Thunderstorm watch:

A watch means that conditions are favorable for thunderstorms to produce wind gusts to 58mph or stronger, or hail to 3/4 inch or larger in the watch area. Indications for this may be a sudden change in barometric pressure, wind direction or other event.

For such an event that comes and goes quickly, a watch means that the odds are good for the dangerous weather, but it's not yet happening. These watches are issued for 4 to 6 hours at a time and for a number of counties Sometimes a severe thunderstorm happens so quickly that warnings can't be issued in time.