First, a quick recap. Hurricanes – defined by their rotating structure, their low internal pressure and (primarily) their wind speed – are geographical variants of tropical cyclones, which is the generic name for them. As the name suggests, they form over the tropics, build up strength over warm, summer oceanic waters, and miss or slam into parts of the Americas.
Hurricanes Harvey and Maria, for example, were tropical cyclones. This latest storm, however, does not share their characteristics. Not only is it forming at a strange time of year – in the frigid winter – but it's also appearing at an unusually high latitude.
According to meteorologist Ryan Maue, and the Weather Channel’s meteorologist Greg Postel, it’s likely to become, or already is, an extratropical cyclone, a cyclonic storm system with a lower cold core that forms outside of the tropics.
Remember, though, that just because it's cyclonic, it doesn't mean it's a hurricane; it is just a pretty potent storm. Claydon tells us that "you can describe storms as zones of cyclonic low pressure. It's just talking about the way the wind is flowing.”
Yes, December/January cyclones in this part of the world do sometimes occur, but genuine hurricanes are extremely uncommon, and they require unseasonably warm ocean waters or extremely chilly atmospheric temperatures – or both. In this particular situation, various atmospheric wedges have conspired to generate an unexpectedly energetic storm system.
Mashable hints that the temperature gradient required to generate an extremely powerful extratropical cyclone is present right now in the region, but again, it’s unclear as to what this storm will ultimately become by Thursday. In any case, at this stage, then, we can definitely call it a bomb cyclone.
At the very least, it's a phenomenon that will dump a heck of a lot of snow in places that don't normally experience it, which sounds like the result of a wizard casting a particularly powerful spell, or even the emergence of a potentially zombified ice dragon.
Update: Meteorologist Eric Holthaus of Grist has since pointed out that the bomb cyclone is the most rapidly deepening winter storm in this part of the world since at least 1980, and perhaps earlier.