When should a warning shot (or a "less than lethal" shot) be fired? Short answer: NEVER. Here's the reasoning:
There is NO WAY to use a firearm or an edged weapon that is not lethal force.
Warning shots are never advised and firing a pistol into the air is criminal negligence. Similarly a pocket knife is not a lower level of force.
More: http://www.mississippigunnews.com/when- ... ning-shot/