Twenty day weather forecasts are generally unreliable because weather has a tendency of being chaotic and unpredictable. According to Time and Date, the longer the weather forecast is, the less accurate it will be.
According to Time and Date, there are four different types of weather forecasting. The first is short-range, and predicts weather patterns in a forty-eight hour time frame. An extended forecast predicts weather from three to five days, while a medium range predicts from three to seven days. The least accurate is a long range forecast which predicts weather for seven or more days. Because of the length of time, the weather has the potential to change before the predicted date because it is difficult to accurately predict over a long period of time.