Rain-on-snow, or a Rain-on-Snow Event, is what happens when it rains on fallen snow. Pretty simple, actually. Besides the connotation of the term means something a bit more than some chilly morning drizzle splashing on last night's slush. The term refers to a warm front moving over an area where a cold front was recently, and dropping enough precipitation to significantly alter the underlying snow.
This isn't a common circumstance, for a simple reason. For it to snow, it has to be cold, and for it to rain significantly, it has to be warmer. Thus, for a full rain-on-snow event to occur, an area has to be where it can be under the influence of both arctic and tropical air masses, which limits these events to places between about 40 and 50 degrees latitude, and within a hundred miles or so of an ocean. It also has to be the right time of year, which narrows it down further. It happens in a mild form annually in the Pacific Northwest, and in February 1996, in its catastrophic form. A catastrophic rain-on-snow event requires something of a perfect storm: a large amount of snow has to fall, and then before that snow can naturally melt, a large mass of warm, wet air has to arrive. While researching this writeup, I found that a chart has been made on how much rain needs to melt snow pack. Cold air followed by warm, wet air doesn't happen that often, and when it does, the flooding can be extreme.
During my research for this, I also found out that there is a rain-on-snow event that occurs in the arctic, when rain falls on snow, freezes underneath it, and blocks herbivores from getting to grasses, with disastrous consequences. Events such as this, or just a common mid-latitude rain-on-snow, may be related to larger scale patterns of climate change, but are also an unavoidable fact of living on a planet with continents, oceans, a tilting axis, and a substance that chances between gaseous, liquid and solid forms.