High-risk U.S. severe weather days are exceptionally rare and dangerous
Intense tornado outbreaks and derechos are possible on days when a high risk is in the forecast
Severe thunderstorms are a year-round risk south of the border. Anyone visiting the United States should be familiar with the country’s severe weather forecasts and alert systems.
While the vast majority of severe thunderstorms only produce 100 km/h wind gusts and hail about the size of a quarter, some thunderstorms grow intense enough to produce exceptionally destructive winds, very large hail, and violent tornadoes.
Days when those high-end severe thunderstorms are possible may warrant a scale-topping high risk for severe weather. Here’s what those high-risk days entail.
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Severe weather on a five-category scale
The U.S. Storm Prediction Center (SPC) is responsible for issuing severe thunderstorm outlooks south of the border. The agency uses a five-category scale to relay the threat for severe weather at any given location.
A marginal risk is a level one out of five, issued when a couple of low-grade severe thunderstorms could produce damaging winds, large hail, and possibly a tornado or two. The scale rises in severity from there, climbing all the way to a high risk.
What is a high risk?
Forecasters reserve a scale-topping high risk for the days with the greatest potential for life-threatening thunderstorms.
“This risk is reserved for when high confidence exists in widespread coverage of severe weather with embedded instances of extreme severe,” the SPC says on their website.
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Generally speaking, the SPC will issue a high risk for severe weather when forecasters are confident in the potential for a tornado outbreak with intense, long-lived twisters, or if a derecho is in the forecast. A derecho is a long-lived squall line that produces widespread significant wind damage along a path measuring hundreds of kilometres long.
High risk days are exceptionally rare
Severe thunderstorms are a year-round threat across the U.S. The SPC highlighted a risk for severe thunderstorms somewhere in the U.S. on 279 separate days in 2024. Nearly 70 per cent of those days were for low-end marginal or slight risks.
High risk days are exceptionally rare. As of April 2, 2025, the SPC had only issued a high risk on 67 occasions since 2000—accounting for less than one per cent of all days since the turn of the millennium.
A day with a high risk for severe weather often includes enhanced wording to severe weather watches and warnings.
Tornado watches mean that conditions are favourable for the development of tornadoes over the next couple of hours. The SPC will issue a “particularly dangerous situation” (PDS) tornado watch when multiple intense and long-track tornadoes are possible.
Tornado warnings are issued when a tornado is occurring or imminent. Local National Weather Service (NWS) offices can upgrade a tornado warning to a “tornado emergency” when a large and destructive tornado is moving into a populated area. There have only been a little over 300 tornado emergencies issued since 1999.