Severe weather field guide

    What Makes a Thunderstorm Severe? The Official U.S. Criteria

    In the United States, a thunderstorm reaches severe criteria with one-inch hail, winds of at least 58 mph, or a tornado. Lightning and heavy rain remain dangerous but are not the baseline thresholds.

    Originally published November 26, 2017Updated August 5, 2026By the WXObservation team
    "Severe" has a specific operational meaning in U.S. weather warnings. It does not simply mean a storm looks dramatic or produces frequent lightning.The thresholds help forecasters communicate the expected wind and hail threat consistently, while tornadoes, flash flooding, and lightning have their own warning and safety decisions.
    Quick answer
    A U.S. thunderstorm is classified as severe when it produces hail at least 1 inch in diameter, wind gusts of at least 58 mph (50 knots), or a tornado. In operations, a tornado threat is communicated with a Tornado Warning rather than relying on a Severe Thunderstorm Warning. Heavy rain and frequent lightning can be life-threatening even when the storm does not meet the hail or wind thresholds.
    Severe thunderstorm and lightning over a city at sunset
    A thunderstorm can contain several hazards at once, the U.S. severe classification is tied to specific wind, hail, and tornado criteria.

    Official severe thunderstorm criteria

    The National Weather Service definition is based on the hail, wind, or tornado thresholds below.
    HazardSevere thresholdWhat to know
    WindAt least 58 mph (50 knots)Sustained wind or a gust can qualify. Structural or tree damage may support the estimate
    HailAt least 1 inch in diameterApproximately quarter-size, measure the largest stone across its widest point
    TornadoAny tornadoA Tornado Warning is the operational warning when a tornado is indicated or reported
    LightningNot a severe thresholdEvery thunderstorm has lightning and every lightning-producing storm is dangerous
    Heavy rainNot a severe thresholdFlash flooding uses separate flood products and can be deadly without severe hail or wind
    A storm only needs one qualifying hazard. It does not need both large hail and damaging wind.

    Why 58 mph wind matters

    Fifty knots converts to about 57.5 mph and is normally rounded to 58 mph in public guidance. At and above this range, winds can break large branches, topple trees, damage roofs, and create dangerous debris.A backyard weather station can document a gust at its sensor, but siting, exposure, instrument limits, and distance from the strongest part of the storm affect the reading. Do not wait for your station to confirm 58 mph before taking shelter.

    Why one-inch hail is the threshold

    One inch is roughly the diameter of a U.S. quarter. Hail at this size can damage crops, roofing, vehicles, and exposed equipment. When reporting hail, measure the widest diameter when it is safe to do so. Coin comparisons are more consistent than "marble size," because marbles vary.
    One-inch hailstones beside a ruler
    One-inch hail is approximately quarter-size and meets the U.S. severe-thunderstorm threshold.

    Watch versus warning

    ProductMeaningYour action
    Severe Thunderstorm WatchConditions are favorable for severe storms in and near the watch areaReview shelter plans, secure loose items, and monitor reliable alerts
    Severe Thunderstorm WarningSevere weather is occurring or imminent in the warning polygonMove indoors and away from windows. Follow the stated wind and hail threat
    Tornado WarningA tornado is indicated by radar or reported by spottersGo immediately to the lowest, most interior safe location available
    Flash Flood WarningDangerous flash flooding is occurring or imminentMove to higher ground and never drive through floodwater

    What does not make a storm officially severe?

    Frequent lightning, loud thunder, torrential rain, vivid cloud color, and small hail do not by themselves meet the baseline severe-thunderstorm definition. That is a classification distinction, not a safety reassurance. Lightning can strike before rain begins, and flash flooding can become life-threatening in a storm whose wind and hail stay below severe criteria.

    What to do during a severe thunderstorm warning

    Act on the warning, not on what you can see

    • Go into a substantial building and move away from windows.
    • Keep shoes, a flashlight, and a reliable warning source available.
    • Do not shelter under trees, open-sided structures, or tents.
    • Delay outdoor activity until 30 minutes after the last thunder.
    • If a Tornado Warning is issued, move to the lowest interior room immediately.
    A SAME-capable NOAA Weather Radio can provide an overnight alert without relying on a phone data connection.See the current NWS severe-thunderstorm safety guidance for shelter, driving, and post-storm hazards.

    Frequently asked questions

    Is every thunderstorm with lightning severe?The answer is no. Lightning makes it a thunderstorm, but the baseline U.S. severe criteria are one-inch hail, 58 mph wind, or a tornado.
    Does heavy rain make a thunderstorm severe?Not under the wind-and-hail definition. Heavy rain can trigger a Flash Flood Warning and may be life-threatening even when the storm is not classified as severe.
    Is quarter-size hail severe?The answer is yes. A U.S. quarter is about one inch in diameter, which meets the hail threshold.
    Can a severe thunderstorm produce a tornado?The answer is yes. Severe storms can produce tornadoes. When a tornado is indicated or reported, the National Weather Service issues a Tornado Warning for that threat.

    Sources

    Set up NOAA Weather RadioReceive official warnings through a dedicated, battery-backed receiver.
    Become a trained spotterLearn what to report and how ground truth supports warning decisions.
    How lightning detection worksSee why lightning data and severe-storm classification answer different questions.
    Improve station accuracyUnderstand how exposure and siting change wind and rain measurements.