Factors Supporting Strong Tornadoes

Tornadoes can form in many different types of thunderstorms, including in supercells, squall lines, and even tropical cyclones. However, violent tornadoes are most likely to form in supercells. In addition to an environment supportive of supercell-type thunderstorms, it is easier for tornadoes to form when the thunderstorm is forming closer to the ground. Recall from the sounding lesson that the LCL (or Lifted Condensation Level) refers to the height at which clouds begin to condense within thunderstorms. The lower the LCL (and therefore the closer it is to the ground), the more likely tornadoes are to form. LCL values most supportive of tornadoes are less than one kilometer above ground level. From this perspective, low-level and mid-level lapse rates are not as important as they were with large hail as moister low levels will help lower the LCLs but also lessen the lapse rates.

Strong wind shear is preferred to separate updrafts and downdrafts within thunderstorms. Research has shown that a mature rear-flank downdraft is often a key component of large, violent tornadoes. A lot of the factors which support tornadoes also support large hail and a mature supercell will often be producing both at the same time. A strongly rotating updraft is also a key component for strong tornadoes. The ability of the atmosphere to spin is measured by storm relative helicity (SRH). Higher SRH values (above 100 m^2/s^2) are more supportive of the formation of tornadoes.