In earlier spotter training material, thunderstorms were classified based on their destructive potential (non-severe, severe, and tornadic). 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. The thunderstorm spectrum is shown in figure 6.

Figure 6: The thunderstorm spectrum. The four main storm categories are listed in the boxes. The bar graphs indicate the frequency and the threat with storms of various updraft strength.

The 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). Figure 7 illustrates the life cycle of a pulse severe storm. 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.

Figure 7: Cloud outlines and radar intensity of a single cell storm (top) and radar intensity of a "pulse" severe storm (bottom)

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