ASK THE METEOROLOGIST
“What does ANY percent chance of rain really mean?”
Bud Fuller, Grants Pass
In some weather forecasts, you have probably heard the phrase “There is a ____ percent chance of rain today.” But what does that really mean? Does it depend on coverage area? Does it depend on likelihood? Well, it’s a little bit of both. There is actually an equation that signifies the probability of precipitation, as it’s called (PoP). This equation is PoP = C x A, where “C” signifies confidence that precipitation will occur somewhere in the forecast area. “A” represents the percent of the area that will receive measurable rainfall, if it rains at all. So, in essence, “C” represents likelihood and “A” represents coverage area.
Forecasters don’t really write out this equation when predicting rain chances, but instead weigh the possibilities as they go. Each forecaster is unique in how they interpret the equation and how they use it in their forecasts. It’s important to note that in our forecasts, we stray away from using definite numerical values to represent rain chances. We instead use words like “isolated” or “scattered” to describe BOTH the likelihood and coverage area of precipitation. We do this, in part, because of the wide misinterpretation of the percentages in forecasts used for probability of precipitation.
Chief Meteorologist Kate McKenna