Not necessary because the .com sells, a name will sell.
These days (anythinghalfdecent).com goes for about 2-3K.
In this case I do believe you are targeting the mobile market by having a possible site detailing the weather globally or just in a certain country, region ...
Bit more then Reg fee at this time, but both time and development (!) could bring more value to this name and answer your question.
Good for the .com! I hope you get at least a small fraction of that for your .mobi! That would be nice
Don't forget .com is king and probably will be for many years to come. Don't lose sight of OVT, wordtracker, and possible mobile uses when registering names at this stage of the game.