A tornado debris signature (TDS) is a dual-polarization radar pattern in which a low correlation coefficient sits colocated with a strong low-level velocity couplet and sufficient reflectivity, consistent with a tornado lofting debris into the parent storm.
The three ingredients
A TDS requires three colocated features in the same beam volume. First, reflectivity has to be high enough to indicate meaningful returns, often 40 dBZ or more where a hook or a debris ball has organized. Second, storm-relative velocity has to show a tight rotational couplet at low levels, marking the tornado circulation itself. Third, and diagnostically, the correlation coefficient has to collapse to values well below what real precipitation ever produces, commonly near 0.7 or lower.
The correlation coefficient drop is the key. Rain and hail are populations of similarly shaped hydrometeors, so their CC stays high, generally above 0.95. Debris is a jumble of boards, insulation, dirt, and torn vegetation of every size and orientation. The mix returns power that varies wildly between horizontal and vertical polarizations from pulse to pulse, and CC falls.
How forecasters use it
Warning meteorologists watch the correlation coefficient field beneath every rotating storm during severe events. When CC drops into the TDS range inside the couplet, and reflectivity is present, the warning language shifts to tornado emergency or a confirmed tornado tag. That decision drives higher-tier alerts to broadcast partners and to Wireless Emergency Alerts.
A TDS is also a strength indicator. Weak tornadoes lift dust and small debris that may not produce a resolvable signature. Long-track, high-end tornadoes loft framing lumber, roof material, and vehicles into the storm and can produce a TDS visible through many volume scans, sometimes climbing to heights well above the ground as the debris is pulled up into the updraft.
What it does not prove
A TDS is a debris detector, not a tornado detector. It requires that debris exists to loft, which means the tornado has already caused damage by the time it appears. Tornadoes over open country with no structures to destroy may never produce a resolvable TDS even at intense strength. Small or brief tornadoes at long range from the radar may not fill the beam enough to depress CC.
The signature also drifts upward and downstream. The beam is high above the ground at long ranges, and debris blown by storm-relative winds arrives at a slightly different location than the tornado itself. A persistent TDS aloft after a tornado has lifted can reflect debris still suspended in the updraft, not a continuing surface circulation. Read a TDS alongside ground truth and low-level velocity, not as a standalone verdict.