A suspected tornado linked to a “supercell thunderstorm” caused damage to about 100 properties in Greater Manchester this week, and a second of these rare storms was confirmed to have hit Morecambe Bay in Lancashire just hours later.
Supercells are rare storms that cause some of the most extreme weather phenomena on the planet. Every thunderstorm has an updraft – a current of air that pulls in moisture from the surrounding area – and a downdraft, which sends rain and hail back to the surface.
But with the right environmental conditions, including strong vertical wind shear (a wind that strongly increases with height), the updraft and downdraft of a storm can begin to twist around and reinforce each other and can keep going for two to six hours.
Updraft speeds in supercell storms can exceed 100mph and are capable of suspending hailstones as large as grapefruit.
In some cases, the supercell can fuel a tornado. “Eventually the rotation may become so strongly focused that a narrow column of violently rotating air forms. If this violently rotating column of air reaches the ground a tornado is born,” the Met Office website says.
It adds: “The tornado is often visible because of the presence of a condensation funnel – a funnel-shaped cloud which forms due to the much-reduced pressure within the tornado vortex. Dust and other debris lofted by the intense winds can also help to make the tornado visible.”
The supercell storm can die down once solar heating decreases (at night), reducing the storm’s supply of warm, moist air close to the surface. It can also weaken if it moves to an area with low wind shear.
Supercells are most common in the Great Plains in the central United States, an area known as Tornado Alley. They are much rarer in the UK, although they have been linked to previous extreme weather events including flash flooding in Northern England and a tornado in the Midlands in 2012.
