Attempt to enter Route 10 and you’ll be told that there is a storm raging on Route 7, and we wouldn’t be much of a trainer if we didn’t check it out would we? Fly back to Nimbasa and run east into Route 7.
It’ll soon start absolutely chucking it down but don’t fret! Simply follow the route until you pass the Pokémon Center house, you’ll be invited in to hear an old tale about a legendary Pokémon. Leave and you’ll meet said Pokémon very briefly.
This now means that your version’s roaming legendary is available for capture: Tornadus in Pokemon Black or Thundurus in Pokemon White. Exciting times!
If you've never experienced a roamer, they basically roam from route to route, hence the term. If the roamer is in your current route, there's a chance of encountering them when you step in the grass. It's a bit easier this time, since a heavy rainstorm is present if the roamer is around.
Should you encounter a roamer, they will flee the battle on their first turn, so you need fast Pokemon to damage them or inflict a status condition (like Sleep). Roamers retain damaged HP, so if they flee before you catch them, it's just a matter of finding them again and weakening them until your Poke Balls can snare them.
With that out of the way, it's time to get back to Route 10 and Victory Road. The quickest way is to Fly to Opelucid City.