One should be wary of secret and 'proprietary' methodologies, and check with government and academic sources for the validity of extraordinary forecasting claims. Forecasts can be made for areas as large as states or regions, or can be made site-specific, down to one weather station in a city. Critical days forecasts, such as the number of days the maximum temperature will exceed a threshold, and multivariable forecasts, such as the number of days the minimum temperature and snowfall exceeds their thresholds have been done. End-users of these forecasts include industries from retail to agriculture to energy. A meteorologist starts the seasonal long-range forecasting procedure with historical climate records, or a climatological database.
Climate data provides the all-important foundation from which a forecast is whittled.
The data should be as long, complete and as 'clean' as possible. Adjustments for warming or cooling trends, station moves and equipment changes all help clean abnormalities. For example, when the Sioux Falls, SD weather equipment was upgraded, it was also repositioned, moving a quarter of a mile closer to the edge of a river bank. The new site was much cooler, and often enveloped by fog. Without this knowledge, forecasts based on the older climate record will be too warm.