And what are the chances of seeing a Mass Outbreak Pokemon in places they are occurring?(please provide source)
They begin occuring after you enter the hall of fame and yes they occur in B/W. Every day there is a new Pokemon and they change everyday. The daily Pokemon will be shown on the gates that connect routes to cities. During winter sometimes the gates do not show any Pokemon. This is because route 8 is frozen over so there is no swarm. The levels of the Pokemon are 15-55 in B/W and 40-55 in B2/W2.
The chances of finding them is 40% in both games for each Pokemon in swarms.
For a list of Pokemon in mass outbreaks in their locations look here: