A severe thunderstorm watch (SVA) is an advisory issued by weather agencies indicating that atmospheric conditions are ripe for severe thunderstorms to develop within a region over the next several hours. Unlike a warning, which signals imminent danger, a watch encourages people to stay alert, as the area might even experience deceptively fair weather before storms hit. The criteria for "severe" vary by country, but in the United States, it typically means winds exceeding 58 mph, hail larger than one inch in diameter, or the presence of tornadoes. These storms can bring large hail, intense straight-line winds, torrential rain, and flash flooding, potentially causing structural damage or injury. Watches can be upgraded to tornado watches if conditions become more favorable for tornadogenesis, but they are not a prerequisite for issuing a severe thunderstorm warning.