"Imagine a rotating sphere that is 12,800 kilometers (8000 miles) in diameter, has a bumpy surface, is surrounded by a 40-kilometer-deep mixture of different gases whose concentrations vary both spatially and over time, and is heated, along with its surrounding gases, by a nuclear reactor 150 million kilometers (93 million miles) away. Imagine also that this sphere is revolving around the nuclear reactor and that some locations are heated more during one part of the revolution and other locations are heated during another part of the revolution. And imagine that this mixture of gases continually receives inputs from the surface below, generally calmly but sometimes through violent and highly localized injections. Then, imagine that after watching the gaseous mixture, you are expected to predict its state at one location on the sphere one, two, or more days into the future. This is essentially the task encountered day by day by a weather forecaster."
—On the difficulty of weather forecasting, Bob Ryan, Bulletin of the American Meteorological Society, 1982.
A forecast is simply a prediction of what may happen in the future. Currently there are different methods of obtaining a forecast and there are different types of forecasts produced depending on the time range. A weather forecast is produced usually for a single or couple of days. For periods longer than a week and less than a month, the term mid-range forecast is used and those that are a month to longer in the future are called monthly or seasonal forecasts.