The radar used by the NWS works via signals sent out by a radar tower transmitter that reflects or bounces back off of precipitation. The radar will then try and determine the type of precipitation, such as sleet, snow, rain, etc. Also the radar shows areas of more intense precipitation as well as the speed and direction in which the precipitation is traveling. 

If the storm is perceived as severe then forecasters can tell where more intense parts of the storms are located, where hail cores could be found, predict where the storm will be going and when it will get there, and even see where possible tornadoes could appear.

Interpreting Radar

The following radar images are of severe storms as they moved into central Kentucky on February 5 and 6 of 2008. The areas of blue and green are places of small amounts of reflectivity which means minimal to moderate precipitation, while areas of yellow and red are areas of high reflectivity which means heavy to very intense rainfall, and areas of pink and white are places of even higher reflectivity and are areas that likely contain hail. 

This radar image is a perfect example of a squall line that is moving from west to east. The squall line is represented by the red band of reflectivity shown going from north to south. Squall lines can have high winds, intense rainfall, hail, and even tornadoes.  
The following is an example of a thunderstorm classified as a supercell that moved across the Tennessee border into Kentucky. The yellow arrow in the bottom left corner is pointing to the part of the storm known as the hook echo that spawned an EF3 tornado that lasted 12 miles and had a path width of 200 yards. The pink shaded area near the hook echo is an area producing hail. Supercells like this one have the potential to be very damaging.  
This radar image was taken at the same time as the reflectivity image above. The only difference between them is that this image is of wind velocities. The green shaded area is showing winds that are blowing towards the radar and red shaded areas are of wind blowing away from the radar. The brighter the shade of the color means the faster the wind is blowing. This information is taken by measuring the velocity of precipitation in the storm. The yellow circle is showing the place where the tornado was located because it is an area where winds are going in opposite directions and are in a hook form. 

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