Severe weather is identified and tracked in many different ways. The National Oceanic & Atmospheric Administration (NOAA) has centers for tracking severe storms in Norman, Oklahoma and Miami, Florida which gather and analyze the observations of weather stations to predict the areas where severe weather may happen. The Storm Prediction Center in Oklahoma handles most types of severe weather, while the National Hurricane Center specializes in Atlantic and eastern Pacific Hurricanes. The amount of advanced warning varies by the type of storm. The movement of low pressure areas is more predictable, so people usually can be alerted to the possibility of hurricanes or large snowstorms a few days in advance. The cold fronts which cause tornadoes and severe thunderstorms can also be followed, but formal watches for these types of storm are usually issued the same day.
Hurricanes are the largest severe storms, so there is a lot of effort to detect them early and track them. Weather systems over the ocean are watched by satellites and reported on with readings from ships and buoys. When a storm is detected, Hurricane Hunter airplanes, and recently areosondes (drone aircraft used to take weather readings) are sent to investigate them. Hurricane watches are issued 48 hours in advance of landfall so people will have time to prepare or evacuate.
Radar is used extensively for tracking all types of storms over land. The main radars currently in use are called NEXRAD, which stands for Next Generation Radar. NEXRAD is a Doppler radar, which means it can detect winds by measuring the speed of droplets using the Doppler effect. But has a limited range because the surface of the Earth curves down away from the beam, so parts of storms are below the horizon. Tornadoes can be recognized by detecting rotation in clouds with Doppler radar, or looking for a hook shaped echo with older radars. Radar is very useful, but it has problems seeing the bottoms of storms where tornadoes form when they are below the horizon. So when there is a tornado watch, spotters are also deployed to look for tornadoes. These spotters include scientists who study tornadoes, police, and volunteers. Thunderstorms are also tracked by radar. Another method of tracking unique to thunderstorms is tracking by plotting where there are lightning strikes. Each lightning bolt sends out a burst of energy, which can be detected. In the days before cable and digital TV, there would be a burst of snow on your TV screen when lightning struck, and continuous snow was said to be a sign a tornado was coming. Doppler radar can also be used to determine if the precipitation in clouds is rain, freezing rain, or snow, and it can be used to judge the amount of rain that has fallen so flood warnings can be issued.