Severe Weather Watcher Handbook

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92  W/NE,  93  N/NE,  94-95  NW/NE   Four classic supercell tornadoes in a moist region. Al four examples show an F2-F3 tornado at the back end of the storm, under the flanking line a few kilometres from the precipitation. In 92 and 95, the flank is advancing as a weak gust front, pushed along by outflow behind the line. In a moist region, the wall cloud will be low, soft and appear larger (95), resulting in a shorter visible tornado (93). There are often scud tags attached to it, and the tornado's position and structure may be harder to see because of rain, low cloud and a mix of debris and condensation in the vortex. 95 is a more distant view of 94 and a few minutes later.

The much tighter tornadic circulation is still largely a mystery. It is known that the column's spin increases as the updraft narrows (the "ice-skater effect") and stretches due to the upward acceleration of air induced by wind shear. Once the storm is fully formed, it acts as a barrier to the flow, enhancing the rotation. A final influence comes when a severe phase leads to an intense downdraft that interacts with the adjacent updraft to create a much smaller vortex within the mesocyclone. (This last "trigger "can be seen from a distance as a burst phase and overshooting top that then collapses). At the same time, the flow at the back of the storm is deflected toward the ground (as the rear-flank downdraft) where it begins to push the flanking line forward and wrap around the mesocyclone, further tightening the rotation. This last step can take from 10-20 minutes after a major updraft dome has flattened again, and will be seen as a speeding-up of wall cloud rotation followed by the emergence of a funnel that may lengthen downward and widen.

This all sounds complicated - and it is!! There are many more questions than answers, and the things you see and relate in your report will greatly help in recognizing these severe processes. Most weak tornadoes occur with an intense phase of a multicell type storm. Supercells, too, can produce brief, weak ones but they are also responsible for almost all strong, large, or long-lasting tornadoes. Tornadoes are rated on the Fujita-scale, from F0 (weak) to F5 (devastating), based on damage assessment. Most tornadoes in Canada are F0-F1.(from 64-179km/hr)

No two are alike

There is no such thing as a typical-looking tornado because so many things contribute to the shape and appearance. Humid conditions cause larger wall clouds, lower cloud bases (short, fat tornado), while drier weather leads to higher bases and relatively thinner, taller tornadoes. The storm severity affects their diameter; terrain determines the type and thickness of debris visible from a distance; and low-level winds and moisture affect shape and the presence or absence of a traditional condensation funnel cloud. It can be said that most weak tornadoes are the slender type while strong ones are generally wider and messier overall.The really big supercell types (such as the Edmonton tornado) can be over a kilometre in diameter, with winds reaching 200-300km/hr in the separate vortices swirling around the perimeter of a rotating wall cloud that has descended right to the ground!

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