Question: What does the probability of precipitation really mean?

Answer: The probability of precipitation is often referred to as PoP. It is the percentage used to signal the certainty precipitation will form or move into the specified area. This is the percentage seen on weather graphics to signify the chance of rain for a certain day, hour, or time period.

Now often people misunderstand what the percentage means. It does not stand for the percentage of the area at a time that will be seeing the precipitation and it also does not mean you will see precipitation for that percentage of time in the forecast window for the area.

What the probability of precipitation means is that is the probability at any point in the area that there will be precipitation. The official definition from the National Weather Service states the “Probability of Precipitation simply describes the probability that the forecast grid/point in question will receive at least 0.01″ of rain.

It is actually calculated by a formula which is the certainty that it will either form or move into the area multiplied by the expected area to be covered by the precipitation. That number is then changed into a percent by moving the decimal to the left two places.

Now here is an example of how this is applied. If when forecasting for Chemung county, there is a 10% chance of rain for the day. There is a 10% chance of rain at any given point in Chemung county that it will be raining at least 0.01″ during that daytime period.

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