What Makes a Storm "Severe"? — The 1-Inch Hail and 58 MPH Wind Thresholds
What officially makes a thunderstorm severe?
By National Weather Service definition, a thunderstorm is severe when it produces at least one of three things: hail one inch or larger in diameter (quarter size), wind gusts of 58 mph (50 knots) or greater, or a tornado. Nothing else qualifies — not deafening thunder, not continuous lightning, not blinding rain. The thresholds mark the point where storms begin damaging well-maintained property: 1-inch hail dents metal and cracks shingles; 58 mph gusts snap large branches and down weak trees onto lines and vehicles. Heavy rain has its own separate product line (flood advisories, watches, and warnings) because its danger — flooding — behaves differently from wind and hail.
Where do the exact numbers come from?
The wind threshold is a unit conversion in disguise: 50 knots — a round number in aviation and marine units the weather service inherited — equals 57.5 mph, rounded up to 58. It approximates the gust strength where tree and minor structural damage becomes consistent.
The hail threshold was raised from 3/4 inch to 1 inch in 2010, nationwide, after damage research (much of it insurance-driven) showed hail below about an inch rarely damages even shingles, and after years of warnings for penny-size hail diluted public response. That change is why older references disagree with current ones. Quarter-size remains the floor at which roofing, siding, and vehicle damage becomes commonplace.
Why don't lightning and rain count?
Not because they're safe — lightning kills more Americans in a typical year than hail does — but because they don't discriminate. Every thunderstorm produces lightning by definition; a "severe" label triggered by lightning would apply to all of them and mean nothing. The severe criteria exist to flag the minority of storms that damage property and threaten life through mechanical force.
Rain is excluded for a different reason: its danger depends on what's downstream — soil saturation, terrain, drainage — not just the rate falling. A stationary two-inch-per-hour storm over a burn scar is catastrophic; the same storm over dry flat farmland is irrigation. So rainfall hazards run through the dedicated flood product chain — flood advisories, flash flood watches, and flash flood warnings and emergencies — which accounts for those factors.
How are severe reports measured and verified?
Two streams feed the record. Measured values come from airport anemometers, mesonet stations, and other instruments. Estimated values come from people: trained spotters comparing hail to coins and sports balls (the standard scale runs quarter 1.00", half dollar 1.25", ping-pong 1.50", golf ball 1.75", tennis 2.50", baseball 2.75", softball 4.00"+) and gauging winds by damage produced. After events, NWS damage surveys distinguish straight-line wind from tornado damage by the debris pattern — divergent versus convergent.
These Local Storm Reports flow into the Storm Prediction Center's daily severe weather database (spc.noaa.gov/climo) — the dataset behind verification statistics, climatology, and the storm-event archives that power surfaces like VORTEX's tornado history pages.
What does "severe" look like at each step up the scale?
- At criteria (58 mph / 1" hail): broken large limbs, shingle and vehicle dents, isolated trees down. Danger is real but localized — mostly to people outdoors or under trees.
- Considerable (70 mph / 1.75"): healthy trees uprooted, widespread power outages, golf-ball hail breaking windows and destroying roofs.
- Destructive (80 mph / 2.75"+): hurricane-force winds moving in a line — structural damage, mobile homes rolled, baseball hail penetrating roofs. This tier triggers WEA phone alerts.
- Top-end events: derechos with 100+ mph gusts and softball hail from supercells — thunderstorm weather that outdamages many tornadoes.
The tiering is the practical answer to "severe compared to what": the word is a floor, not a description, and the warning's damage-threat tag tells you where today's storm sits above it.
How should the definition change how I read forecasts?
Three habits. When the SPC outlook mentions "severe storms," read it as the wind/hail/tornado trio, then check which hazard dominates — outlooks publish separate probabilities for each. When a severe thunderstorm warning arrives, check the tag before deciding it's routine. And never let "not severe" read as "not dangerous": sub-severe storms still bring the lightning and flash-flood risks that the definition deliberately handles elsewhere. Basic thunderstorm safety, per the NWS, applies to every storm: weather.gov/safety/thunderstorm.