Because of the complexity of these models, forecast verification goes a good deal beyond simple measures of statistical association or mean error calculations.
Most weather situations will cycle, since the Earth is forced by a highly regular energy source.
This output, however, adds no information content, since the same cycles are easily predicted from climatological data.
Previous computer models of weather were based on persistence alone and reliable to only 5–7 days into the future.
Christensen et al. demonstrated the ability to predict the probability that precipitation will be below or above average with modest but statistically significant skill one, two and even three years into the future.