You want a met forecast of the prevaling wind in six weeks time ? H'mmm, that'll be as accurate as a forecast of the lottery numbers. Why don't you try a slightly different approach (1) contact airfields in the area and ask if they maintain records/trends for previous years; (2) Ask the balloon people what their thinking is. Whilst I haven't been ballooning I'm sure they can make use of the coriolis effect (seem to remember the adage that "height is right" - i.e. wind veers with increasing altitude). Therefore they have an element of directional control. Take a look at the Met Office synoptics too - probably a good a way as any of guessing what the wind will be a few days later.