How meteorologists and forecasters calculate your daily rain chances

Fidel Moreno-Meza

Updated Mon, August 25th 2025 at 9:41 PM

Forecast decoded: how meteorologists and forecasters calculate your daily rain chances

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LAS VEGAS (KSNV) — We have all seen it in a forecast: "30 percent chance of rain", but what does that number actually mean?

It doesn't mean it'll rain for 30 percent of the day or that 30 percent of the area will get wet.

It actually refers to your odds of experiencing rain at your location during the forecast period.

Forecasters use a specific formula to calculate rain chances: Probability of Precipitation (PoP)= Confidence (C) x Area with rain (A).

Here are two examples:

If forecaster Fidel Moreno-Meza is 100 percent confident that 30 percent of the area will see rain, he would forecast a 30 percent chance of rain.

100 percent (confidence) x .3 (Area with rain)= 30 percent

If forecaster Fidel Moreno-Meza is 60 percent confident that 50 percent of the area will see rain, he would still forecast a 30 percent chance of rain.

60 percent (Confidence) x .5 (Area with rain) = 30 percent

That 30 percent means there is a 3 in 10 chance you'll see rain where you are, whether that is in Summerlin, Henderson, or North Las Vegas.

Think of it like this:

10 percent: A long shot (most of the valley will stay dry)

50 percent Coin flip (some neighborhoods get rain, others don't)

90 percent Almost certain (grab the umbrella, almost everyone could get wet)

The probability of Precipitation does not include the duration of rain or the intensity of rain.

You may also hear forecasters use terms like Isolated, Widely Scattered, Scattered, and Numerous.

10 percent: Isolated/ Few

20 percent: Widely Scattered

30-50 percent: Scattered

60-70 percent: Numerous

Next time you check the forecast, think of it this way: rain chances are about your chance of getting wet, not how long or how much it rains.