Probability of Precipitation: What is the Chance That People Get it?

Meteorology is the study of hydrometeors- precipitation. In the United States, communicating precipitation in weather forecasts has traditionally been by way of probability, or percentage chance, known as a probability of precipitation (POP). As defined in the Glossary of the American Meteorological Society (AMS,) a probability is, "The chance that a prescribed event will occur." One southeastern National Weather Service Forecast Office (NWSFO) office defines POP as, "the likelihood, expressed as a percent, of measurable precipitation (≥ 0.01 inch) at a particular point during a specified time period." Another southeastern NWSFO says, "most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage."

What variables are involved in POP; do all forecasters use it in the same way; and how does the public interpret it? Even when a forecaster has a clear picture in mind of when and how precipitation may be distributed over an area, it may be difficult to express that in a single number. It is possible that forecasters are using statistical probability; areal coverage; intensity; duration; quantity; impact; and overall forecast confidence in arriving at a single percentage.

Through a simple online survey to professional meteorologists in the US, we look at what factors government, broadcast, and private-sector forecasters use in creating a POP, to determine which items are most-often considered and to examine if there is consensus among individuals or regions.

A similar online survey is administered to the general public in the US to examine their perspective of POP.

In these two surveys, both groups are also given the same forecast scenarios for precipitation from convective, and stratiform clouds to gauge how perspectives may or may not differ.

This paper is intended to spur discussion on how the weather forecasting enterprise POP messages may be constructed with different parameters, and how or if those messages might be misinterpreted by the user.