He didn't intentionally make it snow. It's was like when young wizards don't know how to control their powers and when they have strong emotions they make something happen. I don't really remember what happened in that scene, but I know he didn't make it snow on purpose.
in this scene he was looking at Lavender who was basically giving him a death glare because Ron had said Hermione's name while he was unconscious. He was feeling uncomfortable confused and maybe a little bad and his emotions made him subconsciously make it snow
