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X. SUPERCELL VARIATIONS

The supercell discussed in chapter IV is considered a "classic" supercell and serves as a baseline when discussing supercell types. Much has been made recently of "low-precipitation" (LP) and "high-precipitation" (HP) supercells, which might lead some to believe that these are truly different kinds of supercells. In actuality, all supercells are fundamentally the same. They all possess a mesocyclone, they are all long-lived, and all are capable of producing extremely dangerous weather. The only difference in these supercells is the amount of visible precipitation which falls out of the storm. Although variations in precipitation will pose different problems for the NWS radar operators and for spotters, the underlying theme is that "a supercell is a supercell, be it LP, classic, or HP."

Low-Precipitation (LP) Supercells

Low-precipitation supercells are most commonly found on the High Plains near the dryline (sometimes they are called "dryline storms"), but they have been documented in the Upper Midwest as well. LP supercells are difficult to detect on radar. The radar echoes are usually small and weak (low reflectivity values). There may not be evidence of rotation within the storm as detected by conventional radar. Figure 36 shows a diagram of an LP supercell. LP storms are fairly easy to identify visually, however. The typical low-precipitation supercell has a translucent main precipitation area. The main storm tower is usually thin, bell-shaped (flared out close to the cloud base), and has corkscrew-type striations on the sides of the tower. Figure 37 illustrates the typical visual appearance of LP supercells.

Figure 36: Schematic diagram of a low-precipitation supercell. The main precipitation area to the right of the updraft tower is usually very little.

Figure 37: Typical appearance of a low-precipitation supercell. View is to the west. Photo - Steve Tegtmeier.

High-Precipitation (HP) Supercells

High-precipitation supercells can occur in any part of the country. It was once thought that HP supercells only occurred in the Southeast, but they have been documented in the Great Plains as well. HP supercells are easy to detect on radar. They usually have a large radar echo with evidence of rotation within the storm. Figure 38 shows a diagram of an HP supercell. In some high-precipitation supercells, the mesocyclone is displaced to the southeast or east side of the storm. This displacement, coupled with the copious amounts of precipitation falling from the storm make HP supercells difficult for spotters to identify. The heavy precipitation may obscure some (or all) of the "rain-free" base area and obscure the important cloud features that are found in this area. However, HP supercells will usually have striations around the main storm tower and will probably have a beaver's-tail and a mid-level cloud band. Thus, although events under the cloud base will be difficult to discern, ample evidence will exist to confirm that it indeed is a supercell. Figure 39 depicts the visual characteristics of HP supercells. Hybrid Storms.

Figure 38: Schematic diagram of a high-precipitation supercell.

Figure 39: Typical appearance of a high-precipitation supercell. View is to the west. Photo - John McGinley.

Hybrid Storms

It is rare for a storm to fit perfectly into one of the four storm categories (discussed in chapter IV) for its entire life. Rather, it is common for a storm to evolve from one storm type to another. It is also common for a supercell's precipitation rate to increase during its life, resulting in its "evolution" from an LP to an HP supercell. See figure 40 for an example of an LP-to-HP "evolution".

Figure 40: LP-to-HP evolution. (a) LP supercell with developing wall cloud.

Figure 40 (b): HP supercell with updraft base nearly totally obscured. Photos - Alan Moller.

One of the more common evolutions a storm may undergo is a multi-cell-to-supercell transition. Figure 41 contains an example of this transition. As the multicell storm moves along, it may encounter an environment more conducive to supercell formation. One of the updrafts in the cluster may become dominant, and the storm may evolve into a supercell. In fact, numerous supercells with multicell characteristics have been documented!

Figure 41: Multicell-to-supercell evolution. (a) Multicell storm with (at least) 6 updraft elements.

Figure 41 (b): Supercell with one dominant updraft. Photos - Tim Marshall, NSSL.

The multicell characteristics in some supercells may give rise to the cyclic nature of some supercells. A cyclic supercell is a supercell which undergoes the mesocyclone formation-tornado formation-RFD formation process a number of times. In the April 3,1974, tornado outbreak, one supercell produced eight tornadoes as it tracked across Illinois and Indiana. While it is rare for a supercell to produce this many tornadoes, it serves to illustrate the extremely dangerous nature of cyclic supercells. Figure 42 contains an example of a cyclic supercell.

Figure 42: Cyclic supercell. As tornado #1 dissipates, inflow is refocused into the new wall cloud (right). Tornado #2 then develops from the new wall cloud. Photos - Gary Woodall.

Besides the possibility of a storm "evolving" from an LP to an HP storm, it is also possible for a supercell to have both LP and HP characteristics at the same time. Figure 43 shows an example of such a storm. The main precipitation area, to the right of the storm tower, had a thin, translucent appearance. Beneath the base of the storm, however, a heavy precipitation curtain obscured any important cloud features which may have been present. These LP-HP hybrids are yet another example of the continuous spectrum of storm types that may be encountered in the spotting arena.

Figure 43: LP-HP hybrid storm. The "main precipitation area" is translucent, but heavy precipitation is visible beneath the updraft base. Photo Gary Woodall.