FIRST ALERT: What does it mean when meteorologists provide rain chance percentages?

Published: Jan. 25, 2023 at 5:59 PM EST
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MYRTLE BEACH, S.C. (WMBF) - When you’re watching our newscasts you will see our WMBF First Alert Weather team forecasting a percent chance of rain.

But what does that actually mean?

We’re breaking down the science behind the “Probability of Precipitation.”

WHAT IS 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. Basically, your rain chance is an expression of how likely it is to see rain over the coverage area during a 24-hour time period.

Probability of Precipitation
Probability of Precipitation(WMBF)

EXPLAINING AREA COVERAGE

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.

How rain chances are divided up in our area
How rain chances are divided up in our area(WMBF)

HOW IT WORKS

Now that we have the coverage area explained, let’s bring in the confidence part of our equation. One hundred percent confidence means that we as meteorologists trust the model’s outcome 100%. This is rarely the case we know, but to keep it simple let’s say this is true. 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?

Calculating PoP with 100% Confidence
Calculating PoP with 100% Confidence(WMBF)

REALISTICALLY HOW IT WORKS

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%.

Calculating PoP with less than 100% Confidence
Calculating PoP with less than 100% Confidence(WMBF)

HOW US METEOROLOGIST CALCULATE PoPS

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

For example, if you hear us say, “The Pee Dee will definitely see widespread showers and storms,” then you know that your WMBF First Alert Weather Team Weather is very sure that it is going to rain over most of the Pee Dee on that day.

Let’s try another.

“The Pee Dee has a chance of seeing widely scattered storms.”

From this, you can infer that we are 30-50% confident that 30-50% of the Pee Dee could see storms on Tuesday of that week.

In this case, the models most likely not agreeing with each other so our confidence is not that high.

How does PoP relate to the forecast
How does PoP relate to the forecast(WMBF)

Remember the models are only used as guidance. We as meteorologists have the final say in the PoPs we display on the seven-day forecast.

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.

However, we do 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 our hour-by-hour forecasts or by downloading the WMBF First Alert Weather app.