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Of more than 40 types of clocks, warnings and advisories issued by the National Weather Service, one stands out. The severe thunderstorm warning is issued more often than any other warning, but it could also be the most misunderstood.
Most people don’t know it, but the presence of frequent lightning or heavy rain does not affect whether a thunderstorm is classified as severe. The weather service only issues a severe thunderstorm warning if it produces winds of at least 90 km/h or hail at least 2.5 cm in diameter.
How well the public understands severe thunderstorms plays a big role now as they begin to progress from their winter lull to a summer peak. According to consumer research firm ValuePenguin, many states conduct severe weather awareness weeks in March to educate people about these storms and their warnings, which are issued more than 80,000 times a year on average across the United States.
To better understand what the public knows about severe thunderstorm warnings, researchers from the University of Oklahoma surveyed more than 1,400 U.S. adults about the hazards considered by the Weather Service when issuing severe thunderstorm warnings. Eighty percent of respondents correctly identified wind, but only 58 percent correctly identified hail. Perhaps more worryingly, 77 percent identified lightning, 76 percent rain and 67 percent flooding, none of which form part of the criteria for a severe thunderstorm warning.
Understanding Storm Warning Confusion
The confusion surrounding severe thunderstorm warnings “isn’t particularly surprising,” said Makenzie Krocak, a researcher at the University of Oklahoma, during a presentation at the American Meteorological Society’s annual meeting in January. “If you see [forecasts] Speaking of severe thunderstorms, what symbols do you see often? A lightning bolt and a raindrop. So it is not surprising that people associate these warnings with lightning and water.”
To add to the confusion, severe thunderstorm warnings are sometimes issued for showers that produce strong winds but no lightning or thunder, like the warning issued Wednesday by the San Francisco and Monterey Bay Area Weather Service Office for winds up to 80 km/h.
Despite misunderstandings about what hazards play into severe thunderstorm warnings, Krocak isn’t necessarily proposing a name change.
“In general, I would say that we do not yet have evidence that changing the name of the alert would affect understanding or response,” Krocak said in an email. “There’s also a lot of accumulated knowledge with the current name, so we would need to understand if and how a name change would affect people with that institutional knowledge and their intended response.”
Chris Wirz, who studies weather risk communications at the University Corporation for Atmospheric Research, notes that the alerts include descriptions of expected impacts and recommended safety precautions.
“Even if people are confused about what hazards are covered, the alert tells them what to expect and what they can do to stay safe,” Wirz said in an email. “There is also a lot of other communication that is taking place before these warnings are ever issued. So if warnings are needed at all, many people have an idea of what to expect and how to respond.”
Barring misunderstandings, about 80 percent of survey respondents said the weather service issues “the right amount” of severe thunderstorm warnings, and most said they plan to take action to protect themselves if they receive a warning.
Why lightning isn’t among the criteria for a severe thunderstorm warning, meteorologist Jim Duncan explained in a story he wrote for the Washington Post last year that it only takes lightning to kill or to hurt, and lightning strikes may or may not a storm is considered violent. That’s why the weather service and security experts advise: “Go indoors when there’s thunder” and stay there for 30 minutes after the last clap of thunder. Lightning has killed an average of 23 Americans and injured hundreds each year for the past 10 years.
Because severe weather is not just severe weather, the Weather Service has introduced “damage risk” categories for severe weather warnings starting in 2021, with the aim of better communicating the strength and potential impact of thunderstorms and hail. The three categories are defined as:
- Base – Thunderstorms expected to meet only the minimum criteria of winds of at least 58 miles per hour or hail one inch (about as much as a quarter) in diameter. Expected impacts include damage to vehicles, trees, roofs and sidings.
- Significant – Thunderstorms expected to generate winds of at least 70 miles per hour or hail at least 1.75 inches in diameter (about the size of a golf ball) causing injury to people and wildlife outdoors. Other anticipated impacts include damage to roofs, sidewalls, windows, vehicles, trees, RVs, and outbuildings.
- Destructive – Thunderstorms expected to produce winds of at least 80 mph or hail at least 2.75 inches in diameter (about the size of a baseball), creating life-threatening situations and seriously injuring people and wildlife outdoors . Other expected impacts include power outages, broken windows, and extensive damage to trees, roofs, siding, vehicles, and RVs.
Only severe thunderstorm alerts marked as destructive will now trigger emergency weather alerts, which are delivered via cellular carriers.
The introduction of severe thunderstorm damage threat categories is part of a larger effort by the Weather Service to simplify and clarify its monitors, alerts and advisories by reducing the number of alert types and refining the alert language.
For example, the Weather Service is preparing to replace alerts such as “wind alert” or “coastal flood alert” with “plain language” headlines that convey the threat more clearly and quickly. Planned for no earlier than 2025, the change is based on social science research pointing to a public misunderstanding of warnings issued when hazardous weather is occurring or imminent, but the expected impact is less severe than a warning.
What to do if there is a severe thunderstorm warning?
Social science research has found that not only do people not understand the criteria for a severe thunderstorm warning, but they often confuse a storm warning with a gale warning, two very different things.
A severe weather warning is issued when severe weather is possible but not certain. If your area is under surveillance, experts advise sticking to trusted sources for weather information and being ready to act when an alert is issued. This is usually a good time to secure outdoor objects that could be blown away or cause damage or injury, transport light objects inside, and consider postponing outdoor activities.
A severe weather warning is issued when severe weather is imminent or is already occurring. If you receive an alert for your area, stay in a sturdy building or shelter and stay away from windows and doors, bring pets, pull shades and blinds over windows, and avoid electrical appliances and plumbing. It’s also a good idea to turn off the air conditioner and unplug appliances, computers, and other electronic devices to prevent damage from power surges.
If you can’t make it safely to a stable building or shelter, a hard-top (non-convertible) vehicle can offer you some protection with the windows closed. (However, in the event of a tornado, cars are not at all safe.) Avoid open spaces, high ground, water, picnic shelters, shelters and bleachers, and tall, pointed objects such as trees and flagpoles.
