If you pay attention to weather forecasts, you might notice new terms making their way into storm predictions this fall.
The National Weather Service’s Storm Prediction Center is adding two new categories to its list of threat levels. Officials with the Norman-based agency say the new categories will help residents better understand how likely storms are and avoid being taken by surprise.
Under the old threat level system, National Weather Service forecasters predicted slight, moderate or high levels of risk of tornadoes, severe thunderstorms and other weather. Beginning Oct. 22, forecasters will be able to predict a “marginal” or “enhanced” risk.
Greg Carbin, the center’s warning coordination meteorologist, said the new system breaks the old “slight risk” category into three new categories: marginal risk, slight risk and enhanced risk. The “moderate risk” and “high risk” categories won’t be changed.
For example, if forecasters predict a 2 percent chance of a tornado touching down within 25 miles of a point on the map, weather bulletins will now show a marginal risk of a tornado. In years past, a 2 percent chance would have been categorized as a slight risk, Carbin said.
National Weather Service officials have for years fielded complaints from their partner agencies that the “slight risk” category was misleading or difficult to define, Carbin said.
Like the old system of categories, the new threat level designations are tied to percent chances for severe weather, Carbin said. Meteorologists and other weather experts deal mainly in percentage chances, he said, but those figures can be confusing to the casual reader.
For example, a 2 percent chance of a tornado might not sound threatening to most people, he said, but any chance greater than 0 percent could be a cause for concern. Categorical threat levels like “marginal risk” and “slight risk” can be more helpful for people who are trying to plan for severe weather.
But those categorical threat levels are imperfect and can lead to confusion, as well, Carbin said. The “slight risk” category was meant to explain that a chance for severe weather existed, but the storm was too scattershot or isolated to predict with much certainty which towns or neighborhoods would be affected, he said. So a thunderstorm or tornado would occasionally take residents by surprise on “slight risk” days.
“The word ‘slight’ doesn’t convey the right message when it comes to severe weather,” he said. “What we want to do is better define that ‘slight’ in terms of risk.”