The monthly prediction is based on an atmopsheric-ocean-model because the Ocean gains more and more influence on the atmosphere if you look further than 10 or 15 days. It is true that the quality of prediction becomes less exact with a longer range to forecast, but there are many weather constellations where it is possible to make good forecasts for more than 10 days. The forecast is updated once a week. At the graphic you can see the forecast (red line), the mean values (blue) and the 80% range of ensemble forecasts.