What is a Severe Thunderstorm?

The National Weather Service defines a severe thunderstorm as a thunderstorm that produces winds of at least 58 mph, and/or hail at least 1″ in diameter (the size of a quarter). 

There are different types of severe thunderstorms and Iowa sees them all.

  • Single-cell thunderstorms
    -supercells
  • Multi-cell thunderstorms
    -Derecho
    -Bow Echo
    -Squall Line
    -QLCS (Quasi-Linear Convective System)

Single-cell thunderstorms

Most of the thunderstorms that we see in Iowa are single-cell thunderstorms. They are simply one convection cell. These typically last less than 45 minutes in duration and cover a small area such as a few blocks within a city or town.

While small and short in duration, they can still pack a punch. Wind speeds of 30-50 mph are possible as well as small hail that can briefly reach 1″ in diameter. Brief, weak tornadoes are also sometimes possible.

A supercell is a larger and stronger single-cell thunderstorm and also the least common type of thunderstorm. They are unique in that they have a deep and rotating updraft.

Supercell thunderstorm. photo credit: nssl.noaa.gov

Supercells are capable of lasting up to several hours and typically occur in the late afternoon and evening hours. They can stretch from a few counties to a couple hundred miles.

Supercell thunderstorms produce heavy rain which can lead to flash flooding, hail from pea size to baseball size, damaging winds over 60 mph, and sometimes strong tornadoes. The majority of EF-3 or stronger tornadoes come from supercells.

Multi-cell thunderstorms

Multi-cell thunderstorms are multiple convection cells within one larger cluster. They are sometimes referred to as a bow echo, squall line, or QLCS which stands for Quasi-Linear Convective System. Derechos are also stronger and longer-lived multi-cell thunderstorms.

These types of storms are long-lived and usually last several hours. They can travel over 100 miles and in the case of derechos, sometimes over 600 miles.

Multi-cell thunderstorm

Multi-cell thunderstorms can produce hail 1″ in diameter or larger and small tornadoes, but their primary threat is damaging straight-line winds over 60 mph.