What do rain chances in a forecast really mean? (2024)

ORLANDO, Fla. – When the forecast calls for rain and you see the percentage is 60%, the immediate thought might be, “It’s going to rain at my house or my job,” wherever you may be during the day.

When it doesn’t rain in those areas, you may be upset or think the forecast was wrong.

The last thing we want you to do is not trust the forecast. So the PinPoint Weather team is here to help explain what those percentages mean so you can plan better for inconvenient downpours.

Closer look at percentages

Let’s use 60% as the rain chance in the forecast. That means 60% of the entire area (Central Florida) could see measurable rain at some point in the day, while the other 40% won’t. That goes beyond the city that you live in. In fact, the entire area we cover here at News 6 consists of nine counties: Marion, Lake, Sumter, Volusia, Flagler, Seminole, Orange, Osceola and Brevard. That’s a large area covered by three National Weather Service Bureaus: Melbourne, Tampa and Jacksonville.

Yes, math is involved

Meteorology is full of math equations. Some are very complex, while others aren’t. The formula to calculate the percentage of precipitation, or POP%, looks like this:

Precipitation chance (%) = Confidence of precipitation developing x percent of area affected

Meteorologists look at several models of different layers of the atmosphere to determine if all the elements needed are available over different parts of the day to be confident there will be rain. Often times we are not 100% confident in model data. It’s science and when you’re dealing with a very complex fluid atmosphere that has chaotic moments we call weather, it’s hard to agree with computer data in it’s entirety all the time. Models are guidance, not the absolute final word.

For simplicity let’s say we are 100% confident in the model data that it will rain in 60% of the coverage area.

Doing the math (1 x 0.60) x 100 = 60 or 60%. Easy peasy, right? Now let’s make this more realistic since we rarely have that much confidence in the data.

Let’s say that after looking at all the models and the data the confidence is only 50% that 60% of the coverage area (the nine counties listed above) could see measurable rain during some point in the day. That math looks like this:

(0.50 x 0.60) x 100 = 30 or 30%

Do you see how different that is? Based on the POP%, it’s kind of hard to figure out if you’re included in the rain potential, right?

Our team takes it one step further. We break down the POP and PinPoint the rain chance on a hyperlocal level. You may have seen the News 6 PinPoint Weather team do this in order to better communicate the weather story for the day.

For example, the coast might have a POP of 20%, while areas west of I-4 may have a 60% POP for the day. That’s a big difference and we want you to know that difference because it can throw off your entire day.

By breaking down the map and highlighting the coverage areas and then placing a number over that area, we show the percent of the area highlighted and the chance it could see measurable rain at some point in the day. Let’s say the highlighted area is all of Seminole County. In that highlighted area, we have placed 40%. That means 40% of Seminole County could get measurable rain, while 60% won’t.

Also, 20% is NOT ZERO. It’s often assumed a 20% rain chance means it won’t rain. Based on the information above, that isn’t true, so don’t be surprised if it rains on a day with a low POP.

It’s all about timing and location.

The percentage of precipitation does not tell you how long or how much it will rain. Often, when a higher rain chance is seen in a forecast, it is assumed that the entire day will be rained out. That’s not how this works.

This is why our weather team uses future graphics or hourly daypart forecasts to express when you are most likely to see rain and how long it will impact your day.

Keep in mind, if you move from one location or the other or two multiple locations throughout the day like postal workers or delivery drivers, that rain chance changes based on where you will be at what time of the day. While there might be a lower POP for the area you call home, where you work may have a higher POP. Even if your destination for the day has a higher POP, if you are not in the area during the time period that the rain is expected, you could go all day without seeing any rain at all or you could get rained on all day long. It’s all about timing and location, location, location.

So, next time you see a forecast for rain and you plan on being in three different places throughout the day, be sure to check the rain outlook for those areas and the times of day you plan to be there. It will really make a difference in how the forecast is viewed and utilized and we hope you are able to get the most out of what we present to you.