Unbalanced class priors

After several sunny days, your predictive model tells you that there is a very high chance of the following day also being sunny, yet it rains. Why? This is simply because your model has not seen enough instances of both prediction classes (sunny and rainy days) to accurately assess the chance of there being rain. In this case, it is said to have unbalanced class priors, which misrepresent the overall weather pattern. According to your model, there are only sunny days because it has only seen sunny days as of yet.