||Weather forecasts provide information about the
weather expected over the next few days.
Seasonal forecasts provide information about long-term trends and
normally provide outlooks for 3-month-averaged temperature and precipitation
up to 6 months ahead. The outlooks are based on predicted probabilities
for three categories; below normal, near normal and above normal
for the region and time of year. Skill for these long-range outlooks
is substantially lower than for the shorter-range predictions, and
the techniques used here for seasonal forecasting are still at an
early stage of development. With a knowledge of current sea surface
temperatures in the tropical ocean basins, and model predictions
of how the oceans will likely evolve during the next several months,
numerical models of the atmosphere are used to predict mean seasonal
patterns of temperature and rainfall. But because the atmosphere
is a chaotic system, small changes in the current conditions can
result in big differences in the seasonal patterns the model predicts.
So, a single prediction does not tell the whole story. Each prediction
is just one possible outcome. This problem is solved by doing a
number of predictions. The combination of several of these predictions
is the best estimate of the next season's mean climate.