Adam Grossman, not giving too much away on the inner-workings of the Dark Sky app:
Weather is chaotic. It's the quintessential non-linear dynamical system. Complicated fluid dynamics combined with a spinning globe, uneven terrain, and energy pumped into the system from the sun makes the weather extremely hard to predict with any accuracy. Meteorologists have spent a century coming up with increasingly sophisticated models and they still can't reliably talk about the weather – even generally – much more than a week in advance.