A severe thunderstorm warning is an urgent alert issued by weather agencies when a severe storm is detected by radar or spotters, indicating a direct impact is expected within the hour. In the United States, the National Weather Service (NWS) issues this warning when a storm produces wind gusts of at least 58 mph or hail of 1 inch in diameter—a threshold upgraded from 0.75 inches in January 2010 to reduce public complacency. Unlike a watch, a warning signals immediate danger, and while these storms can cause significant wind and hail damage, they can also spawn tornadoes; if rotation is detected, a separate tornado warning is issued. These alerts are broadcast through NOAA Weather Radio, the Emergency Alert System, and Wireless Emergency Alerts, often displayed as polygonal shapes on weather maps. Criteria vary globally, with agencies like Environment Canada also factoring in high rainfall rates, while others may issue flash flood warnings instead.