A tornado vortex signature (TVS) is the Doppler radar velocity signature of a tornado or of a tornado-like circulation aloft, and it is also the label the WSR-88D tornado detection algorithm puts on such a feature. In velocity data it shows as strong values of opposite sign side by side, about one beamwidth apart, that may extend through a considerable depth of the storm.
Why it looks the way it does
The signature appears when the radar beam is wider than the tornado, so the radar cannot resolve the vortex itself. The measured winds come out weaker than the real ones, and the core looks wider than it is.
What remains is a sharp shift between adjacent radar gates, the gate-to-gate shear that marks a velocity couplet at its tightest. When the beam centers on the vortex, a zero velocity value can sit between the two extremes.
How the algorithm flags one
The tornado detection algorithm developed at the National Severe Storms Laboratory (NSSL) for the WSR-88D searches for strong shear between gates that sit side by side in azimuth at the same range. It does not require an algorithm-detected mesocyclone first.
A detection is labeled a TVS if it meets minimum strength and depth rules and its base reaches the lowest elevation angle or a set height above ground. One that meets every rule except the low base is labeled an elevated TVS (ETVS).
Aloft before the ground
NSSL notes that a TVS can appear several kilometers above the ground before a tornado touches down. That makes it a lead-time clue for forecasters, not a confirmation, and it is one reason the depth and base of the signature matter.
What it does not tell you
A TVS is not a confirmed tornado. In the 1998 evaluation of the NSSL algorithm, on a test set of 31 tornadoes, its false alarm ratio was 48 percent. Nonweather targets can trigger detections too, as an NWS Western Region case showed with a moving train in Utah.
Forecasters judge any algorithm marker against the raw velocity data, the storm's structure and range from the radar. Confirmation comes from reliable reports or a tornado debris signature.