Probability of Precipitation: What does 40% chance of rain actually mean?

QUAD CITIES, Iowa/Ill. (KWQC) - Have you ever wondered what the terms “isolated” or “scattered” actually mean, or how meteorologists come up with the percentages for the chance of rain?

A lot of times during the summer months you will hear the First Alert Weather team say “the potential for an isolated shower or thunderstorm.”

This typically means that less than 20% of the tv6 viewing area is affected by rain.

Widely scattered showers and thunderstorms means 20% to 40% of the area is experiencing rain.

Scattered is when 40% to 60% of people are affected.

Numerous rain and thunderstorms is described when 60% to 80% of the area will be experiencing rain.

When we say widespread precipitation, we are talking about 80% or more of the area covered in precipitation.

However, that is only part of the puzzle for probability of precipitation.

Here in the weather world, we call these “PoPs”. It is actually a simple calculation of the confidence percentage, multiplied by the percentage of the area that is expecting the rain or snow, or whatever precipitation.

Watch the video above for an example.

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