Updated: Jun 19, 2020
There's no doubt that you have heard a meteorologist talk about a "chance of" this
or a chance of that. Perhaps there's a 30% chance of thunderstorms this afternoon or a 60% chance of snow this evening. Typically it's mentioned in association with specific precipitation events rather than sensible weather elements such as temperature or wind. But what does a 20% chance of afternoon thunderstorms really mean?
Does it mean 20% of the time you'll get thunderstorms this afternoon? Or perhaps it means that 20% of the area will see a thunderstorm? Well, sort of, but not quite. It's a bit more complex and it all comes down to a principle called quantifying uncertainty. Meteorology is not about black and white, but more about shades of gray. Forecasters are rarely 0% or 100% certain about an event. In order to communicate their uncertainty to you they use a probabilistic approach.
The official answer provided by the National Weather Service (NWS) is...
"The likelihood of occurrence (expressed as a percent) of a precipitation event at any given point in the forecast area. The time period to which the probability of precipitation (PoP) applies must be clearly stated (or unambiguously inferred from the forecast wording) since, without this, a numerical PoP is meaningless."
Essentially PoP is the average point probability for any given area and equals the expected areal coverage of measurable precipitation. For a 20% PoP, this means that 20% of the area will receive measurable precipitation at the surface. The key word is expected. It's not saying that the areal coverage equals the PoP, but that the expected