Thus, while the spectrum is very useful, it is neither perfect nor a final solution. Nevertheless, arrangement of storms within the spectrum is dependent on updraft strength, here represented by different colors; relative frequencies of these updraft strength categories, as indicated by differing lengths on the upper bar graph; and relative threats of the updraft categories, here represented by the lengths on the bottom bar graph.

Thus, while a "strong" updraft is less common than a "weak" updraft, the relative threat to life and property is greater with the "strong" updraft storm. Similarly, "intense" updraft storms are quite rare but inflict a disproportionate amount of damage and personal injury.

The breakdown into single cell, multicell, and supercell covers the major storm types within the spectrum. One "cell" denotes one updraft/downdraft couplet. Thus, there are several updrafts and downdrafts in close proximity with a multicell storm. Multicell storms can be broken down further into the categories of multicell line and multicell cluster storms.

The "intense" updraft storm is almost invariably the supercell, a storm capable of producing the most devastating weather, including violent tornadoes.

With the two multicell storm categories, we have defined four basic storm types from the thunderstorm spectrum. The supercell is always severe, whereas the others can be non-severe or severe. We stress that a "severe" storm is a somewhat arbitrary National Weather Service definition of a storm with one or more of the following elements: 3/4 inch or larger diameter hail, 50 KT downbursts, and tornadoes.

Before reviewing these storms, it is important to emphasize that real thunderstorms do not always fit neatly into the categories we have just described. Research has suggested that the most basic distinction among storm types is between supercells and everything else, the so-called "ordinary" cells.

Non-supercell storms consist of one or more ordinary cells, and we have described three basic ways in which ordinary cells commonly occur: as isolated cells, as clusters of cells, and in lines of cells. Even though real storms can have physical traits that cross the boundaries of these categories, this classification scheme still has considerable value. This is because the intensity and type of weather events produced by a storm tends to be dependent on which category it fits most closely. We should also point out that a given storm may change its type one or more times during its existence.

Single cell storms 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 on 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.

Photo by Moller

The bottom sequence depicts the radar history of the severe pulse storm. Note that the initial radar echo in the pulse storm develops at higher levels than in the non-severe single cell storms. Stronger radar reflectivities aloft with the pulse storm cascade down, resulting in a quick burst of severe weather, possibly hail but more likely downbursts, just before storm dissipation.

Although both non-severe and severe single cell storms typically occur in weakly-sheared, summer atmospheres, the pulse storm usually occurs in a more unstable environment. Its stronger updraft allows for a slightly longer lifetime.

Severe storm radar detection methods such as the Lemon Technique, developed for vertically-sheared environments, will not work very well with pulse storms. The exception to this is the case where the radar operator detects unusually strong mid-level reflectivities prior to updraft collapse.

Multicell cluster storms frequently look similar to this, 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.

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 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.

This low-level, horizontal cross-section depicts a severe multicell storm or marginal supercell where the gustfront typically has moved out ahead of and "undercut" the updraft area and possible wall cloud.

Although the storm might well be severe, tornado production from the updraft/wall cloud area is unlikely.

A multicell cluster storm, the most common of the four basic storm types, evolves as an organized sequence of cells in various stages of development and decay at any given time.

When multicell storms form in environments with winds which veer from southerly to westerly and increase with height, new updraft development usually occurs in the upwind (usually southwest) quadrant of the complex, with older cells decaying in the downwind storm, looking north from about 12 miles. The new development, called the flanking line, is at the left (southwest) side of the complex. The rain-free base disappears beneath the twin towers on the right-hand side of the photo, since precipitation is falling from these "glaciated" thunderstorm cells.

Glaciation refers to the transformation of cloud particles from water droplets to ice crystals. The visual cloud appearance often changes from rock-hard to soft during the glaciation process. The northeastward tilt of the multicell complex indicates the presence of vertical wind shear.

Photo by Doswell

Radar (PPI mode) often reflects the multicell nature of these storms, as seen with the central echo mass and its three light red (in this case VIP 5) cores in this photo. Occasionally, a multicell storm will appear unicellular in a low-level radar scan, but will display several distinct tops when a tilt sequence is used to view the storm in its upper extremities.

The close proximity of updrafts within the multicell cluster storm results in updraft competition for the warm, moist low-level air. Thus, updrafts never attain extremely strong vertical velocities and each has a short life span when compared to a supercell updraft. Naturally, multicell severe weather usually is less intense than that from supercells, but still can be quite potent, with marble to golf ball size hail and 60 to 80 MPH winds that can rearrange your garden furniture is not uncommon..

This illustration portrays a portion of the life cycle of a multicell storm. As cell 1 dissipates at time = 0, cell 2 matures and becomes briefly dominant. Cell 2 drops its heaviest precipitation about 10 minutes later as cell 3 strengthens, and so on.

Thus, severe multicell storms characteristically produce a brief period of hail and/or downburst damage during and immediately after the strongest updraft stage. Later updraft resurgence may or may not result in further damage, leading to a spotty damage pattern.

Note that the development of new cells causes the storm in the diagram to move from right to left with time. However, if the winds in the storm environment are blowing from left to right, it can happen that the storm motion arising from new cell development nearly cancels the motion arising from the environmental winds. Thus, new cells reach maturity over the same location, repeatedly.

This is the "train-echo" pattern of flash flood-producing rainfall, although train echoes also may occur as different multicell thunderstorm complexes moving across an area with a slightly greater time interval. Not having the benefit of radar, it will seem to citizens living in an area receiving repeated, short-term precipitation bursts that the storm is "backing up and moving across again and again." This is a popular but erroneous notion.

A closer view at T = 20 minutes from in the last slide shows that cell 3 still has the highest top, but precipitation is undercutting the updraft in the lower levels. New echo development is occurring aloft in cells 4 and 5 in the flanking line, with only light rain falling from the dissipating cells 1 and 2 on the northeast side of the storm cluster.

The inset shows what the low-level PPI radar presentation might look like. This storm appears to be unicellular but the several distinct echo tops tell us otherwise. Note that the greatest risk of severe weather at this time extends from beneath the heavy precipitation areas of cell 3 (hail and downbursts) into the area of the leading gustfront (downbursts and, on rare occasions, weak gustfront tornadoes or "gustnadoes").

From the backside we watch as a cluster moves away to the east.

The storm was definitely multicellular, although not as "clear-cut" about preferred updraft locations as other multicell storms we have viewed. Again, nature does not always allow us to label and catalog everything neatly!

Concerning storms in northwest flow aloft, it has been observed that the updraft area frequently shifts to the southeast flank, when rain-cooled air keeps warm, southerly winds from providing a continual feed to the northwest flank updrafts. Thus, with this storm it is possible that the leading (southeast flank) updraft area became predominant once heavy precipitation began, with the northwest updraft area no longer benefiting from the "prime" air.

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.

Actually, the distinction between multicell and single cell storms is not nearly as important as that between multicells and supercells.

The multicell flash flood threat can be significant, in fact most flash floods probably occur with multicell complexes. As with all thunderstorms, the threat to the aviation community is quite high.