There is often confusion as to what a specific percentage chance of rain means. The official National Weather Service definition of the Probability Of Precipitation is: "The probability of at least 0.01" of precipitation occurring in a specific forecast area within a given time frame". For example, in Atlanta if there is a 40% chance of rain that means there is good confidence in 40% of the area getting some measurable rainfall. If there is a 80% chance of rain that means there is good confidence in 80% of the area getting some measurable rainfall. Confidence in the percent chance of rain is often high between 3 and 5 days out but beyond 5 days the confidence is lower. Confidence in the percent chance of rain is often higher in the summer because the pattern and humidity. In the summer we have pop up rain showers and thunderstorms pretty much every day. In the Fall and Winter the confidence is lower sometimes due to the timing of fronts and frequent changes in the weather.
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