A better way to classify storms is to base the categories on their actual physical characteristics. There is actually a continuous spectrum of thunderstorm types, but there are four broad categories of storms that will be discussed single cell storms, multicell cluster storms, multicell line storms, and supercell storms.
Single Cell Storm
Single cell thunderstorms have lifetimes of 20-30 minutes. They usually are not strong enough to produce severe weather. A true single cell storm is actually quite rare. Even with separate appearing storms in weak vertical wind shear, the gust front of one cell often triggers the growth of another cell some distance away.
Although most single cell storms are non-severe, some single cell storms may produce brief severe weather events. These storms, called pulse severe storms, tend to form in more unstable environments than the non-severe single cell storm. Pulse severe storms have slightly stronger draft speeds and typically produce marginally severe hail and/or brief microbursts. Brief heavy rainfall and occasional weak tornadoes can also be expected (it should be remembered that any thunderstorm is theoretically capable of producing a tornado). Because single cell storms are poorly organized, and because they seem to occur at random times and locations, it is difficult to forecast exactly when and where severe weather will occur.
Multicell Cluster Storm
The multicell cluster is the most common type of thunderstorm. The multicell cluster consists of a group of cells, moving along as one unit, with each cell in a different phase of the thunderstorm life cycle. As the cluster moves along, each cell takes its turn as the dominant cell in the cluster. New cells tend to form at the upwind (usually western or southwestern) edge of the cluster. Mature cells are usually found at the center of the cluster with dissipating cells at the downwind (usually eastern or northeastern) edge of the cluster.
Although each cell in a multicell cluster lasts only about 20 minutes (as with a single cell storm), the multicell cluster itself may persist for several hours. Multicell clusters are usually more intense than single cell storms but are much weaker than supercell storms. Multicell cluster storms can produce heavy rainfall (especially if a number of cells mature over the same area), downbursts (with wind speeds up to about 80 miles an hour), moderate-sized hail (up to about golfball size), and occasional weak tornadoes. Severe weather will tend to occur where updrafts and downdrafts are close to each other (i.e., near the updraft- downdraft interface (UDI) associated with mature cells).
Multicell Line Storm
The multicell line storm (or "squall line," as it is more commonly called) consists of a long line of storms with a continuous, well-developed gust front at the leading edge of the line. The line of storms can be solid, or there can be gaps and breaks in the line. As the gust front moves forward, the cold outflow forces warrn unstable air into the updraft usually at the leading (eastern) edge of the storm, with the heaviest rain and largest hail just behind (to the west of) the updraft. Lighter rain, associated with older cells, often covers a large area behind the active leading edge of the squall line.
Squall lines can produce hail up to about golf ball size, heavy rainfall and weak tornadoes, but they are best known as prolific downburst producers. Occasionally, an extremely strong downburst will accelerate a portion of the squall line ahead of the rest of the line. This produces what is called a bow echo. Bow echoes are easily detected on radar but are difficult (or impossible) to observe visually.
As with multicell cluster storms, squall lines usually produce severe weather near the UDI. Recall that this is near the leading (eastern) edge of the storm. If tornadoes are associated with a squall line, they will usually develop in cells that are just north of a break in the line or in the line's southemmost cell (sometimes called the "anchor cell"). Cells in these locations tend to behave more like supercells than typical squall line cells.
The supercell is a highly organized thunderstorm. Although supercells are rare, they pose an inordinately high threat to life and property. Like the single cell storm, the supercell consists of one main updraft. However, the updraft in a supercell is extremely strong, reaching estimated speeds of 150- 175 miles an hour. The main characteristic which sets the supercell apart from the other thunderstorms we have discussed is the element of rotation. The rotating updraft of a supercell, called a mesocyclone helps the supercell to produce extreme severe weather events, such as giant hail (more than 2 inches in diameter), strong downbursts of 80 miles an hour or more, and strong to violent tornadoes.
Recall that the supercell environment is characterized by high instability, strong winds in the mid and upper atmosphere, and veering of the wind with height in the lowest mile or so. This environment is a contributing factor to the supercell's organization. As precipitation is produced in the updraft, the strong upper level winds literally blow the precipitation downwind. Relatively little precipitation falls back down through the updraft, so the storm can survive for long periods of time with only minor variations in strength. As mentioned earlier, the veering winds with height assist the mesocyclone formation within the supercell.
The leading edge of a supercell's precipitation area is characterized by light rain. Heavier rain falls closer to the updraft with torrential rain and/ or large hail immediately north and east of the main updraft. The area near the main updraft (typically towards the rear of the storm) is the preferred area of severe weather formation.
Last Modified: Mon June 29, 1998