40% chance of rain? Here’s what the ‘probability of precipitation’ actually means

BUFFALO, N.Y. (WIVB) — In a typical forecast, you often hear or see the words “probability of precipitation.” This value is given as a percentage. Although this value is frequently seen and used, there is often confusion on what it truly means in a forecast.

Misconceptions about the meaning of the phrase range from the area of coverage to the confidence of precipitation occurring, or even how intense the precipitation could be. The actual meaning is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. Measurable precipitation is equal to or greater than 0.01 inches.

On Thursday, May 9, the probability of precipitation increases for the point location of Buffalo. The percentage increase through the 12-hour period represents the likelihood also rising at the specified times.

Now, the probability of precipitation is a percentage value, so what does each percentage mean in terms of likelihood?

10% means isolated or few areas of precipitation.

There is a slight chance for precipitation when 20% is used, and it represents the likelihood of precipitation being widely scattered.

It becomes a chance for precipitation when the value is 30%, 40% or 50%, which represents scattered showers.

When a value is listed as 60% or 70%, the precipitation is categorized as likely with numerous areas of precipitation being forecasted.

As the value gets higher, a percentage of 80%, 90% or 100% means a period of precipitation, on and off, or occasional precipitation is expected.

As the percentage increases, so does the likelihood of the precipitation at a specified time.

Here is another perspective on this topic. A 20% value is a 1 in 5 chance you will see 0.01″ of precipitation at your location in a set period of time. On the higher end of things, a 100% indicates that you will receive precipitation at some point that may last the whole time period or just an individual window of time.

Keep this in mind the next time you see a chance for rain!

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Nick Guzzo is a meteorologist who joined the 4 Warn Weather team in October 2023. See more of his work, here.

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