Lightning disarming spell

This was a spell Dumbledore used on Severus Snape in 1978 or 79 right before Snape revealed that Lily Evans son was the target of Lord Voldemort. It manifested itself as a jet of jagged white light, and Harry Potter thought it was lightning, but it only made Snape's wand fly out of his hand. Apperances;deathly hallows