Probability of precipitation- what it means

We talk about the chance of precipitation and show it on our forecast graphics to give you an idea of the likelihood of precipitation occurring at your location. This “chance of precipitation” is called a POP or probability of precipitation. It is the likelihood of occurrence of precipitation, stated as a percentage for a measurable amount of liquid of .01″ or more for a specified time period, for an given point in the forecast area. It is a statistical probability.

For example, a 30% chance of rain does not mean that 30% of the area will be covered by precipitation or that you will get rain 30% of the time during the given time period at your location. What it does mean is that there is a 30% chance of rain at any point in the forecast area during a given time period.

The POP is determined by multiplying two factors: the certainty that precipitation will occur and the areal coverage of precipitation that is expected. So for instance, if we are 80% certain that rain would develop but would only cover 50% of Mid-Missouri, then we would give a 40% chance of rain for that time period. Or, if we expect all of Mid-Missouri would be covered by rain but only 40% certain it will develop, then we would give a 40% chance of rain.

ABC17 Stormtrack forecasts for a large part of Missouri which includes 23 counties and our chance of precipitation takes any point in this area into account. Here is a look at the counties we cover.

We often use descriptive words to go along with the POP. When the probability is in the 80-100% chance, we will use terms to describe the duration of precipitation such as brief, periods of, occasional, intermittent, frequent, etc.