Have you used Yahoo's own weather app? The design is practically identical, excepting Yahoo's app providing more information.
I have used Yahoo's weather app, and it's very nice.
The designs are similar (thin, white fonts with thin, simple weather symbols over a "live" backdrop), but hardly identical. There's huge differences in the way each app looks and behaves once you move past superficial first impressions of "
hey the font looks similar!".
The iOS 7 Weather.app is something Apple seems to be quite proud of. They specifically showed it off during WWDC, and if you have access to the WWDC session videos this year, the new Weather.app is frequently used as an example of iOS 7's new icon and design philosophies. Not once is it mentioned that Apple talked to Yahoo for design advice. The Weather.app is very much an Apple original.
So let's just put this to rest: Yahoo has nothing to do with the default Weather app's design.
With regards to the OP, Yahoo never got a sneak-
peek(!) at iOS 7.
Why would Apple want to throw around their top-secret mobile OS just so some designers at Yahoo could make a 3rd-party weather app which matched its design? No reason at all.
The similarities between iOS 7's general aesthetics and the Yahoo weather app are purely coincidental. Do you think Apple gave The Iconfactory access to iOS 7 so that they could make Twitterrific 5 along the same aesthetic? No. Of course they didn't. These are simply examples of clever designers following industry trends.