In this example, I would  feel quasi-comfortable issuing a tornado warning before a tight couplet appears on velocity, given the increased Az Shear values that occur about a volume scan before the low level velocity tightens up. The caveat is that the forecaster must fully know the atmospheric setup before focusing purely on Az Shear data.

These images indicate, from top to bottom, the initial enhancement from Az Shear before the velocity couplet really started to tighten up. The following two images are the two subsequent scans.