Probability of Precipitation: What is the Chance That People Get it? - Presented at the 43rd AMS Broadcast Conference. Also presented at the 40th NWA Annual Meeting. Survey results and comments are linked below.
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." Broadcasters and private forecasters are not bound to NOAA definitions of PoP.
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, between April and May of 2015, 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 was administered simultaneously to the general public in the US to examine their perspective of PoP or rain chance... In these two surveys, both groups were also given the same forecast scenarios for precipitation from convective clouds, and stratiform clouds, to gauge how perspectives may or may not differ. This study was 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.