A Severe Thunderstorm Warning is issued when a severe thunderstorm is occurring or about to occur. A severe thunderstorm is a storm that has winds of 58 MPH (93 KPH) or greater and/or hail 1 inch in diameter or larger, torrential rainfall, continuous cloud to ground lightning, or other conditions that may warrant the warning.
https://emergencyalertsystem.fandom.com/wiki/Severe_Thunderstorm_Warning