Weather 101: What does a 30% chance of rain really mean?

Weather 101

The science behind the probability of precipitation

As meteorologists, one of the topics we get asked the most is what the chances of rain actually mean on our 7-day. On this episode of Weather 101, our own Chief Meteorologist Dan Skoff explains the probability of precipitation.

What Exactly is a PoP?

The probability of precipitation (PoP) is defined as the likelihood of a measurable precipitation event (0.01″) at a location during a specific time period.

To make that definition less scientific, your rain chance is an expression of how likely it is to see rain over the coverage area during a 24 hour time period.

The official mathematical formula to calculate a PoP or Precipitation Chance (%) = Confidence of Precipitation Developing x Percent of Area Affected.

Visualizing PoPs

To visualize PoPs let’s break down the viewing area into 10 equal-sized blocks. All 10 blocks together represent 100% of our coverage area. This means, of course, each block represents 10% of the coverage area.

Now that we have the coverage area explained, let’s bring in the confidence part of our equation. 100% confidence means that we as meteorologists trust the model’s outcome 100%. This is rarely the case we know, but for simplicity’s sake let’s say this is true.

Calculating PoPs with 100% Confidence

So in this case we have 100% confidence that 30% of the coverage area will see measurable precipitation. The math is (1 x 0.30) x 100 = 30 or 30%. See how that works?

Let’s look at another example.

This time we have 100% confidence that 80% of our coverage area will see measurable precipitation. The math is (1 x 0.80) x 100 = 80 or 80%.

Calculating PoPs when Confidence is Less Than 100%

So now you know how we calculate PoPs when we have 100% confidence in the models, BUT…as was mentioned already we rarely have 100% confidence in their outcomes. So now let’s look at a more realistic example.

The confidence has been lowered to 50% and only 40% of the coverage area will see measurable precipitation. The math is (0.50 x 0.40) x 100 = 20 or 20%.

Language That Meteorologists Use to Describe Rain Chances

Chart created by the National Weather Service

Obviously, we can’t run through an entire math lesson on air every time we mention PoPs so we use certain language to express our amount of confidence and how much of the coverage area will see measurable precipitation.

If you hear us say for example, that “Northwest Arkansas will definitely see widespread showers and storms on Friday.” Then you can know that your Weather Authority team is very sure that it is going to rain over most of Northwest Arkansas on Friday of that week.

Let’s try another. “The River Valley has a chance of seeing widely scattered storms on a Tuesday.” From this, you can infer that we are 30-50% confident that 30-50% of the River Valley could see storms on Tuesday of that week. In this case, the models most likely are diverging on a solution and so our confidence is not that high. Or the models could already be off and we don’t trust their outcome.

Remember the models are ONLY guidance. We as meteorologists have the final say in the PoPs we display on the 7-day.

Keep in mind the probability of precipitation does NOT tell you how long it will rain or how much it will rain. It only tells you how likely the coverage area is to pick up at least 0.01″ of precipitation. Although the PoP doesn’t tell you how long it will rain, we try to show you the most likely time it will rain during that 24-hour period. You can also get a good idea of the precipitation timing by looking at the PoPs in the hourly forecast during our newscast and by downloading our Weather app.

Hopefully, you can now understand what sort of thinking lies behind the 30% chance of rain you see on the 7-day.

For other exciting and educational digital weather content, check out other Weather 101 and Weather Blog posts.

Copyright 2021 Nexstar Media Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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