At first glance, it may seem difficult to tell a severe thunderstorm from a "garden variety" thunderstorm. There are, however, a number of visual clues which can be used to gain an idea of a thunderstorm's potential strength and organization, and the environment in which the storm is developing. Many of these visual clues are interrelated, but for discussion's sake, we will classify these clues as upper-level, mid-level, and low-level features of the storm which is being observed.

Upper-Level Features

Most of the upper-level clues are associated with the thunderstorm's anvil. Recall that the anvil is a flat cloud formation at the top of the storm (figure 14). Air (and cloud material) rising in the updraft reaches a point where it begins to slow down. This level is called the equilibrium level. The air (and cloud material) rapidly slows its upward motion after passing the equilibrium level. As the air (and cloud material) spreads out, the anvil is formed.

If the storm you are watching has a vigorous updraft, a small portion of the updraft air will rise higher than the surrounding anvil. This will form a "bubble" of cloud sticking up above the rest of the anvil. The bubble is called an overshooting top (again, see figure 14). Most thunderstorms will have small, short-lived overshooting tops. However, if you observe a storm with a large, dome-like overshooting top that lasts for a fairly long time (more than 10 minutes), chances are good that the storm's updraft is strong enough and persistent enough to produce severe weather.

The anvil itself will also provide clues to the storm's strength and persistence. If the anvil is thick, smooth-edged, and cumuliform (puffy, like the lower part of the storm), then the storm probably has a strong updraft and is a good candidate to produce severe weather. This is also
shown in figure 14. If the anvil is thin, fuzzy, and glaciated (wispy, similar to cirrus clouds), then the updraft is probably not as strong, and the storm is less likely to produce severe weather (figure 15). If the anvil is large and seems to be streaming away from the storm in one particular direction, then there are probably strong upper-level winds in the storm's environment. The storm will be well ventilated, meaning precipitation will probably be blown downstream away from the updraft rather than fall through the updraft.

Mid-Level Features

Most of the mid-level cloud features are associated with the storm's main updraft tower. If the clouds in the main updraft area are sharply outlined with a distinct cauliflower appearance, then the clouds are probably associated with a strong updraft which may produce severe
weather (figure 14). If they have a fuzzy, "mushy" appearance to them, then the updraft probably is not as strong as in figure 15. If the updraft tower itself is vertical (almost perfectly upright), then the storm probably has an updraft strong enough to resist the upper-level winds blowing against it (again, see figure 14). On the other hand, if the updraft leans downwind (usually northeast), then the updraft is weaker (figure 16).

Figure 14: The overshooting top, thick anvil, vertical updraft tower, and "hard" texture to the updraft tower suggest storm severity. Photo - Tim Marshall.

Figure 15: The glaciated anvil and the "soft" updraft tower (behind the towering cumulus in the foreground) suggest a lack of severity. Photo - National Severe Storms Laboratory (NSSL.).

Figure 16: Updraft strength vs. environmental wind speed. Compare the vertical severe storm at left to the tilted updraft of the towering cumulus at right. Photo - Howard Bluestein.

Thunderstorms with good storm scale organization typically have a series of smaller cloud towers to the south or southwest of the main storm tower. These smaller towers are called a flanking line and usually have a stair-step appearance as they build toward the main storm tower. This is shown in figure 17.

Some supercells, as their mesocyclones develop, will show signs of rotation in the updraft tower. You may see striations on the sides of the storm tower. Striations are streaks of cloud material which give the storm tower a "corkscrew" or "barber pole" appearance and strongly suggest rotation (figure 18). A mid-level cloud band may also be apparent. The mid-level cloud band is a ring of cloud material about halfway up the updraft tower encircling the tower like a ring around a planet. This is another sign of possible rotation within the storm.

As a storm increases in size and intensity, it will begin to dominate its local environment (within about 20 miles). If cumulus clouds and other storms 5-15 miles away from the storm of interest dissipate, it may be a sign that the storm of interest is taking control in the local area. Sinking motion on the edges of the storm may be suppressing any nearby storms. All of the instability and energy available locally may be focused into the storm of interest which could result in its continued development.

Figure 17: Flanking line of a supercell. View is to the southeast. Photo - Charles Doswell III.

Figure 18: Striations are evident as the corkscrew-type markings on the side of this supercell updraft tower. View is to the west. Photo - Alan Moller.

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