According to California ISO's Integration of Renewable Resources draft report (http://www.windaction.org/documents/12510) wind generation on a typical summer day peaks during periods of low demand and is at its lowest production levels when electricity demand is high.
This graph (http://www.windaction.org/pictures/12515) shows the variation of average hourly wind generation and actual wind generation (red dots) at the time of the daily system peak demand for the month of July 2006.
The report further states that while the daily summer pattern of high demand and low wind is predictable, the actual hourly wind generation output can vary significantly from one day to the next. As more wind is added to a grid region, the impact of this variability becomes more pronounced. Even with advanced wind forecasting in place and available five minutes ahead, wind could vary by hundreds of megawatts in the time it takes to respond to the forecast.