Why Pop-Up Summer Thunderstorms Are Among the Hardest Weather to Predict

Brian Donegan
Published: June 8,2017
We all want to know if it's going to rain on any given day, and when you check your local forecast on weather.com or The Weather Channel app, you notice there's a 30 percent chance of thunderstorms this afternoon.
But what does this chance of thunderstorms really mean? Is it going to rain or not? Should you cancel your round of golf or tee off as planned?
Unfortunately, pop-up thunderstorms in the warmer months are among the hardest weather to predict, and it's difficult for forecasters to provide a yes or no answer as to whether or not you'll see rain today at your exact location.
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If there's a 30 percent chance of rain, that means there's a 70 percent chance of dry weather during your outdoor event. Those odds sound pretty favorable – certainly not a reason to cancel any plans, you think to yourself.
Then, when the event begins, so does the rain. You're wet, your family and friends are wet, and no one is happy. So you send an angry tweet to The Weather Channel.
According to the National Weather Service, the chance of rain, or probability of precipitation (PoP), describes the chance of precipitation occurring at any point in the forecast area during a given time period. Therefore, the correct way to interpret the forecast is that there's a 30-percent chance rain will occur at any given point in the area.
PoP also incorporates a forecaster's confidence that it will rain or stay dry at a given location, so anything above a 50 percent chance of rain means there is higher confidence it will rain rather than not. Less than a 50 percent chance of rain means there is higher confidence that it won't rain.
PoP does not depict the duration or intensity of the rain. In other words, a 100 percent chance of rain does not mean it will rain all day, nor will it necessarily downpour the entire time.
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Photo of a rain shaft over Dallas captured by Toddy Jack on Sunday, June 4, 2017.
(Toddyism/Twitter)

Most meteorologists would agree summer is the most challenging time of year to forecast the chance of rain. There is typically plenty of moisture and instability to help fuel the development of scattered showers and thunderstorms, particularly in the South.
The triggers for thunderstorm development can be tough to pinpoint in the summer. Rather than distinct warm and cold fronts, summer thunderstorms sometimes flare up on subtle outflow boundaries from previous thunderstorms, sea-breeze fronts, higher terrain or in a more random pattern.
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This all makes the task of giving a yes or no answer about whether it will rain difficult. Occasionally, storms may be so isolated that a small part of a county gets drenched, while the rest of the county stays dry and possibly even sunny.

In this radar image from 4:30 p.m. MDT June 7, 2017, isolated showers and thunderstorms are drenching small portions of various counties in northeast Colorado.

As a result, summer thunderstorm forecasts are often broad, making use of PoP or terms like isolated and scattered.
Isolated corresponds with a 10 percent chance of measurable (0.01 inches of greater) precipitation at your given location, the National Weather Service says. Scattered indicates there will be 30 to 50 percent coverage of convective weather, such as thunderstorms, in the forecast zone. "Widely scattered" is used for a forecast of 20 percent coverage.
"In Florida, it's well understood that during the summer it may rain on your house one day and not the next," said weather.com digital meteorologist Jonathan Belles, a Florida State University alumnus. "In many cases, the forecast of a simple 50 percent chance of rain, or just removing that PoP altogether and saying scattered rain, solves a lot."

Key Takeaways

• Don't cancel or postpone any outdoor plans due to a forecast of isolated or scattered showers and thunderstorms.
• Always have a plan to briefly move indoors in case you're one of the unlucky ones who gets soaked on the day with a 10 or 20 percent chance of rain.
• You should also have an app with radar on your smartphone and check it every once in a while during your outdoor event, assuming you’re in range of cell service, to see if any thunderstorms are nearby.

Brian Donegan is a digital meteorologist at weather.com. Follow him on Facebook and Twitter.

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