“This risk is reserved for cases where there is high confidence in the widespread coverage of harsh weather with embedded instances with very serious instances embedded,” SPC said on its website.
Related: How Supercells grow to produce huge hail and intense tornadoes
Generally speaking, SPCs are at high risk of severe weather when forecasters are confident in the possibility of a tornado with a severe, long-life twister, or when Dereco is predicted. The Derecho is a long-life squall line that produces significant wind damage over a wide range of areas along paths that are hundreds of kilometers long.
High-risk days are very rare
Severe thunderstorms are an annual threat across the US, with the SPC highlighting the risk of serious thunderstorms somewhere in the US on 279th in 2024. Almost 70% of the day was due to low-end limitations or slight risks.
High-risk days are extremely rare. As of March 2025, SPC had issued high risks on 66 occasions since 2000.
Days at high risk of bad weather often include reinforcing bad weather clocks and warnings.
A tornado clock means conditions will be advantageous for tornado development over the next few hours. The SPC issues “particularly dangerous situations” (PDS) tornado clocks when multiple intense, long truck tornadoes are possible.
