Supercell structure: updraft, wind shear and the flanking line

What defines a supercell

This is a general guide to the conceptual structure of a supercell, not a forecast for Italy or any other specific territory. A supercell is a highly organized thunderstorm distinguished by an updraft rotating counterclockwise. Size does not define it: the storm can be large or small, as well as tall or short. [s4]

Rotation does not automatically mean that a tornado will form. Nearly all supercells produce some kind of severe weather, such as large hail or damaging winds, but 30 percent or less produce tornadoes. The thunderstorm category should therefore be kept separate from one of its possible effects. [s4]

How wind shear contributes

Wind shear is the change in wind speed and/or direction with height. Both components matter in supercell formation. Speed shear tilts the updraft and carries precipitation downwind, reducing the likelihood that it will fall back into the rising current and weaken that updraft. [s4]

Speed shear alone will usually not produce rotation. Directional shear is also needed, with wind direction turning clockwise as height increases. Together, these components create horizontal rolling motion. An updraft can lift this motion into a vertical orientation, allowing cyclonic rotation to become dominant within the cloud. [s4]

The updraft and flanking line

Supercell updrafts can exceed 100 mph (160 km/h), velocities capable of supporting very large hail up to grapefruit size. South or southwest of the storm, a flanking line may develop as a series of new updrafts merging into the main one. This persistent supply can help a supercell continue for several hours. [s4]

When conditions allow, successive flanking-line updrafts replenish the supercell with moisture and instability. The storm can consequently pass through cycles of weakening and renewed strengthening. Diagrams are simplifications, however. Hypothetical example: failure to see a flanking line does not by itself exclude a supercell, because some features may be absent or hidden; the rotating updraft remains the defining element. [s4]

Sources consulted

1. NOAA/NWS — struttura di una supercella — Source: NOAA / National Weather Service