Concerning your first question: This is largely a topological problem: If you're surrounded by mountains, the weather behaves distinctly different than on the open plain. In the second case, high wind speeds lead to rapid changes in conditions. For that scenario, such an app might give you some useful information.

I, myself, happen to work in a meteorological company and have an easy indicator: When our meteorologists all take off at the same time, rain is imminent.