Wave resource characterization is an essential step for Wave Energy Converter (WEC) development. The West Coast is one of the top regions in the U.S. with an energetic wave energy resource and great potential for early market development. This paper presents a study using a multi-scale modelling approach combined with shallow water wave measurements to improve the accuracy of wave resource characterization. The multi-scale wave modelling was conducted with a nested-grid WaveWatchIII (WW3) model from global to regional scales. The Unstructuredgrid Simulating Waves Nearshore (UnSWAN) model was used to provide accurate wave hindcast with a resolution of approximately 300 m, which meets the requirement recommended by IEC for wave resource assessment and characterization for the feasibility class. Extensive model validation for a period of 32 years was conducted using measured data from wave buoys maintained by National Data Buoy Center, as well as from three recently deployed nearshore buoys along the Oregon and California coasts. Inter-annual and seasonal variations of wave characteristics along the entire West Coast was analyzed. In addition, a sensitivity analysis was carried out to evaluate the quality of wind forcing on the accuracy of model prediction for large waves.