Pokemon Black and White Pokedex - Thundurus

Thundurus
#148 Unova / #642 National
Type: Electric/Flying
Species classification: Bolt Strike Pokemon
Ability: Prankster – Non-damaging moves get higher priority
Dream World ability: Defiant – User's Attack stat raises two stages for every stat lowered by an opponent (including Attack)
Location found (Black): Trade with White
Location found (White): Roaming Unova (once you receive all eight badges and talk to the old woman at the healing house on Route 7)
Wild hold items: None
Egg groups: Cannot breed
Capture rate: 3
Gender ratio: Male only
Experience at lvl 100: 1,250,000
Base stats: 79 HP / 115 Atk / 70 Def / 125 SAtk / 80 SDef / 111 Spd / 580 Total
Effort values: 3 Special Attack
Evolution family: None

Ah, roaming legendaries. Diamond and Pearl had two roamers,MespritandCresselia, so we're lucky that Black and White only have one each. Thundurus is exclusive to Pokemon White, and you can go after him once you have all eight gym badges.

As you pass through the gate between Opelucid City and Route 10 after receiving your final badge, the woman manning the info desk stops you to tell you about a storm brewing on Route 7 - that's your cue that Thundurus is available for capture.

Fly back to Mistralton City and head north onto Route 7. Whoa! That lady wasn't kidding when she said it was storming. The effects are really cool here – take a moment to soak it up. When you're ready, head to the house east of Dragonspiral Tower (the healing house) and the old woman will come out to talk to you as you approach. She'll invite you in and tell you about the legendary, and when you leave the house, it will appear and then fly away.

The easiest way to encounter Thundurus is to find a spot where you can go back and forth between the border of one route and the next area, and continue going back and forth until you enter the route and it's storming. The spot between Route 6 and Chargestone Cave is a good one, for example.

Before you start though, make sure the Pokemon leading your party either knows the move Mean Look (WatchogandCofagrigusboth learn this via level up) or has the ability Shadow Tag, otherwise chances are your target will flee before you can catch it. Roaming Pokemon are always a good candidate for using your Master Ball, too.