Let me start off with saying that this is not new, although I never really tried asking Google for the weather by saying. "Ok Google, what is the weather in Chicago this coming Friday?"
But it does indeed work both on desktop search and mobile search.
It will fast forward to the day and show you the weather on that day. Will Google be right? Who knows. But it is nice you can ask questions like this.
Yahoo and Bing do not return answers like this but Wolfram Alpha does.
Forum discussion at Google+.