Nova powers can be used in Close combat it is a nova not a witchfire type

[Xaric]

A nova power automatically targets and hits all enemy units (including flyers and flying monstrous creatures) within the psychic power's maximum range, regardless of line of sight, being locked in combat, intervening models/terrain and so on Otherwise, a nova is treated like a shooting attack, and indeed will have a profile like a ranged weapon. Only one unit that is within the power's maximum range can attempt to deny the witch.

Now I read the category of witchfire and most of the stuff in nova make the rules to witchfire contradicted for one it said the target point as for nova they auto hit regardless of line of sight, being locked in combat, intervening models/terrain and so on Otherwise also at the bottom it said use the rules for the subtype if it has it in its profile so the power sunburst clearly state's its a Nova not a witchfire so you would follow the nova sub-type category rather then the witchfire rule.

So basically sub-type (nova) > main type (witchfire)

Now where it says treat it like a shooting attack its just stating use the rules as you would when shooting basically like how a flame template works because it automatically hits.

Also think of it this way with say wizards that cast nova's in films or games they are always in the thick of the battle when they cast a nova power also the factor most nova ability's are 6" range.