Lets Talk PoP: Probability of Precipitation

Updated: Jun. 24, 2019 at 3:57 PM CDT
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MANKATO, Minn. (KEYC) - Rain, it happens whether we like it or not. Understanding the likelihood of rain helps us plan our day, week and any big events we may have. So what does it really mean when meteorologist say there is a 20%, 50% or 70% chance of rain?

This type of forecasting is referred to as probability of precipitation or PoP. Probability is a mathematical representation explaining the likelihood or chance of an event occurring. In this case, the probability represents the chance of rain at any given point in the selected forecast area.

How you find the probability
How you find the probability

The mathematical equation is broken down like this:

PoP = C x A

Where C = the Confidence a meteorologist has that precipitation (rain) will occur somewhere within the forecasted area.

And A = the percent of the the same forecasted Area receiving measurable precipitation (rain).

For example: let us say that a meteorologist is 100% confident (C) that rain will occur over Mankato, MN. However only 40% of Mankato will see measurable rain. Take our equation, break down our percents into decimal form and you have C = 1 (100%), A = 0.4 (40%). Multiply it out and the final answer reveals there is a 0.4 or 40% chance of rain at any given point across the city.

Another examples may show a meteorologist only being 80% confident (C) and that measurable precipitation will only cover 50 percent of the area (A). Again using our awesome equation we come up with a final answer of once again 40%. Still confused? That’s OK.

Another form of showing PoP
Another form of showing PoP

Numbers and probabilities are not always that easy to explain. That is why meteorologist will even use words to represent these confusing probabilities. It breaks down like this:

20-30% = slight chance

30-50% = chance

50-80% = likely

Anything above 80% results in NO terminology before the event. So if a CHANCE for showers represents a 30-50% then showers alone represents 80-100%.

Forecasting undoubtedly comes with some uncertainty. That is why meteorologists rely on probability. The probability helps cover the uncertainty that comes with any forecast. So when it comes to planning the day and you see there is a chance for rain, it is best to plan in advance.

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