In Pokémon Black and White, after obtaining the Legend Badge, when entering the gate leading to Route 10, the woman behind the desk will alert the player about the big storm on Route 7.
Tornadus(B) or Thundurus(W) will make the weather more extreme in Route 7 when entering the route. Heading towards the second house on the route will cause an old lady to come out and pull the player in. She will then recite the story of either Tornadus(B) or Thundurus(W).
Upon exiting the house, the player will be approached by the respective Pokémon, which will then fly off and will roam around Unova until being caught or defeated.
Source: Bulbapedia/ Roaming Pokemon