Well, Thunderus/Tornados appear randomly based on the time.
If it's in the morning, the Pokémon may be found in the southern areas.
If it's in the afternoon, the Pokemon may be found in Northern areas.
You can check the boards inbetween cities/routes to see which route a thunderstorm is occuring in.
Both are found at Level 40, Thunderus on White and Tornados on Black.
They're run-arround Legendaries, so it's best to use Quick Ball to catch them.
Dunno if there's anything else to be said D;