Watches may be issued hours before a storm. Remember a watch just means that conditions are favorable for severe thunderstorms and/or tornadoes.
Severe thunderstorm warnings
Severe thunderstorm warnings often precede tornado warnings, providing you with extra time to prepare. In addition, severe thunderstorm warnings issued by the National Weather Service will almost always contain details about the expected hail size and wind speeds. Severe thunderstorms can produce destructive hail — ranging in size from dimes to softballs or larger — winds in excess of 100 mph, flooding rains and deadly lightning.
Tornado warnings cannot be issued for every single tornado that occurs. This is why you must take responsibility for your personal safety any time storms threaten. Do not wait until you get an official warning, through TV, radio or siren. If you feel threatened, you should take your tornado precautions. It is better to be safe than sorry. Tornado warnings contain information that lists the cities and towns in the path of a tornado. While the National Weather Service strives to provide the most detailed and accurate information possible, a small town or community may be in the path of a dangerous storm but may not listed in the warning text. This also holds true for television forecasts. You should be cautious when using detailed forecasts of time and location.
Severe weather statements
Severe weather statements are issued to update warnings, and include new information about an ongoing severe storm.
Source: National Weather Service