As with anything in life, what goes up must come down (apart from taxes) and the same principle occurs in thunderstorms and shower clouds.
A thunderstorm has an updraft (where the air rises into the system) and a downdraft (where the air exits the system)
The downdraft will occur on the frontal boundary of the system in the direction which it is moving in, which is why when a thunderstorm approaches it is usually the wind which picks up first (the downdraft)
There are three factors which decide the strength of the downdraft :-
1. The air at height being much colder than the air at the surface which means the upper air is more dense.
2. The amount and type of precipitation from the thunderstorm. If hail or large hail is possible (strong updraft) then the intensity and size of these objects in effect "drag" down the colder air from aloft and increase the speed of the downdraft.
3. Converging upper air winds. If upper air patterns show converging winds then any showers/thunderstorms underneath will be able to tap into this pool of "extra" air.
The atmosphere can be unstable for updrafts but stable for downdrafts, stable for updrafts but unstable for downdrafts, stable for both, or unstable for both.
Weak updrafts and strong downdrafts often occur with elevated thunderstorms and tend to increase the risk of microbursts of heavy rain.
Strong updrafts and weak downdrafts due to warmer/moist air aloft will produce thunderstorms with a lack of wind, but heavy rain and hail (depending on the upper air temperature).
Relatively weak updrafts and downdrafts are found with non-severe showers and thunderstorms. The last possible combination is a storm with strong updrafts and downdrafts. These are the big guys... Severe convective wind gusts around the system with heavy rain/hail and tornadoes.
So, if you know the updraft and downdraft risk of a thunderstorm you will know what type of storms are likely to develop.
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