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What does a 20% or 40% or 60% chance of rain mean?

Rain chance forecasts can be confusing, especially when you really only want to know whether to grab the umbrella or leave it at home.

MEMPHIS, Tenn. — You hear it every day in the weather forecast... "We'll have a 20% chance of rain this afternoon... There's a 60% chance of thunderstorms overnight tonight." But what does that mean? If rain chance forecasts confuse you, you're not alone!

Rain chances are actually a pretty controversial topic in the weather world. In fact, many meteorologists come up with a percentage using their own method, only leading to more confusion. 

There is technically an official way to do it. According to the National Weather Service definition, rain chances should be determined using a formula, where the percent chance of rain equals the amount of area that will see rain multiplied by the confidence in the forecast. Let's break that down. 

The amount of the area that will see rain

Every meteorologist is responsible for forecasting the weather for a certain area. At Local 24 News, we are responsible for Western Tennessee, Northern Mississippi, and Eastern Arkansas. This means if we know it's going to rain in Oxford but not in Memphis, we would still have to include rain in the forecast.

If we're just expecting a small storm or two, we'd say 20% of the area will see rain. On the other hand, if we're expecting more widespread rain, the area that will see rain would be more like 70% or 80%. For the sake of this formula, we represent this variable as a decimal value. Let's say today is a regular summertime day where a few storms will pop up in the afternoon. For this example, we'll say 30% of the area is going to see rain. 

Confidence in the forecast

You don't need me to tell you that weather forecasts aren't perfect. Sometimes, we just aren't sure if it's going to rain or not, especially if we're forecasting for 6 or 7 days down the road. Let's say today we're pretty confident in our forecast... and we are 80% sure that we're going to be correct.

Putting it all together!

Okay, so we're 80% sure that 30% of the area will see rain. Again, the confidence is represented as a decimal, so 30% times 0.8 gives us a rain chance total of 24%. We'd round that up to say a 30% chance of rain for Memphis and the entire Mid-South. 

What is NOT included in the rain chance percentage?

Unfortunately, there's no way for a rain chance forecast to be pinpointed to your exact location. Science isn't advanced enough yet to tell you if or exactly when it's going to rain in your backyard. We have to give a percentage for the entire area.

The rain chance percentage also has nothing to do with cloud cover... we can have an 80% chance of rain and still have a little sunshine peeking through. 

The rain chance percentage also does not take into account how heavy the rain is going to be or how long the rain will last. This means if the meteorologist knows with 100% certainty that it will rain at your location, we'd have to call that a 100% chance of rain, even if it only rains for 30 seconds and is dry for the rest of the day. 

Confused yet? Yep, we are too. 

Although there are official guidelines on how to come up with a rain chance percentage, many viewers still don't get it. Most people don't want to sit at home and try to figure this all out in their head, they just want to know if they can mow the yard or not. 

You may notice that many meteorologists have stopped using rain chance percentages all together, and now use words such as "morning showers", or "isolated storms". We've also moved toward a more detailed graphics to better explain when and where it's going to rain without using percentages.

At the end of the day, all that matters is making sure you understand the forecast as clearly and quickly as possible. And meteorologists are working every day to not only be more accurate, but to make our forecasts easier to understand.