Tis the season for severe weather, but what actually makes a storm severe? There are three things: >58mph winds, tornadoes, and >1 inch in diameter hail. When any of these are radar indicted or reported by storm spotters, the National Weather Service will issue a warning.
When an updraft feeds warm, humid air into a thunderstorm, it creates the dynamics needed for severe development. As far as hail is concerned, the stronger the updraft, the larger the hailstone. When moisture is pushed into a storm cloud, it collects and becomes rain. If a strong updraft is present, that rain gets pushed higher in the cloud and freezes and collects more H2O the longer it stays in the cloud. Once the hailstone becomes too heavy for the updraft to hold it up, it falls to the ground. If you see hail that's the size of a quarter or larger, the storm is severe.
With strong updrafts come strong downdrafts and with that, strong winds at the surface. When winds gust over 58mph, the storm is considered severe as this can do significant damage to roofs, power lines, and trees, but it doesn't stop there. Some downdrafts have been recorded at over 160mph, which is the equivalent to the damage that can be done by an EF-3 tornado.
When a tornado is spotted (via radar or spotter), the National Weather Service will issue a tornado specific warning, as opposed to just a severe warning. The updraft plays a vital role to tornado formation as well. As the warm, moist air gets pulled in by the updraft, cooler air gets pushed out by the retrieving downdraft. This convergence of airflow, along with the strong horizontal winds of the jet stream, creates a rotating wall cloud under the base of the cumulonimbus cloud. With an intensifying updraft, a small area of low pressure is created and pulls down condensation in the form of a funnel cloud. Once it reaches the ground, it is classified as a tornado and can do a multitude of damage.
A WATCH is issued when conditions are favorable for a severe weather event. Preparations should be made in the event severe weather become imminent.
A WARNING is issued when severe weather occurring or imminent. Measures for protection should be taken.
An ADVISORY is issued when hazardous weather is imminent or likely BUT is less serious than a warning. These are events that can cause significant inconvenience to life and property if caution is not exercised.