Explainer: What meteorologists mean by a ‘chance of rain’

What does it mean when a meteorologist tells someone there is a 40% chance of rain? Meteorologist Charles Roop explains.
Published: Sep. 13, 2021 at 5:47 PM EDT
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TALLAHASSEE, Fla. (WCTV) - Weather forecasters tell the public what to expect for the days ahead, including the conditions, high and low temperatures and the POP. If the last element throws one off, it has nothing to do with a dad or the type of carbonated beverage in different parts of the United States.* POP is an acronym for Probability of Precipitation or, simply put, the chance of rain (or sleet, snow, sleet, etc.).

There may be either confusion or a misunderstanding on what it means. As defined by the National Weather Service, it’s determined by the confidence that a forecaster has of at least a 0.01 inch of rainfall in a given area multiplied by the coverage that is expected. If one prefers equations, it’s as follows,

POP = confidence x coverage

Confidence can come from either climatology and/or a weather pattern for a given time of interest. A trough of low pressure in the mid- to upper-levels of the atmosphere, for example, can induce lift as long as there is warmer air and moisture to work with and, therefore, increase the chance of rainfall. A ridge of high pressure and drier air can lower rain chances.

To break it down a bit more, a hypothetical scenario can be used. Say a trough is arriving to the Southeast U.S. in the late spring/early summer along with high atmospheric moisture content ahead of it. Usually, that calls for better rain chances as long as the features are to the west. With the larger-scale pattern, there is a 95% chance of rain somewhere in the Tallahassee-Thomasville television market. But, to complicate things, the lower-level pattern will keep the sea breeze pinned to the Big Bend of Florida - the southern-half of the viewing area. That would call for approximately 50% coverage. Going back to the aforementioned equation,

POP = (0.95) x (0.5)

POP = 0.48 = 48%

For the sake of having whole numbers, it can be rounded to 50%. That would lead to a 50% chance of rain for the entire viewing area for that specific day (sunrise to sunset). This would not be the rain chances for a specific city or county. The coverage and confidence would depend on what location would have the best setup. In the example given, Leon County would be given rain chances higher than 50% while Miller County would have rain odds lower than 50%.

Also, low rain chances do not mean that one will not get wet, and vice versa. If someone is not pleased that it rained at their house when the POPs were at 20%, meteorologists didn’t say it wouldn’t rain. Instead, nature just decided to rain at the house.

*Carbonated beverages are called soda. Don’t @ the writer.

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