In the pantheon of HPC grand challenges, weather forecasting and long-term climate simulation rank right up there with the most complex and com- putationally demanding problems in astrophysics, aeronautics, fusion power, exotic materials,and earthquake prediction, to name just a few. Modern weather prediction requires cooperation in the collection of observed data and sharing of forecasts output among all nations, a collabora- tion that has been ongoing for decades. This data is used to simulate effects on a range of scales— from events, such as the path of tornados, that change from minute to minute and move over distances measured in meters, to turnover of water layers in the ocean, a process that is measured in decades or even hundreds of years, and spans thousands of miles.