There are plenty of weather apps, and lots of navigation apps, but there are few apps that allow you to plan a road trip taking current conditions or forecasts into consideration.
Road Trip Weather does that. The US $0.99 app allows you to input your current location or starting point, a mid-point on your trip, and your destination. After some quick calculations, a map will show your route and some color codes will show good weather, bad weather, and weather that is worth noting. If you click on a point on the map, you'll get some details about conditions.
This app is not a navigation app, just a quick way to get an idea what to expect. You can set it for a start time on the current day, the next day, or the following day.
I found the information accurate and a good place to turn if you are planning a road trip. The app points out some issues I'd love to see others tackle. Most navigation apps have no built in weather. Both Garmin and Navigon, for example, do have some bare bones weather data, but I think there is a need for more information in navigation apps. Why can't a severe thunderstorm that crosses an interstate I'm on get noted? How about tornado warnings along my route? Many nav apps include traffic warnings, but in-depth weather warnings don't seem to be widely available.
Road Trip Weather is a nice little app, that serves a good purpose. The app is universal, and works fine on any iOS device and requires iOS 5.0 or later. Some screen shots are below.