Watch or warning? What changes when the alert changes
A watch asks you to get ready. A warning means the threat is happening or imminent in the warned area.
A watch covers the possibility of dangerous weather. The National Weather Service issues a tornado or severe thunderstorm watch when conditions favor those storms in or near the watch area. It is time to check your plans, know where you would shelter, and keep a reliable way to receive later alerts. A watch does not mean every location in the area will see a storm.
A warning is more urgent. For a tornado warning, a tornado is imminent or occurring; seek safe shelter immediately. A severe thunderstorm warning means severe thunderstorms are occurring or imminent in the warning area. Read the alert's location, timing and instructions carefully rather than assuming that the entire watch area faces the same immediate threat.
Flash flooding has its own warnings. A flash flood warning means rapidly rising water is imminent or occurring. Because conditions can change quickly, follow local emergency instructions and never drive through floodwater.
The practical distinction is simple: use a watch to prepare, and act immediately when a warning covers your location. Always consult the latest National Weather Service alert for the exact area and instructions.