This statement of probability is not like rolling dice, where the probability of getting a certain number is known with statistical certainty that a sufficient number of rolls would converge to. It is, rather, based upon the observed frequency of rain occurring with conditions similar to those predicted by the weather model, e.g. at a certain temperature, relative humidity, and warm front coming in, rain is observed to occur xx% of the time. There is some additional conditional probability from cumulative predictions of precipitation such that a day in which each hour is predicted to have a 50% probability of rain may have a cumulative probability of 60% or 70%, but it is all based upon largely empirical models and some simplified atmospheric behavior modeling.
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