Recognizing that more people without a meteorology background are looking to it for information about severe weather, the National Oceanic and Atmospheric Administration’s Storm Prediction Center plans to make adjustments to its outlooks.
The changes, which are being tested this spring and are projected to be made official later this year, add two categories to the risk spectrum to improve clarity in severe weather threats, said Bill Bunting, chief of operations at the Storm Prediction Center in Norman, Okla. They also include a weather summary for those who have not studied meteorology.
The old outlook system classified the risk for severe weather as “slight,” “moderate” and “high.” The changes add a “marginal” risk at the lowest end of the scale, followed by “slight” and then “enhanced.” “Moderate” and “high” will follow “enhanced.”
Suzanne Fortin, meteorologist-in-charge at the Wichita branch of the National Weather Service, said the changes address some perceived holes in the outlook spectrum.
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“They felt there was a gap,” Fortin said.
Officials wanted to address “the upper end of the slight risk spectrum,” Bunting said.
That included up to a 10 percent chance of tornadoes and a 30 percent chance of hail or strong winds within 25 miles of a given location.
“‘Slight’ isn’t the word that best conveys the risk” in those circumstances, Bunting said.
Meteorologists will tell you any time there’s even a 5 percent chance of tornadoes, people need to pay close attention to the weather. But it’s human nature for at least some people to all but dismiss the threat of severe weather when they hear the risk is “slight.”
Recent history has demonstrated the danger of that response. May 22, 2011, began as only a “slight” risk day – but an EF-5 tornado decimated the southern half of Joplin, Mo., late that afternoon, killing more than 160 people. The two biggest outbreak days in recent memory in the Dallas-Fort Worth metropolitan area occurred on “slight” risk days, Bunting said.
In each of those cases, Bunting said, forecasters knew storms could rapidly intensify and become dangerous if conditions came together just right – or they could remain garden-variety thunderstorms if the timing of the convergence was off.
The new “enhanced” risk category would mostly likely be issued on days with such potential, he said.
Generally speaking, “marginal” will cover days when storms aren’t expected to become severe. That would include storms with winds of less than 58 miles per hour and hail smaller than an inch in diameter.
Under the old system, a 10 percent chance of tornadoes was considered a “slight” risk. In the system now being tested, it would be an “enhanced” risk.
Along with the new risk categories, the Storm Prediction Center will feature a short public summary in each outlook, free of the acronyms and meteorological shorthand that has long been a staple of its outlooks.
Bunting said the summaries will be written with the public – not working meteorologists – in mind, allowing residents to answer a basic question: “What should I prepare for where I live?”
“Whether more specifics will cause people to take action, I don’t know,” Fortin said. “In general, I think it will hopefully bridge the gap” in the outlook spectrum.
The new system will have little impact on the local weather service branches, Fortin said.
“It gives us a little insight as to what they’re thinking” about a given severe weather threat, she said.
Fortin said she is looking forward to hearing what people think of the new system after it has been in place for a while.
“That would be an interesting type of study after they roll this out,” she said.