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Single Cell Thunderstorms (Also known as pulse thunderstorms) Single cell storms typically do not produce severe weather and usually last for 20-30 minutes. Also known as pulse storms, single cell storms seem quite random (perhaps because of our lack of understanding) in the production of brief severe events such as downbursts, hail, some heavy rainfall, and occasional weak tornadoes. image unavailable

The "degree of predictability" is extremely low as forecasters are never quite sure which storm will produce severe weather and from which portion of that storm the severe events will occur. However, the microburst threat to aviation cannot be over-emphasized. image unavailable

Photograph by: NSSL This is a single cell storm, looking east from about 15 miles. The storm was moving east (into the photo). Some of the anvil cloud has been left behind the storm, but the greater portion of the anvil is blowing off in advance of the storm and is not observable from this perspective. (May storm in the Texas Panhandle near Amarillo.)

Multicell Lines (Also known as squall lines) Multicell line storms consist of a line of storms with a continuous, well developed gust front at the leading edge of the line. An approaching multicell line often appears as a dark bank of clouds covering the western horizon. The great number of closely-spaced updraft/downdraft couplets qualifies this complex as multicellular, although storm structure is quite different from that of the multicell cluster storm.

Photograph by: Doswell

Multicell line storms are better known as squall lines, which is the term that we will use from here on. The former name is for positioning squall lines in the thunderstorm spectrum.

Squall lines most frequently produce severe weather near the updraft/downdraft interface at the storm's leading edge. Downburst winds are the main threat, although hail as large as golf balls and gustnadoes can occur. Flash floods occasionally occur when the squall line decelerates or even becomes stationary, with thunderstorms moving parallel to the line and repeatedly across the same area. image unavailable

Multicell Cluster Storms (A cluster of storms in varying stages of development) A multicell cluster consists of a group of cells moving as a single unit, with each cell in a different stage of the thunderstorm life cycle. As the multicell cluster evolves, individual cells take turns at being the most dominant. New cells tend to form along the upwind (typically western or southwestern) edge of the cluster, with mature cells located at the center and dissipating cells found along the downwind (east or northeast) portion of the cluster. Multicell cluster storms frequently look similar to the one pictured in the photograph below, (assuming that low visibilities and/or intervening clouds, trees, or hills do not obscure the view). Looking north from about 10 miles, note the three distinct updraft towers at the left (west) portion of the storm. The heaviest precipitation likely falls beneath the highest cloud top. The right (east) side of the complex is dominated by anvil outflow, moving with the storm from left to right. image unavailable

Photograph by: Moller Multicell severe weather can be of any variety, and generally these storms are more potent than single cell storms, but considerably less so than supercells. Organized multicell storms have the higher severe weather potential, although unorganized multicells, which are simply conglomerates of single cells, can produce pulse storm-like bursts of severe events. image unavailable

Supercell Thunderstorms (Thunderstorms with deep rotating updrafts) The last of the four major storm types is the supercell. We define a supercell as a thunderstorm with a deep rotating updraft (mesocyclone). In fact, the major difference between supercell and multicell storms is the element of rotation in supercells. As we shall see, circumstances keep some supercells from producing tornadoes, even with the presence of a mesocyclone. image unavailable

Photograph by: Moller Even though it is the rarest of storm types, the supercell is the most dangerous because of the extreme weather generated. This storm was producing baseball hail east of Carnegie, Oklahoma, as it was photographed looking east from 30 miles. From right to left (south to north), we note the flanking line, main Cumulonimbus clouds, and downwind anvil above the precipitation area.

The flanking line of the supercell behaves differently than that of the multicell cluster storm, in that updraft elements usually merge into the main rotating updraft and then explode vertically, rather than develop into separate and competing thunderstorm cells. In effect, the flanking updrafts "feed" the supercell updraft, rather than compete with it.

"Image/Text/Data from the University of Illinois WW2010 Project."