DB, focus on the word "intense", that's the key in this question. Icing can very dangerous, though generally accumulates predictably, and not with intensity. Similarly IMC, or tropical storms, they don't spring up intensely, you know they are there as you enter, and, unless you're walking, they won't engulf you!
I'm not sure about the UK, but squalls in Canada can come and pass in 20 minutes, and do high wind damage not unlike minor tornado damage. A half hour later, calm and sunshine again - they're intense, and very hazardous to planes.