LogFAQs > #970062236

So it turns out that both location and sandwich encounter power effect the mass outbreak spawns. If you're looking for a specific outbreak to hunt, go to a location where that Pokemon spawns normally, eat a level 2 encounter power sandwich for its type, and an outbreak of that Pokemon will be more heavily weighted than other Pokemon. Still might need to do some date skipping to see it spawn, especially if you're near multiple "biomes" like snow and river and town all at the same time. But sandwiches and location will cause you to see outbreaks of Pokemon you would pretty much never see otherwise.

Edit: Like I tried to do a hunt for a Cetoddle yesterday, so I went up in the mountains and started date skipping. While I never saw the Cetoddle outbreak, I saw multiple Ice type outbreaks I had never seen before like Frigibax, Snorunt, Cryogonal, and Bergmite. I think if I go a bit south, like the hills between the mountain and Area Zero and pop an Ice sandwich and date skip, I just might see that Cetoddle outbreak.