What Makes a Thunderstorm Severe? The Actual Threshold
Direct answer: in the United States a thunderstorm is classified as severe when it produces hail at least 1 inch (2.5 cm) in diameter, wind gusts of at least 58 mph (93 km/h, 50 knots), or a tornado. Any one of the three is enough. Heavy rain is not on the list, however torrential it is — which is why a storm dropping enough water to drown a street can be, in the technical sense, not severe.
The threshold is a measurement, not an impression
“Severe” sounds like a judgement about how frightening a storm is. It is not. It is a line drawn at specific numbers, and a forecaster issues the warning when a storm is producing them or is expected to.
| Criterion | Threshold |
|---|---|
| Hail diameter | 1 inch / 2.5 cm or larger |
| Wind gust | 58 mph / 93 km/h / 50 kt or stronger |
| Tornado | Any |
One inch is roughly the width of a large bottle cap. That number is not arbitrary: it is approximately where hail begins to damage vehicles and roofing rather than simply making noise on them.
Fifty-eight miles an hour is approximately where healthy trees begin to fail and unsecured objects become airborne. Again, the threshold marks a change in what the wind does, not a change in how loud the storm is.
The gap that catches people: rain is not a severe criterion
This is the most consequential thing in the whole subject, and it is genuinely counter-intuitive.
Flooding kills more people in most years than hail and thunderstorm wind combined. Yet rainfall, at any intensity, does not make a thunderstorm severe. A storm can sit over one catchment for three hours, drop a month of rain, flood every underpass in a city — and never meet the definition.
The reason is administrative rather than scientific: flooding is handled by a separate warning stream. Rain produces a flood watch, a flash flood warning or a flash flood emergency, issued on different criteria by people looking at catchments rather than at storm structure.
So the absence of a severe thunderstorm warning tells you nothing about your flood risk. Those are two different questions answered by two different products, and reading one as covering the other is how people end up driving into water. If you are trying to work out which kind of flooding a warning is about, flash flooding and river flooding behave very differently and are warned about differently too.
Watch, warning, and what each is really telling you
A severe thunderstorm watch means conditions are favourable across a broad area over a period of hours. It is a statement about ingredients: instability, moisture, wind shear. Nothing has necessarily happened yet.
A severe thunderstorm warning means a specific storm, in a specific polygon, is producing or is about to produce the threshold values — usually within the hour. A radar signature or a ground report has crossed the line.
The distinction applies across most alerting systems and is covered in more depth in alert levels and colour codes explained.
Damage threat tags, and why some warnings sound louder
Not every severe thunderstorm warning is equal. Many now carry an explicit damage threat tag — typically base, considerable or destructive. The top tag can trigger a Wireless Emergency Alert on phones inside the polygon, which is why one storm makes every handset in an area sound and the next one does not.
A destructive tag generally implies hail around 2.75 inches or wind near 80 mph. If a warning woke your phone and the last one did not, that difference is usually the reason rather than a fault in your settings. The mechanics of why a particular alert did or did not reach you are covered in why you did not get an emergency alert.
Outside the United States the numbers change
The one-inch and 58 mph thresholds are American. Elsewhere the concept exists but the line sits in a different place, and sometimes the framing differs entirely.
European services frequently work from impact-based warnings, which combine likelihood with expected consequences rather than triggering on a single measured value. Others set their own hail and gust thresholds, or fold thunderstorm risk into a general severe weather category. There is no international severe thunderstorm standard, so a storm described as severe in one country is not necessarily comparable to one described that way in another.
That is worth remembering when comparing events across borders on any global feed, including this one.
What a severe warning does not tell you
- It does not mean a tornado is coming. Most severe thunderstorms never produce one. A tornado gets its own warning, and if a tornado is the threat the message says so.
- It does not mean the storm is over your house. Warnings cover polygons that usually contain many places the storm will not reach.
- It says nothing about lightning. Lightning occurs in every thunderstorm, severe or not, and is a significant cause of death in ordinary storms that never approach the threshold.
- It says nothing about flooding. As above, that is a different product entirely.
How this site fits
Severe thunderstorm warnings shown here are reproduced from the issuing meteorological agency, with that agency named on the event. We do not issue warnings and are not an official warning system. When a warning is active where you are, the issuing service is the authority and its current text supersedes anything on this page.
Thresholds described here are those used by the United States National Weather Service and were current at the time of writing. Criteria are revised periodically and differ by country — check your national meteorological service for the definition that applies to you.
Frequently asked questions
What makes a thunderstorm officially severe?
In the United States, hail of at least 1 inch (2.5 cm), a wind gust of at least 58 mph (93 km/h), or a tornado. Any one of the three qualifies. The classification is a measurement, not a description of how intense the storm appears.
Is heavy rain part of the severe thunderstorm definition?
No, and this is the most important gap to understand. Rainfall of any intensity does not make a storm severe. Flooding is handled by a separate warning stream, so the absence of a severe thunderstorm warning tells you nothing at all about flood risk.
What is the difference between a severe thunderstorm watch and a warning?
A watch means conditions across a broad area favour severe storms over the coming hours. A warning means a specific storm is producing or about to produce the threshold values, usually within the hour, inside a defined polygon.
Why did my phone alert for one storm and not another?
Many warnings carry a damage threat tag, and only the highest tag, typically destructive, triggers a Wireless Emergency Alert on phones in the polygon. A storm warned at the base level is a real warning that simply does not sound the handset.
How big is one inch of hail?
Roughly the width of a large bottle cap. The threshold sits there because it is approximately where hail starts damaging vehicles and roofing rather than just making noise on them.
Do other countries use the same threshold?
No. The one-inch and 58 mph figures are American. Many European services use impact-based warnings that weigh likelihood against expected consequences instead of a single measured trigger, and other countries set their own values. There is no international standard.
Does a severe thunderstorm warning mean a tornado is possible?
It means the storm meets the hail or wind criteria, and most severe storms never produce a tornado. If a tornado is the threat, a separate tornado warning is issued and the message says so explicitly.
Sources
- NOAA / National Weather Service - severe thunderstorm definitions and warning criteria
- NOAA Storm Prediction Center - watches, outlooks and convective criteria
- NOAA - impact based warnings and damage threat tags
- WMO - severe weather forecasting and impact-based warning guidance
- EUMETNET Meteoalarm - European impact-based warning framework
- GDACS - global severe weather alerting
About the author
WND Editorial Team — The World Natural Disasters editorial team curates disaster information from official scientific and humanitarian sources, reviews safety guidance against public-agency recommendations, and maintains the historical database.
Written and reviewed under our editorial policy. Factual claims cite the sources listed above.
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