Route 7 (Optional)
It starts off as a downpour on the whole route, but quickly grows into a raging storm. You'll want to make your way halfway through the route, over to the house you could heal in (just to the east of the Celestial Tower).
When you try to pass it, an old lady will open the door and invite you inside. She'll then ask if you'd like some soup. Mmmm, soup! Can you really turn down soup from an old lady? I didn't think so! Answer yes and she'll make you some, telling you about one of Unova's legendary Pokemon — Tornadus in Black or Thundurus in White.
Now, as soon as you leave the house, Tornadus or Thundurus will swoop down from the sky, causing some more extreme weather before flying off again. Yikes! And then... then it all clears up.
So what happened, you ask? And how do you get that legendary Pokemon? Well, it's a bit of a pain in the rear to do. It's a lot like the older versions, where you have to chase it down on a route.
Depending on the time of day, you'll find it either on the northern routes or the southern routes. In the mornings, you'll find it in the south, so you'll want to head to Route 5; in the evenings, you'll find it in the north, so check out Route 9. You'll be able to see on the bulletin board if there's an "unusual storm" in the area to also see what route it's on.
When you enter that route, you should see a strong storm like the one you encountered when you were on Route 7. If you don't, exit and then reenter the route until you do. You can't miss it. Then, after it's storming, get into the field with wild Pokemon and you might run into your version's legendary Pokemon. Believe me, you'll know it when you do!
Like Latias/Latios, Mesprit/Cresselia, and Entei/Raikou from the previous generations, Tornadus and Thundurus both run from you during the first turn of the battle. However, any damage you do to them will persist between each encounter, so you should definitely weaken them with a hit or two before you start chasing them down.
Again, the best way to encounter it is to just entering and exiting a route by a gate. You can encounter it once a minute without too much effort, plus it's really easy to spot. However, you may still run into other wild Pokemon, and this may cause it to flee to a different route (you'll notice it's no longer storming if this happens).
Since Tornadus and Thundurus flee so often, it's recommended you set a limit to the number of balls you wish to throw to catch it. Quick Balls or Dusk Balls are your best bet against it, but Ultra Balls are possible to use as well if you're lucky. After you've gotten it weakened to the point you want it at, save your game (do this often anyway), and then promise yourself you'll only encounter it, say, 5 times, tossing a ball at it each time, before you turn off your game and turn it back on. Otherwise, you're just wasting money, because it's not like you have to worry about weakening it again like other legendary Pokemon you fight.