Long-term Wind Speed Forecasting: reality or wishful thinking?
Warning: "long-term wind-speed forecasting for generation" has (at least) two very different meanings. One refers to forecasting a distribution of wind speeds; the other refers to hour-by-hour (or half-hour by half-hour) forecasting of wind speeds.

Generally, when we talk about long-term forecasting of wind speeds for wind generation, we're talking about estimating the distribution of wind speeds over the next few years. That is to say, we're not talking about what the wind speed will be exactly two years from today, at 15.00 UTC. We're talking about what the shape of the distribution of the wind speeds will be - maybe in terms of the two parameters of a Weibull distribution, together with some indication of how the distribution varies by time of day, and by season of the year.

There is some skill in hour-by-hour wind speed forecasts up to about two weeks away. But for longer than that, there's very little skill in the models at all. In that regard, if we are talking about hour-by-hour wind speed forecasts, then short term is the next 0.5 - 4 hours, and long-term is 1-2 weeks.

In both cases, I'd look at recent publications from Reading University - David Brayshaw and others - and specifically on the accuracy of wind forecasts for wind generation, I'd also recommend work by Henrik Madsen and Pierre Pinson.