>Pokemon Mass outbreak (Swarm) Mass Outbreaks, are a feature introduced in Generation II that allows the player to catch Pokémon that are usually rare or otherwise unobtainable.The gates in Unova have a monitor inside that will alert the player where an outbreak is taking place.Unlike previous games, the Pokémon outbreaks can be manipulated by soft resetting the game: turning the console off without saving the game beforehand, but only if the player has not saved their game at all that day. When the game is restarted, there will be a different Pokémon swarming. In Pokemon Black/white all have the level range of 15-55.
Source: Experience and Bulbapedia
Link: http://bulbapedia.bulbagarden.net/wiki/Pokémon_outbreak
//Jofly