases. This requires a method of forcing the SSFM via coupling to three-dimensional NWP model output; such a method is described. It is shown that when observational data at the site of interest is incorporated into the model's initial state humidity measurements provide the greatest impact on model evolution. However, their influence is short-lived (typically less than 6 hours) implying the importance of initialisation time in the correct forecast of radiation fog.