Outbreak is certainly a subjective term but IMO, the best way to sort of quantify it is (speaking for tornadoes here)
1. Climatology
2. Geographic area
I think...don't quote me on this but I think usually an event which spawns like 3-4+ tornadoes in our region is considered an outbreak (keeping in mind that our climo is very low). Also, let's not forget how small of a geographic area New England represents then the scale which severe weather/tornadoes occur on. I mean we have had 4 F4 tornadoes occur within a relatively small area...Great Barrington, Windsor Locks, CT, Hamden, CT, Worcester, MA. That's actually pretty impressive...when you factor in aerial size. We have also had more than several events produce 4-5+ tornadoes...yes granted most part they're weak ones but compared to our yearly tornado climo that's alot for one event.