Alarmingly is the “how” form of alarming. You use it to say that the way something is — or the way it is changing — makes people worried or frightened. It often sits in front of words like high, low, fast, common, quiet: “The river is alarmingly high.” You also put it after a verb: “Prices have risen alarmingly.” It is at home in news, reports, and careful talk; it sounds a bit serious, like a headline. Though actually, people also use it with a small smile: “He is alarmingly good at this game” means so good it feels almost unfair, not that anyone is in real danger.