The 'mass outbreak' is a swarm of Pokemon that you would not normally find anywhere else, and are at extremely high frequency rates. You can listen to Professor Oak's talk show for what the mass outbreak of Pokemon is and on what route it is on.
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if by 'swarm' you mean a mass outbreak, then turn on your pokegear to the radio and go to the prof. oak and Mary's talk show. here they wil mention breaking news of an outbreak of a random hard to find Pokemon. the Pokemon will be random and be different every day.
mass outbreaks are according to day, not time. So there is a different mass outbreak every day.
route 203 (mass outbreak).
the places vary. check the radio to find out which Pokemon is appearing where.
A mass outbreak in Pokemon Black and White is when a certain species of Pokemon becomes available temporarily that would not normally be there. Most of these are for the older Pokemon, and only after the National Pokedex is received.