One of the many reasons I pay for Carrot is to get the other, more expensive, data sources. If you do stump up, you get access to the Apple Weather API--what once was Dark Sky--as well as Foreca. I've found both of them to be very accurate based on what Carrot reports.

(For what it's worth, I never used the Dark Sky app directly. I've always consumed it via Carrot or a free API key that Dark Sky used to give out for individual developers.)