The Role of Vertical Shear and Storm Severity

• Even in an environment with great CAPE, updrafts are not uniform. Some parts of the updraft will be stronger than other parts. The slightly weaker parts of the updraft will only support smaller hail sizes, which quickly fall, generating a cold downdraft that prevents the hail stones in the stronger part of the updraft enough time to grow to maximum size
  
  
• This is why it is important for a thunderstorm to stay existent as long as possible in order for hail to grow to the maximum possible size. Vertical shear will remove the precipitation that may fall before that happens so that it does not fall into the updraft area and suppress the updraft.
  
  
• The outflow boundary associated with the falling precipitation will also undercut/suppress the updraft if the precipitation is falling too close to the updraft area
  
  
• A key and complicated issue for thunderstorm forecasters is, therefore, how much vertical shear there is, and whether the combination of vertical shear and storm motion (which also increases the stronger the upper winds) will allow the thunderstorm to stay "alive" long enough for severe hail to occur.
  
  
• In a narrow window of circumstances associated with great shear, the motion of the outflow boundaries and of the storm itself results in the outflow boundary on the forward flank of the storm staying fixed with respect to the updraft. It never undercuts the updraft, and the storm may persist for many hours. We will see that such a set of circumstances produces not only a long-lived storm capable of severe hail and winds, but a rotating updraft. The latter is a feature of a class of stormcs called "supercells."