As a meteorologist, hearing and speaking “weather lingo” has become a part of my daily life. Just ask my wife! On a day during the summer when severe weather is possible, I flip some proverbial switch that launches me into full weather nerd mode. When she asks when the storms will hit, I’ll tell her when, but only after I explain how the increasing instability, appropriate wind shear, wet-bulb zero heights near 8,000 AGL and plentiful moisture will make it a busy day at the weather office. At first, she used to tell me to “tone it down.” Now, she flips her filter “switch,” picks up her book, reads a few pages, and waits for the punch-line. Smart woman. But when it comes to expecting precipitation, there’s one term I don’t mention very often around her, or even with my family and friends: “percent chance.”
Knowing the “percent chance” of an upcoming event is important to many people. However, some people who put faith in this number mistakenly interpret the true definition of “percent chance” in weather terms. If I go out on the street and ask people what it means if there’s a 30% chance of rain or snow today, a majority will probably say it means 30% of the area will receive rain or snow, while the other 70% will stay dry. It’s a good try, but a complete miss.
The percent chance of precipitation is also known as the probability of precipitation. (POP) The National Weather Service has been using POPs for decades, assigning them to daily forecasts. Basically, when a meteorologist says “there’s a 30% chance of rain today,” it means there’s a 30% chance any random point in a given area will experience measurable precipitation. And the number isn’t just pulled out of the air. (Pun intended, of course!) The number 30 is determined by forecasters and computer models. It means three out of ten times when conditions were similar in the past, it rained. So yes, the higher the percentage, the more likely precipitation will occur, but it all depends on the size and history of your forecast area.
So why the confusion? A large part of the problem is the lack of explanation by broadcasters and internet resources. You’ll rarely hear from a broadcaster what percent chance means; or, worse yet, the broadcast does not correctly understand the whole concept. On internet forecast pages with percent chance listed, there’s usually no written statement next to the forecast, explaining it’s probability rather than coverage area. The lack of explanation, combined with human nature’s urge to jump to conclusions, can lead to a misunderstanding of what the meteorologist or broadcaster is trying to relay.
The WQOW News 18 forecast team frequently displays the precipitation risk. You’ll hear mentions of “slight, moderate, and likely.” Once in a blue moon, a number is mentioned, but you’ll more likely hear terms such as isolated, scattered, hit and miss, widespread, etc.
I have many friends and colleagues that live by percent chance. That’s perfectly fine and I don’t disapprove of their method. Their forecasts have a lot of good, quality information too. A POP is there to provide the meteorologist an understanding of what the atmospheric setup is like. In my personal opinion, the reason that specific number does not make it to air is because I feel avoiding an exact percent chance as much as possible provides you, the viewer, a better idea what the weather will be like. Remember the whole area thing? News 18′s viewing area is far different compared to other media outlets. It’s also not fair to write on a graphic “50%,” when that chance exists closer to Rice Lake compared to the “10%” that would exist in Eau Claire. Second, it can be very confusing! I am a big fan of taking the complexity of numbers and equations and making a forecast that’s easy to understand. More or less, you’re just trying to find out if it will rain or snow, right? So why make your experience of understanding the weather forecast similar to writing a college mid-term? No thanks!
Thanks for reading, and feel free to post with comments or questions! Stay tuned for more from Beyond the Forecast…
Posted under Hometown Weather
This post was written by Nick Grunseth on January 27, 2012