Even a mild thunderstorm comes with lightning, thunder, wind, and rain; each of which can be dangerous. A severe thunderstorm has all that in spades, with a possibility of hail and tornadoes besides.
Lightning is the most obvious danger of every thunderstorm. With an estimated 16 million thunderstorms every year - 1,800 thunderstorms going on at any given moment - odds are high that something or someone is going to be hit. A fifth of the time, that someone will be killed. Even when lighting does not kill, it leaves the person with lifetime consequences which are only now beginning to be properly studied.
Lightning can also kill indirectly. A struck tree can fall on a house or car. Lightning can zip through the wiring in an unprotected house, shorting out the electrical system and possibly electrocuting anyone who comes into contact with it. The spark can fly feet out from an electrical outlet. In one case, a person was electrocuted in her bath when lightning struck distant wiring and came into contact with her copper plumbing.
Where lightning is accompanied by little or no rain, the strike may start a forest or grass fire. Thunderstorms are especially common in summer, when trees and grass may be very dry.
High areas are struck most often: but they are not struck exclusively. Houses can be struck directly by lightning which has bypassed a much taller tree. Even so, the chance of being struck by lightning drops considerably by avoiding high areas and by staying indoors.
Winds associated with a thunderstorm can come in at tornado-like speeds, even when they are "only" straight-line winds. Most thunderstorms happen in seasons when trees have all their leaves, creating the maximum amount of resistance to the wind. The result can tear even a large tree out of the earth. With this kind of force, simple loss of shingles is only to be expected.
Another kind of wind associated with thunderstorms is the downburst. These unpredictable strong winds can slam an airplane to the ground. Takeoffs and landings are extremely vulnerable to a sudden downburst.
Some thunderstorms produce tornadoes. EF0s, EF1s, and EF2s do about the same amount of damage as strong straight-line thunderstorm winds, but EF3s and higher are of another echelon entirely. A wide EF3 or EF4 will destroy most structures in its path. After an EF5 goes through an area, almost nothing will be left standing.
During most thunderstorms, the rain falls in buckets. Storm sewers will be overwhelmed, low parts of roads will be flooded, and water may be backed all the way into the houses. Insurance is becoming increasingly unwilling to pay these kinds of claims: on the basis that the city is responsible for the consequences of poor planning.
Low-lying areas near rivers are also prone to flash flooding. Where the flood crosses roads, cars which enter the area may be swept away by the raging waters.
Even small hail, falling thickly over a large enough area, can destroy a crop. Wheat, corn, barley, soybeans, and tobacco are most vulnerable: precisely the dominant crops grown in regions most prone to thunderstorms.
Larger hail will smash windows, even automobile safety glass. It can accumulate on a flying aircraft, combining the dangers of projectile force and in-flight ice accumulation, or make a runway far too slick for a safe landing.
The impact velocity of large hail can be over a hundred miles an hour. That size and speed can kill.