What does 'probability of precipitation' really mean?

With rain in the forecast many look not only at whether it’ll rain but for the percentage chance that it will rain. That percentage chance is known as the probability of precipitation, or POP for short.

It all seems straight forward, with around a 40% chance for rain, either 40% of the forecast area will get some rain, or that it will rain for about 40% of the day, or, as some say, it’s definitely going to rain once precipitation percentages reach about 40%.


However, with sciences, and especially predictive sciences involving statistics, it’s a little more complicated and less clear cut than that and meteorology is exactly one of those statistical, predictive sciences.

According to the National Weather Service the probability of precipitation (POP) is "the probability that the forecast point in question will receive at least 0.01" of rain." The 0.01" rainfall total is to ensure measurable rainfall since the smallest precipitation increments are measured in hundredths of an inch.

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That means that any specific point in the "40% pop" forecast region has a 40% chance of getting measurable rain. That also means that there is a 60% chance the same point will get no rain at all.

Think of this 40% chance as a day where thunderstorms are forecast. One town or area may get a huge amount of rain whereas neighboring areas don’t receive any rain at all. That one town that got rained on will be in the minority, or 40% group, of those who, according to the forecast, should have expected rain. The areas that stayed dry will be considered to be in the majority, or 60%, of those in the forecast area.

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The biggest question now will be, how do I know if I’m in the majority or minority group? Well, unfortunately, weather forecasting still deals with a lot of unforeseeable conditions. While weather modeling and forecast skill has increased substantially over the past few years, it can be extremely difficult to forecast precipitation with that kind of precision.

The best way to know is to pay attention to your local forecasters – like Scott Fisher, Zach Shields, Chelsea Andrews, and Carlo Falco – for the dominant weather situation of the day. You can also use a weather app, like the FOX 7 Austin WAPP, for your own hyperlocal weather conditions once the day arrives. 


FOX 7 Austin Meteorologists update the forecast on-air, online, and on social media. You can also receive updates by downloading the FOX 7 Austin WAPP.