Been looking for a site like this for a long time and finally found it:
You enter your starting point and date and time as well as your end point (sorry, only two points allowed) and it will show you weather at points along your trip. By changing a starting point or destination, you may be able to avoid rain. It's based on Google maps, so you can also change the route it selects.
I think it may be broken today because it's showing rain on me all day today, and clearly the Corvette gods wouldn't allow that
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I wanted a Corvette my whole life, but I never dreamed of all the wonderful people I would meet because of it!