Forecasts

What do the precipitation chances mean, and how are they determined?

It depends. If the forecaster is sure there will be "measurable" precipitation (0.01 inches of rain or its frozen equivalent), the probability of precipitation ("PoP") corresponds to the amount of coverage that is expected in the forecast area. This forecast area may be a county, a group of counties, or some equivalent geographic area, and is specified in the Zone Forecast, Area Forecast, and Detailed Forecast (formerly the Revised Digital Forecast). In other cases, the PoP is an approximation derived from a combination of the likelihood that precipitation will occur, and how much area it is likely to affect. For example, if a forecaster is 40% sure that rain will occur, and expects it to cover 70% of the forecast area, the PoP will probably be 30%. At this time, PoPs in official forecasts are always for the given 12-hour period ("today," "tonight," etc.), or what remains of it.

Computer models also produce a PoP. In their case, the probability of precipitation is for a given point location, and represents the likelihood that "measurable" precipitation will occur at that point in the given time period.