What does the probability of precipitation in the weather forecast mean?
The probability of precipitation is the likelihood of measurable precipitation (0.01 inches or greater) for a specified forecast time period, and occurring at any point for which the forecast is valid. The probability is expressed as a percent and can be misleading to the public when decid-ing if scattered rain will occur, or if the day will be a complete washout. The meteorologist assigns a precipitation probability that shows his/ her confidence in the forecast, the areal coverage, and duration of the event. Precipitation probabilities are misinterpreted when thunderstorms are forecast, as there is some degree of uncertainty in the areal coverage and event duration. An example used in the summer follows: “Partly cloudy. A 30 percent chance for showers and thunderstorms”. A forecast contains these terms when specific probabilities are used: (NWS Operations Manual) POP=Probability of Precipitation. POP (%) Expression of Uncertainty Thunderstorm Areal Coverage 20 Slight chance Isolated/
