by David Murphy
Thunderstorms usually form when a warm, moist air mass is invaded by a cooler, drier air mass. The cool air forces the warm air to rise, the warm air cools as it rises, and its moisture condenses and turns into clouds and rain. Additional forcing mechanisms then encourage even more rising air and turbulence until a thunderstorm is born.
What specifically happens: As cooler air moves into warmer air, winds riding up from the humid south bring in additional warm air and moisture, which you can think of as fuel for the developing storm. Meanwhile, farther above the surface, winds from the west and northwest pass over the two air masses, acting like a vacuum, sucking air up from the surface. Because of the different wind directions, the rising air also starts to rotate. Eventually, a spinning column of rising air forms, known as an updraft.
Other factors may also encourage this lifting and spinning (See: What causes a thunderstorm? for a list of Forcing Mechanisms). Great volumes of moisture being drawn into the storm's base rise up through the updraft and begin condensing into liquid water. Heavy downpours result. Eventually, the updraft rises into colder air and ice pellets form. Some of these ice pellets grow and fall out of the clouds as hail. Meanwhile, electrical charges within the violent, developing storm become separated, setting the stage for lightning and thunder (lightning is merely the conduit through which these separated charges are reunited). As cool air tumbles out of the top of the storm, it rushes to the surface with such a force, that strong, gusty winds often push out ahead of the storm.
Now, you have an idea of where heavy downpours, hail, lightning and thunder, as well as gusty winds from a thunderstorm all come from.