Thunderstorms can develop if three important ingredients are important in the atmosphere.
First, you have to have unstable air. This is the warm rising air I talked about on Monday, March 5, when I discussed CAPE.
Finally, you must have moisture for clouds to form. If there is enough warm rising air an updraft is formed.
The updraft is a steady stream of rising air that causes the clouds to grow into supercells. Think of the updraft as the fuel for the storm. It's an inflow of warm moist air. This is the area where hail forms and the process that generates lightning starts. The updraft is also where you look for a tornado.
The downdraft is the area of falling rain and hail.
The area of heavy rain seen on radar is part of the downdraft. The largest hail is seen on radar in bright pink and even white.