This module compiled with information courtesy of the official NOAA Storm Spotters Guide.
GLOSSARY As in the other sections, you can click on the glossary image wherever you see it, and the glossary will open in another window. Just close that window when you are ready to continue.

The difference between this storm and a tornadic supercell is that the latter storm's vigorous rotation and surface low pressure field cause a wave to form on the gustfront.
This allows warm, moist surface-based air to feed continually into the updraft and wall cloud. Cold air is "wrapped up" by the strong circulation and does not immediately undercut the wall cloud. Instead, the wall cloud becomes the location of extreme convergence of warm and rain-cooled air.

Schematic of Mesocyclone Effect on Storm's Gust Front

The two previous examples are combined for comparison's sake. If the "undercut" storm is a relatively weak supercell (rather than a severe multicell storm), then the circulation is not strong enough to prevent cold outflow undercutting.


These 3 pictures illustrate the life cycle of the tornado. Tornado circulation develops at the mid levels (about 20,000 ft) in the storm where the storm's updraft and mesocyclone are the strongest. The circulation gradually builds down and up within the storm. At about the same time, a downdraft develops in the mid levels near the back edge of the storm. This downdraft, called a rear flank downdraft (RFD), descends to the ground along with the tornado circulation. Rapidly lowering barometric pressure near the ground is believed to be the primary means of drawing the tornado circulation and RFD down toward the ground. The RFD may reveal itself as a "clear slot" or "bright slot" just to the rear (SW) of the wall cloud. Sometimes a small shelf cloud will form along this slot. Eventually the clear slot and tornado will reach the ground within a few minutes of each other.

The first of three photos of a tornado in the Lake McClellan National Grasslands, TX in May 20, 1999. Warm inflow and cool RFD outflow arrows superimposed to help clarify the low-level airflow around a tornado. Note inflow wrapping into the tornado from the southeast, east, and northeast. We are looking west-northwest from about 5 miles. A clear slot is becoming visible behind the subtle "outflow lowering."
A few minutes later, the clear slot becomes more obvious, as does the weak shelf-like outflow cloud at the base of the flanking line. This is during the tornado's mature stage, and the twister's warm inflow is already being cut off by the left-to-right advancing cold RFD outflow.
Near the end of the mature stage, we have a spectacular view of the clear slot wrapping around the tornado. The gustfront continues to advance and virtually has isolated the tornado from the rest of the storm and has almost ended it's life.

An entire section could be devoted to this alone, and very may well be at some point in the future. But for now, we will take a brief look at it. Remember the one key thing. Look for sustained rotation!
Experienced spotters/chaser are probably aware that a number of features (both natural and man made) can bear a resemblance to a tornado or funnel cloud. Some of these natural features include rain shafts and scud clouds. Some of the man made features include smoke from oil flares and factories. If a suspicious-looking cloud formation is observed, watch it for a minute or two. Look for organized rotation and a vertical or near vertical axis. Here we have a scud/shelf cloud type formation that is getting almost vertical, and could cause confusion to the untrained eye.
In this image we have a backlit rain shaft. However, later this storm was briefly tornadic.
A strong gustnado formed along the leading edge of the Rear Flank Downdraft boundary. Gustnados can be particularly confusing if they occur near or under the updraft base. Always try to look for some sort of rotation at the cloud base when you see a dirt "spinup" on the ground.
A non-tornadic lowering near Ruma, IL. Notice how the bottom is blocked from view by the trees. This was not touching the ground or rotating, however it did prompt tornado sirens to be sounded in Ruma.
Here we have another dense, narrow rain shaft. From a distance this could appear to be a wedge tornado. Again, look for signs of persistent rotation, lofting of debris, and other signs of rotation in the mid levels of the storm, which the storm is not showing here.

The images below are a series that shows a tornado always doesn't follow the rules as far as the typical life cycle. This tornado in South Dakota actually appeared more rope like in it's earlier life, and skipped the dying rope stage at the end. The last picture in the series was how it appeared shortly before dissipating.

Above images contributed by Andrew Revering


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