Offshore wind energy is developing rapidly in Europe and the trend is towards large wind farms with arrays containing in excess of 80 wind turbines. Considering the capital investment required to develop an offshore wind farm, accurate assessment of the wind resource/power output from the wind farm is a necessity. Accurately predicting the power output requires an estimate of the wind resource over the lifetime of the wind farm (20-50 years) at the turbine hub height. The lack of climatologically representative in situ measurements at typical turbine hub heights and potential nonstationarities in flow regimes introduce uncertainties in the resource assessment which are described herein along with methods that may be used to reduce or quantify these uncertainties.