Deterministic weather forecast are only possible for one to two weeks. (Or in other words, we just can’t predict whether it will be sunny or rainy 14 days from now.) But can we say something about the weather over the next few weeks, for example, will it be warmer and drier than average, even if we can’t say exactly which days will be sunny?
The answer is yes, we can (sometimes) provide informative forecasts on the time scales of a few weeks to a month, or “sub-seasonal” in the weather prediction lingo. Please check our our chapter Sub-seasonal Predictability and the Stratosphere, where we show how the the stratosphere can help us provide useful forecasts on these time scales! This is just one chapter in the epic, 585 page Sub-Seasonal to Seasonal Prediction: The Gap Between Weather and Climate Forecasting.