Weather has been a hobby of mine for years and AFAIK there is no one place that would provide what you're looking for that covers a wide geographical area.
Weather Underground has historical data for specific locations for previous years.
When we are on the road I use this map of
weather advisories to plan travel and hopefully miss any dangerous conditions. It shows all alerts on the map issued by the NWS and makes it easy to spot trouble areas.
This site uses Google Maps to show weather stations that you can check for real-time weather conditions along your planned route or destination. The stations listed are personal weather stations and many will have a webcam so you can "see" the local conditions.
And finally, if you like to watch radar, either the national "big picture" of the entire USA, or pick a specific radar for a location, as well as the ability to see video from storm chaser vehicles, check out
http://www.tornadoalleylive.com/subindex/weather/maps