Severe thunderstorm watch
A severe thunderstorm watch is an alert issued by meterologists when a severe thunderstorm might develop. Who issues the watch and what is required for the watch to be issued depends on the country. For the watch to be issued, there might have to be the possibility of bad rainfall and tornadoes.
A severe thunderstorm watch is not a warning; it does not say a severe thunderstorm is right there, right now. It means that the storm has a possibility to develop severe weather, it's just not imminent, so they don't issue a severe thunderstorm warning on the spot, they will have to wait until the storm develops to severe weather levels.
United States
In the United States, severe thunderstorm watches are issued by the Storm Prediction Center (SPC for short).
The criteria for thunderstorms to be classified as severe varies by country; e.g., in the United States, severe thunderstorms must produce winds exceeding 58 mph [93 km/h], and/or hailstones larger than one inch [2.5 cm] in diameter, and/or one or more tornadoes.) Generally (but not always), thunderstorms that develop within the watch area may contain large hailstones, intense straight-line winds, intense lightning, torrential rainfall and/or flash flooding caused by high rainfall accumulations. Depending on storm cell intensity, severe thunderstorms can cause damage to structures or vehicles; impairment of vehicle and pedestrian travel; flooding to streets, populated neighborhoods, farmlands and other areas of poor drainage; and in extreme cases, injury or possible fatality to people and animals exposed outdoors (from either repeated hail-induced blunt trauma, wind-generated debris or intense flooding.
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