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!
After five generations of Pokemon, Tornadus is actually the first purely Flying-type Pokemon, making it very unique despite having but a single type. It is also a certified legendary Pokemon, so you know it's capable of business. It is available only in Pokemon Black, while its brother is in Pokemon White. Their stats are identical, but there is some difference in their moves and in their types. Both of these legendaries have awesome Sp. Atk, and almost as good Attack and Speed, too! Their defensive stats also aren't too shabby, either, making both Tornadus and its electric brother very solid Pokemon.
Tornadus comes with the Prankster ability, which lets it strike first if it is using a non-damaging attack like Tailwind or Rain Dance. This can be incredibly useful if used in the proper situation, although due to its high Speed, you'll probably find it more useful when helping your partner setup in Double or Triple Battles.
When it comes down to moves, it starts off with Air Cutter for a reliable Flying-type attack with a high chance of a critical hit, Extrasensory for some sweet Psychic-type coverage (definitely a great attack before you have the TM for Psychic post-game!), Revenge to hit with some tough Fighting-type damage (Physical-based), doubling its power if you're hurt on that turn (you go last when using that attack), and then Agility to boost its Speed for some unknown reason. You don't need Agility, because Tornadus is fast as the wind anyway. At level 43, it gets Air Slash, which is a definite staple for Tornadus, hitting hard and having a nice 30% chance of inflicting flinch. It gets Crunch at level 49 to do some heavy Dark-type damage. At level 55, though, it gets one of the more useful moves it can pull off in a Double/Triple Battle: Tailwind, which raises your team's Speed by 50% for the next couple of turns. Since Prankster causes that to activate first no matter what, you can boost your slower Pokemon's Speed right away on that same turn. Hurricane is an amazing Special-based Flying-type attack, although it only has 70% accuracy... unless it is raining, when it is boosted to a perfect 100%. Sadly, it doesn't learn Thunder to complement this. Other great moves it picks up are Dark Pulse at level 73, which is slightly stronger than Crunch, Hammer Arm at level 79, and then Thrash at level 85.
As far as TMs go, Tornadus can learn a lot. Acrobatics is a definite choice to consider for massive, massive power if you don't mind not having an item, but you may find the added chance of flinching that Air Slash has is just fine. It can learn Fly, but that should mostly be for a convenience and you should remove it before taking on the Elite Four, for instance, since Air Slash and Acrobatics are both better. Sludge Bomb and Focus Blast are Special-based options for extra type coverage, while Brick Break is another option for Physical-based attacks (generally preferred over Revenge, since you go first with Brick Break and hit twice while the foe hits just once most of the time). Sky Drop is an option to sweep Pokemon away, but it's rather gimmicky. Rain Dance is a good choice if you want Hurricane to be super accurate, but since Tornadus doesn't get any other benefit from Rain Dance (no Water-type attacks or Thunder), it's not necessarily worth it. Substitute, Taunt, and even possibly Attract are options, since they'll be guaranteed to strike first.
Overall, Tornadus is a Pokemon that greatly benefits from being able to support a partner or a team in Double/Triple Battles, but it will still be a solid fighter for you as you gear up for the final stages of the main storyline.
Thundurus is the other of the roaming legendaries of Unova, available only in Pokemon White. While he and his brother, Tornadus, are both equivalent in their stats (they're exactly even), they differ in their types and learned moves. Thundurus is an Electric/Flying-type that focuses on Electric-type attacks as its primary source of damage. Both have awesome Sp. Atk with very darn high Attack and Speed as well, and even their other stats are very well-balanced, ensuring whichever stormy cloud dude you use will have superior stats to most other non-legendary Pokemon.
Thundurus comes with the Prankster ability, which enables it to use non-damaging attacks before the foe can even strike, essentially turning normal-speed non-damaging attacks into having speed similar to Quick Attack. That can come in handy in a variety of ways, although since Thundurus is so fast anyway, you'll find it isn't quite as obvious in-game as it is when you're battling against other players. Prankster also really comes in handy in Double or Triple Battles to help Thundurus set up something like a Rain Dance, although it is not quite as useful as Tornadus makes it due to their varying movesets.
Speaking of moves, Thundurus comes equipped with Shock Wave, an Electric-type attack with unfailable accuracy, and then Revenge, which is a Physical-based Fighting-type attack hitting for 60 power while also causing you to move last, but if you are damaged this turn, you'll strike with 120 power instead. It also has Heal Block and Agility, although neither are particularly useful in-game, especially since Thundurus is fast enough as-is. It gets Discharge at level 43, which is useful in Double or Triple Battles because it hits multiple targets and has a 30% chance of inflicting paralysis, but it is generally weaker than Thunderbolt is (TM move). Crunch is learned at level 49, which is a solid Dark-type attack, even though it is Physical-based (slightly weaker than its Special-based attacks due to lower Attack generally). Thundurus gets Charge at level 55, but that is fairly gimmicky; Nasty Plot at level 61 is way, way better, and works to boost Thundurus' Sp. Atk by a solid two stages! Use that first, then Discharge or Thunderbolt away. Dark Pulse is learned at level 73 for a better replacement for Crunch, while Hammer Arm and Thrash are both learned later on, at levels 79 and 85 respectively.
As far as TM moves go, you're definitely going to want to slap Thunderbolt on right away, since Shock Wave just isn't strong enough to be used. Whether you want to go with Thunderbolt or Discharge at level 43 is up to you, although generally Thunderbolt is better for in-game purposes. Thunder is an option, especially when comboed with Rain Dance to give it perfect accuracy, although, like his brother, Thundurus learns no Water-type attacks that can combo with Rain Dance, so you are generally better with Thunderbolt unless you're in a long fight. Volt Switch or Charge Beam are both other potential Electric-type options, each having a certain level of strategy they bring to the table, while Wild Charge (in Victory Road) is a Physical-based Electric-type attack, giving Thundurus a chance to attack on both ends of the spectrum. This isn't usually necessary in-game, though, and Thunderbolt is still superior to Wild Charge, but it is at least an option. For both convenience sake and also because it's the strongest Flying-type attack Thundurus learns, you can teach it Fly (HM move) to hit fairly hard, although it forces you to spend a turn in the air (not usually that big of a deal in-game, but it's terrible when battling other players with actual intelligence, because they will just switch). Brick Break is generally more useful than Revenge, because Thundurus' Speed means it will likely attack first; Revenge, while hitting harder (if you're damaged), forces you to go second, so you're going to receive two hits instead of one. The choice is yours, though. Grass Knot, Focus Blast, Sludge Bomb, and Flash Cannon are all other possibilities if you need extra type coverage. Thunder Wave, Swagger, or even Attract can work nicely with Prankster, although you'll find Thundurus would probably rather do damage right away rather than disable the foe. Substitute can help keep it safe if used properly, too. Last, but not least, after you've beaten the game, you have options to some other moves as well: Sludge Wave, Psychic, and U-Turn are all feasible attacking options, while Bulk Up helps build Thundurus' Attack and Defense up.
At the end of the day, Thundurus makes an excellent Electric/Flying-type Pokemon for in-game purposes. It may seem rather lackluster after you've beaten the game, and it may not be the best option for you when battling against other players, but it is really hard to go wrong with its nice spread of different attack possibilities.
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.