The most accurate and detailed forecasts are short term because weather systems change directions, speed, and intensities over time. However, these forecasts are still only able to be predicted probabilistically.
It is impossible for computers to model all of the different variables that affect the weather at a given time and place, long-term forecasts are even less reliable than short-term forecasts. Recall that features on Earth's surface, such as lakes or snow cover, affect the amount of thermal energy absorbed at any location, which in turn, affects the wind. Wind influences cloud formation and virtually all other aspects of the weather in that location. Over time, these factors interact and create more complicated weather scenarios.