I have never seen a forecast system for in-flight structural icing conditions, which tells you anything that is not obvious from one look at the MSLP chart.
Icing is a highly localised phenomenon, in both space and time. For 5 minutes, no ice. Next 5 mins, you get 1mm. Next 5 mins, you get 5mm. I don't see any weather model ever being any good for this.
If somebody finds a good website for this, I'd like to know.