Thunderstorms need certain ingredients in the atmosphere to form: moisture, instability and lift. However, for storms to tip the scale into severe status, additional ingredients are necessary, such as CAPE and wind shear. We discussed CAPE, or energy available for storms to tap into, in more detail here. The key component to developing an organized thunderstorm is wind shear.
There are two kinds of shear meteorologists will refer to: speed shear and directional shear. Both types of shear can occur in the atmosphere at the same time. In fact, having a strong presence of both wind shear types will typically lead to more organized thunderstorms if other necessary ingredients for severe weather are present.
Speed shear is wind speed changing with height. Similarly, directional shear is wind direction changing with height. For example, if winds at the surface are coming in from the south at 30 mph while winds a few thousand feet higher in the atmosphere are coming in from a more southwesterly direction at 60 mph, this would be an example of both speed and directional shear.
Winds that take that clockwise turn with height are what we call veering winds. Winds that veer can be important when forecasting the potential for supercell thunderstorms. Wind shear aids in sustaining the updraft of a storm.
Speed and directional shear will help to tilt the updraft of a storm so that rain-cooled air from the downdraft does not fall back into the updraft and inhibit the storm from strengthening.
For storms to maintain their strength, the updrafts and the downdrafts must stay separated. Tilting the updraft ensures that happens. At a certain point, thunderstorms will become downdraft dominant, leading to the collapse of a thunderstorm. However, having strong wind shear will help to maintain storm structure for a longer period of time.
More organized thunderstorms will thrive in an environment that has strong speed shear and weak or strong directional shear. Strong speed shear and weak directional shear will lead to the development of storms that can be fast-moving and eventually congeal into a line of thunderstorms capable of producing strong, damaging wind gusts. When you add strong directional shear to strong speed shear, rotation becomes possible in the atmosphere which can be ingested into supercell structured storms. These storms could produce tornadoes and large hail.
Weak speed shear and either weak or strong directional shear combinations will lead to less organized thunderstorm development and would support more “garden variety” thunderstorms that produce rainy rumbles, but may not be as organized. Several severe weather parameters need to come together in order for a setup to be favorable for strong to severe storms. Having just wind shear present may not be enough for strong storms to develop, but it certainly is a key ingredient for organized thunderstorm development.
— Meteorologist Erika Paige