One of the most important parameter in ship's weather routeing is the forecasted environmental data used by the optimization algorithm to calculate the most optimum route for the ship, if the characteristics of the error of the forecasted data are known a better routeing could be achieved. In an approach to do so, we proposed to compare the wind and wave forecasted data issued by the Japanese Meteorological Agency (JMA) with observations contained in the International Comprehensive Ocean-Atmosphere Data Set (ICOADS). The period of comparison extends from January 2003 to December 2008. The comparison shows that the JMA data represents the actual weather situations fairly enough, the ME and the RMSE of the forecast are well dependant of the seasons variation and latitude; around the tropics the ME and RMSE are smaller than at higher latitudes. The evolution of the error with the forecast period is quite steady and stable. However the errors becomes really large during bad weather, the JMA data tends to underestimates the wind speed and wave height when strong winds over Beaufort scale 8 blow, so if ever these data would be used for routeing a great care should be taken to deal with that point.