The National Weather Service operates a series of WSR-88D radars. WSR-88D stands for Weather Service Radar 88 Doppler (the radars were deployed in 1988). Doppler radar measures both the reflectivity of clouds and the speed at which the echoes are moving towards or away from the radar (called the radial velocity). Since the size of a thunderstorm is much greater than the resolution of the radar, meteorologists can use these radial velocities to determine if a storm is rotating.
Today there were a couple of great examples of tornadic thunderstorms passing over Alabama. Several tornadoes were reported across the stated. Coincidentally, two of these storms passed over exactly the same path but about 7 hours apart. The first image to the left is a typical reflectivity radar image. In this shot you can clearly see a supercell thunderstorm that is part of a squall line outside of Montgomery. When storms get very strong, they develop a 'hook echo' shape. In the 'fist' of this hook is a feature called a mesocyclone, or a medium size cyclone. This mesocyclone is basically a tight column made up of a rotating updraft. To the right of the mesocyclone is an area called the 'inflow notch' where warm, moist air is being sucked into the updraft. If a tornado were to develop, you would expect it to be located underneath the mesocyclone and track in the direction of the inflow notch. Unfortunately for storm chasers, the only way to typically get a clear view of a tornado is to position yourself in the inflow notch which places you directly in the path of the tornado.
The next image is the storm relative radial velocity image of the same storm. This means that, instead of absolute velocities relative to the radar, the velocities are given in reference to the average velocity of the storm. Green colors represent motion towards the radar and red colors represent motion away from the radar. You may notice that located exactly where the mesocyclone was in the previous image is a large bright red dot embedded within a field of green. This is showing a small area of very fast winds blowing away from the radar and surrounded by fast winds blowing towards the radar. Such a signal is indicative of a rapidly rotating of column of air, like a tornado.
Once again the storm relative radial velocities showed a clearly signal of rotation. In the second storm you can see a clear finger of bright green velocities sticking into the field of red. It is important to realize that this bright green dot represents winds of of at least 74 kts (above hurricane strength) blowing at the radar and is located next to very strong winds blowing in the opposite direction.
A thunderstorm does not have to take the shape of a mature hook echo to produce a tornado. Some echos can take the shape of bows and spawn tornadoes at the tips or front of the bow. In another case today further north near Ashland, Alabama, there was a supercell that was just beginning to develop a hook echo shape when it was displaying the previously mentioned tornadic signature.
The storm relative radial velocities again showed the aforementioned signal. Meteorologists call this a Tornado Vortex Signature (TVS). Weather service radars are not capable of automatically alerting meteorologists if a TVS develops in case they are not actively watching the situation. If a TVS is detected and verified, the weather service will mostly like issue a Tornado Warning.
During a severe weather outbreak, you can refer to the Storm Prediction Center for forecasts and damage reports. For more information on convective weather, the National Center for Atmospheric Research (NCAR) offers a series of online modules. The Weather Underground maintains an excellent online tool for checking reflectivity and radial velocity radar data.