Severe Thunderstorm Criteria are the fixed thresholds the National Weather Service uses to distinguish a severe thunderstorm from an ordinary one, currently defined as hail 1 inch or larger in diameter, wind gusts of 58 mph or greater, or a tornado. Any thunderstorm producing at least one of these is officially severe.
Why the thresholds are what they are
The 1-inch hail threshold, adopted nationwide in 2010, corresponds to roughly quarter-sized hail, which is large enough to dent vehicles, injure people caught outdoors, and cause meaningful property damage. The prior 0.75-inch (dime-sized) threshold had produced too many severe warnings for storms that were not causing substantive damage.
The 58-mph wind threshold, equivalent to 50 knots, is set at the low end of the wind speeds that reliably down small trees, damage roofs, and produce measurable damage. It is a threshold that generally survives evaluation across a wide range of surface conditions and storm environments.
A tornado of any intensity qualifies on its own, because even weak tornadoes present life-safety concerns that outweigh their footprint.
How the criteria are used
Warning meteorologists issue Severe Thunderstorm Warnings when radar signatures, spotter reports, or model guidance indicate a storm will meet one of these thresholds. Local storm reports use the same criteria to confirm severe occurrences after the fact.
The criteria also feed the impact-based tiering. A baseline warning uses the 1-inch, 58-mph thresholds. A Considerable tag applies at 1.75-inch hail or 70-mph wind. A Destructive tag applies at 2.75-inch hail or 80-mph wind and triggers Wireless Emergency Alerts on smartphones in the polygon.
What the criteria do not capture
A storm below severe thresholds can still injure or kill. Lightning is a life-safety threat at any storm intensity. Localized flash flooding, especially in urban areas, can be lethal without meeting any wind or hail criterion. Fifty-mile-per-hour outflow can knock a person over even though it falls short of the severe wind cutoff.
The criteria are also uniform across the country by design. In practice, the same storm can feel very different in an open plain versus a densely populated urban corridor, and the warning meteorologist accounts for that context even though the numeric thresholds do not change.