What is a Severe Thunderstorm Watch?
A severe thunderstorm watch is issued by the National Weather Service when atmospheric conditions make it likely that severe thunderstorms will develop over a specific region. During a watch, the threat is not immediate, but meteorologists have identified that storms could form within the next few hours. A watch typically covers a large area, sometimes spanning multiple counties or states. People in a watch area should stay informed about weather updates and prepare for possible severe weather.
What is a Severe Thunderstorm Warning?
A severe thunderstorm warning is issued when a severe thunderstorm has been sighted by weather spotters, detected on radar, or is about to develop imminently. A warning indicates that dangerous weather is either happening now or will arrive very soon. Warnings cover a much smaller geographic area than watches and are more urgent. When a warning is issued, people should immediately seek shelter and take protective action.
Dangers Addressed
Severe thunderstorm watches and warnings alert the public to potential hazards including large hail, damaging wind gusts, heavy rainfall that can cause flash flooding, and occasionally tornadoes. Watches help people prepare for these dangers, while warnings indicate the danger is imminent or already present.
How to Respond to a Watch
When a severe thunderstorm watch is issued for your area, you should check weather forecasts regularly, have multiple ways to receive weather alerts, know where safe shelter is located, and keep emergency supplies accessible. It is a good time to postpone outdoor activities and keep an eye on the sky.
How to Respond to a Warning
When a severe thunderstorm warning is issued, move to a safe shelter immediately, preferably in a basement or interior room away from windows. Stay away from windows and doors, turn off outdoor air conditioners, unplug electronics, and remain sheltered until the warning is lifted. Do not try to outrun a thunderstorm in a vehicle.
Key Differences Summary
The main difference is timing and urgency. A watch means be prepared; a warning means act now. Watches cover larger areas and last longer, while warnings are specific, localized, and short-duration alerts for immediate danger. 
Answer