[evan_au (OP)]

The recent heavy snowfall along the East Coast USA (including normally-temperate Florida) has been attributed to a “bomb cyclone”. What is it, and what caused it?

I understood that cyclones/hurricanes were most frequent over warm ocean water, which is rare in the North Atlantic in January?

The best explanation I could find was in Wired.

Or Is this just an example of the politically correct Doublespeak we can expect from the current US administration, where NOAA (the US government weather bureau) is headed by a Trump-appointed administrator who cannot say that sea temperatures are on an upward trend?

[alancalverd]

Apart from the usual genuflection towards climate change, the Wired article is pretty good.

100 degree differences between invaginating air masses, even of they are really only 55.6 degrees in proper units, will cause enormous thermal updrafts as the arctic layer rolls over the slower-moving moist tropical air. Air rushing in to replace the updrafts forms a cyclonic vortex thanks to Coriolis (surface air moving northwards has a strong westerly component) , and when the warm moist air accelerates upwards through the cold front, expands, and reaches the freezing level, it will dump all that tropical moisture as snow.

 
The "bomb" epithet just refers to the speed with which it all happens, and the eastern USA is perfectly suited to small, rapid cyclogenesis as the polar continental air mass has had a long fetch over cold ground whilst the tropical maritime mass has had an equally long trip over a relatively warm sea, so has gained plenty of moisture and lost very little heat.

North Atlantic lows are actually very common, indeed the cause of the dominant 3 - 5 day weather cycle in the British Isles, but they usually form further east, are weaker, and tend to dissipate somewhat before they reach these shores, though Storm Eleanor caused a lot of damage in Ireland last week.  A few sudden snowfalls on the US east coast is entirely normal each winter, but this one just happened to be a bit more vigorous than usual.