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5 Day Weather Forecasts are the most accurate weather forecasts. Beyond 5 days weather forecast accuracy is significantly diminished.

According to research done by Robin Stewart, even though computers have increased exponentially in power, weather forecasts have only increased in a linear fashion. Here is a quote from his article Computers Meet Weather Forecasting...

"Yet despite this exponential increase in computational power, the accuracy of forecasts has increased in a decidedly linear fashion. Nothing shows this better than the graph of "anomaly correlations" for forecasts made by the European Centre for Medium-Range Weather Forecasts (ECMWF) (figure 6). Anomaly correlation is an indicator of forecast accuracy based on comparison of predictions and actual weather; anything above about 60% is considered "useful." The vertical axis measures how many days ahead are being forecast, and the colored lines represent constant anomaly correlation over the past 20 years. Thus, in 1980, predictions beyond about 5 days were essentially useless; but by 1998 5-day forecasts were fairly accurate (80% anomaly correlation). Looked at in a slightly different way, we can now predict 5 days ahead with the same accuracy as we could predict only 3.5 days ahead in 1980. (The lines shown are actually averaged out over a running 5 year period to smooth out yearly changes in the natural predictability of the weather.)".

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