Pokemon White/Black [How to catch Thundurus/Tornadus] 

A couple of events need to trigger for you to catch either Thundurus [White] or Tornadus [Black]
After getting 8 badges go through towards route 10, the lady will tell you there is a storm on Route 7
Go to Route 7 and head for the tall grass and you will be stopped by an old lady offering tea, after you head inside she will tell you about the Pokemon with respect to which game your playing
Then if its in the morning go to Route 5, I recorded at night so I went to Route 9 and enter and exit until you see a thunderstrom and head into the grass
This legendary Pokemon will flee after one turn so whittle him down and capture him. Always remember to save and enjoy your new legendary Pokemon