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Behind the Forecast: What do rain percentages really mean?

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.
Published: Jun. 28, 2021 at 9:45 AM EDT
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LOUISVILLE, Ky. (WAVE) - Watch a weather forecast and you’ll most likely hear it: “There’s a blank percent chance of rain in the forecast.” But what does that even mean?!

If a meteorologist says there’s a 30% chance of rain does that mean will it rain 30% of the day? Or will it rain over 30% of the area?

Here’s how the National Weather Service (NWS) breaks it down.

The “Probability of Precipitation” (PoP) describes the chance of precipitation occurring at any point you select in the area.

So the PoP is the chance of a measurable amount of precipitation falling during a designated time frame at any point in the forecast area. For us at WAVE 3, our forecast area consists of 29 counties; 19 in Kentucky and 10 in Indiana. Unless specified, the time frame is usually 12 hours. A measurable amount of precipitation is defined as equal to or greater than 0.01 inches.

NWS explains it mathematically with this equation: PoP = C x A

  • C is the meteorologist’s confidence the precipitation will fall somewhere in the forecast area
  • A is the percent of the forecast area that will receive measurable precipitation if precipitation does fall.

So let’s say I’m putting together my forecast and I am absolutely positive that it will rain on Saturday. C, in that case, would be 1 since my confidence is 100%. If I think that 30% of the 29 counties we cover will get rain then A would be .3 (or 30%). Put it all together and you’d get a rain chance of 30%. (PoP = 1 x .3 = .3)

Sometimes you’ll hear a meteorologist say that we have a chance of isolated or widespread showers? What does that mean?

Here’s a neat little table from the National Weather Service that explains it all.

POP PercentExpression of UncertaintyEquivalent Areal Qualifier
10 percent(none used)Isolated/Few
20 percentSlight ChanceWidely Scattered
30, 40, 50 percentChanceScattered
60 & 70 percentLikelyNumerous (or none used)
80, 90 & 100 percent(none used)Occasional, periods of, or none used

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