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WEATHER LAB | Explaining the science behind rain chance percentages

What does it actually mean when the forecast has a 40% chance of rain?

DES MOINES, Iowa — The percent chance of rain, or probability of precipitation (POP), is one of the most misunderstood aspects of a weather forecast.

Recent viral videos and posts on social media like a tweet from NFL wide receiver A.J. Brown have only added to the confusion.

So what exactly do those percentage chances mean?

Don't overthink it. A POP is simply the chance of at least 0.01" of precipitation in a specific location for a given time frame.

If there is a 40% chance of rain for your location this afternoon, it means there is a 40% chance there will be measurable rain between noon and 6 p.m.

It does not mean:

  • 40% of the area will get rain
  • It will rain 40% of the afternoon

POPs are calculated with a simple formula: confidence in precipitation multiplied by coverage of precipitation.

For example, if the forecaster was 60% confident rain would develop but only expected it to cover 50% of the surrounding area, then the forecast would show a 30% chance of rain.

Forecast POPs change based on the latest data from computer models which update numerous times a day. Rain chances on most phone apps are pulled from just one computer model, often causing wild swings in POPs from one update to the next.

Forecasters take into account numerous weather models to put out a POP that is more realistic.

Since confidence is typically higher in the short term compared to the long term, POPs will often increase as the rain event approaches.

A high POP does not necessarily mean the rain will be heavy. POPs are not in any way a reflection of how much rain will end up in the rain gauge.

To avoid the confusion, some meteorologists opt to not show percentages at all and instead use phrases like isolated, scattered or widespread.

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