You pretty much can't. You can trade with another player that plays Pokemon Black and trade.
Tornadus is unavailable in Pokemon White. It was an Mystery Gift Event for Tornadus in Pokemon White for Japan though. You'll need to trade for one or wait if they are going to do another event for the USA this time.
At this time, there is no legit way to capture him.
In Pokemon black/white you must have all the badges and catch tornadus/thundurus and get someone to trade the other legendary( ex. if you have white, get someone to trade for a tornadus). Than goto the shrine with both the legendary pokemon in your backpack and landorus will come out of the sky. Save then fight. Source: me got him a long time ago.
When you find a thunderstorm on a route, tornadus will be there. It is a roaming Pokemon, so it changes all the time.
You Don't Get Thunderus In Pokemon Black. You Actually Get Tornadus. To Get Thunderus, You Must Trade With Someone From White Version Who Has Him... (Tornadus, And Thunderus Are The Entei, Raikou, And Suicune Of The Unova Region, Meaning They Run Away Every Time You Enter Battle With Them. So I Suggest Saving Your Master Ball For Them.)
Yep! I caught a shiny Tornadus after about 15 times of it fleeing, and it was shiny from the first time
It all has to do with the right timing. I also think it has to do with the number of Pokemon you've caught...if you've caught more, you have a higher chance of getting a critical capture. On my Black version, I've caught almost all of the Unova Pokemon, so I get critical captures all the time. I hope this helps you!
anytime after beating champ.
Tornadus roams around the region after your first encounter on Route 7, of which you will hear of when the assistant in the gate between Opelucid City and Victory Road, notifies you of gigantic storm in the area. It can run from a battle straight after your first attack, similar to other roaming Pokemon in previous generations, so Mean Look (learned by the Zubat evolutionary line, among others) or the Master Ball (if you didn't waste it on Zekrom/Reshiram) is essential. (or a quick ball, as i used, after luckily hitting him down to one life point in my second encounter) You will know it is nearby when heavy rain starts as it did near the location you first encountered it, as this rain will intensify or decline, indicating your proximaty to Tornadus. It will appear in tall grass.
You meet him in 2 places. The first time you have to capture a Murknow and 4 Spinarak. The next time you have to capture the 3 legendery dogs.
Thunderous, Is The (Raikou, Suicune, And Entei) Of The Unova Region (In White Version). It Runs Away Every Time You Enter Battle With It, So Save Your Master Ball For It. But You Can Only Get It AFTER You Beat The Elite 4. The Only Way To Get Landorus Is To Have Both Thunderous, And Tornadus In Your Party And Talk To The Shrine In Which Landorus Is Held... So You Must Trade With Black Version, To Beat The Entire Game. (Tornadus Is The Raikou, Suicune, And Entei Of Black Version).
There is nowhere the player can capture a wild Bulbasaur in Pokemon Crystal. It can be traded using the Time Capsule.