Which in-flight advisory contains information on severe icing?

The in-flight advisory that contains information on severe icing is SIGMET. SIGMETs are specifically designed to inform pilots about significant weather phenomena that may affect the safety of flight. This includes severe turbulence, widespread dust storms, volcanic ash, and importantly, severe icing conditions. SIGMETs are issued for hazardous weather that affects a large area and poses a significant threat to aircraft safety, which is why they provide critical information on issues like severe icing.

AIRMETs, while they also provide information on weather phenomena, focus on less severe conditions that may affect smaller aircraft or may not pose an immediate threat to safety. METARs are routine aviation weather reports that provide information about current weather conditions at airports, and they do not include advisories about severe weather events like icing. TAFs are forecasts for aviation weather that provide outlooks for weather conditions at airports for specific periods, rather than immediate danger alerts during flight. Thus, for immediate alerts regarding severe icing, SIGMETs are the appropriate source of information.