So many weather apps, so many to answer, "Will it rain/snow/flood/today?"
Sky Motion has an interesting approach, with a very localized view and a forecast that only looks forward two hours.
The free iOS app updates your personal forecast every 5 minutes, and claims a very high resolution grid (1x1 km) for the best information. It can detect rain, snow, freezing rain, ice pellets and hail, and their forecast technology works in both the US and Canada.
I had good results here in Arizona. We're in our monsoon season, and there is plenty of rain around in often isolated cells. Sky Motion nailed the forecast accurately, and helped me plan my activities. My colleague Steve Sande tried the app in Colorado, and his results were not so good. He says he did better with Dark Sky, a paid app with similar features.
I've also liked using Rain Aware which is US$3.99. Ultimately, no weather app is perfect, and different people at different locations are bound to see different results. Sky Motion is free, and worth a try. It's worked well for me, but I do wish it had push notifications, something that Dark Sky does include.
Sky Motion requires iOS 5 or later and is designed for the iPhone and iPod touch. It will run on the iPad, but the app is not universal.