A severe thunderstorm watch is issued when conditions are right for severe thunderstorms to develop. If the those potentially severe thunderstorms also have the chance of seeing tornadoes, then a tornado watch will be issued. Watches usually cover large areas, and sometimes all of central Illinois. Typically a watch goes for several hours, sometimes as much as 8 or 12 hours, but also are sometimes cancelled early.
A severe thunderstorm warning is issued when a storm could be, or is producing severe weather (hail of 1" or more, winds at 58 mph or greater). When a storm shows the potential of producing a tornado, or a funnel cloud or tornado is confirmed, a tornado warning will be issued. Warnings are smaller and typically cover part of a counties, or multiple areas in different counties. They're issued using a "polygon" meaning that while there could be a tornado warning for the southern part of the county, but not the northern.
Radar Indicated Severe Weather
Not all severe weather is confirmed before a warning is issued. Most of the time, you'll hear the phrase "Doppler indicated". Through the technology of radar, we see inside of the storm and get a good idea of what's happening on the ground. For example, we can see how heavy the precipitation is and how tall the storm is. From this, we can estimate the size of hail and if it is over 1". Furthermore, Doppler radar doesn't just show us precipitation (what you normally see), but it also can show us velocities of the raindrops falling from the storm. From this, we can gather if the storm is creating 58 mph winds. Not only can we see how fast the raindrops are moving, but we can see which direction they're moving. If we see strong rotation, we can likely assume that a tornado may be produced, and issue a tornado warning.
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