They’re called “PoP’s” in weather geek speak.

‘Probability of precipitation’ forecasts have been around for a long time. They’re not easily understood by the public, and even some forecasters. They cause unnecessary confusion in this forecasters opinion.

Perhaps it’s time for us in the meteorological community to rethink PoP’s and invent a better way to describe incoming rain or snow chances?

For the record, here’s a PoP’s chart from the National Weather Service.


NPR has an interesting piece on POP’s and why they can be so confusing. It turns out it’s not just the public who are confused by POP’s. Even meteorologists who communicate the forecasts can get confused.

1722 npr

Here’s a snippet from the NPR piece.

Forecasts are less philosophical at the National Oceanic and Atmospheric Administration’s National Weather Service, where Vankita Brown works. She’s not a meteorologist, but a social scientist whose job it is to try to better understand how the public interprets weather forecasts.

Brown herself isn’t sure how to interpret a 20 percent chance of rain.

“We think people know what it means,” she says. “I have conversations with my colleagues in meteorology all the time about what that means. And in fact I challenged one today to tell me, in less than five minutes.”

Brown walked away without a clear answer — confirming her suspicions that much of the public is also likely confused.

For the National Weather Service (NWS), of course, this is not a question to puzzle over for fun. It’s serious stuff. They need the public to not only understand forecasts, but to have confidence in them so that people will respond appropriately to weather threats.

And it’s not just a numbers game — words used to describe weather can be just as confusing. Take “watch” and “warning,” for example.

” ‘Watch’ means that conditions are ripe for something to happen. ‘Warning’ means that it is happening — It is imminent,” Brown says. “It’s easy to get them confused.

Brown says the NWS has asked the public if other terms might make more sense. People they’ve surveyed have suggested words like ‘emergency,” “imminent,” “dangerous” and “caution.”

Weather communication

How we communicate weather information is every bit as important as forecast accuracy. For decades I have purposely moved away from using PoP’s in my weather broadcasts. I prefer to use more of a ‘coverage and timing’ model to communicate rain and snowfall chances.

How much of the area will be covered by rain tomorrow? What locations will see the best chance of rain or snow? What time of day is most favored?

Approaching the forecast this way does a better job of communicating the realities often scattered, random precipitation patterns, and the limitations of our science of meteorology. In my opinion it’s also a more “honest” way to communicate uncertainty of weather to our audience. When we know, I’ll say that. When we don’t know for sure, it’s okay to convey that uncertainty.

Localized storm rolls over Lake Minnetonka in June. Paul Huttner/MPR News

Dirty little weather secret?

In spite of some forecasters who confidently proclaim that a severe storms will hit you ‘after 9 pm tonight’, limitations in state of the science of meteorology and the random nature of summertime convection often make that prediction impossible. Mondays predictions by some of severe storms in the metro is a good example. We can show you a cool model-generated map that says we’re going to get smacked by severe storms or tornadoes at 9 pm, but the reality is that doesn’t mean it’s going to happen. The storms might just slide north.

Storms slide north of the Twin Cities Monday night. Weather Underground

Putting those kind of specific maps out that have a high bust potential negatively effects our credibility as a profession. It’s no wonder we get razzed as a profession about “being wrong half the time and still getting paid” when we put out those kind of iffy forecasts.

Maybe it’s time for us a a profession to come up with a better way to describe precipitation potential in tomorrow’s forecast.