Which in-flight weather advisory would contain information on severe icing not associated with thunderstorms?

The correct choice here is a SIGMET, which is a significant meteorological information product that provides details about hazardous weather conditions that may affect flight safety. In particular, a SIGMET is issued for severe weather phenomena, including severe icing, particularly when it is not associated with thunderstorms.

SIGMETs are crucial for pilots because they focus specifically on weather events that can impact aircraft operations significantly, such as severe turbulence, dust storms, or, in this case, serious icing conditions. This information is vital for pilots planning routes or conducting flights in potentially hazardous weather situations.

AIRMETs, on the other hand, cover less severe weather conditions and are typically intended for general aviation, providing advisories for hazards like moderate icing which they specify may not be as dangerous as what is outlined in a SIGMET. METARs are routine aviation weather reports that provide information on current weather at specific locations but do not offer details on severe weather phenomena. TAFs are terminal aerodrome forecasts that predict weather for airports over a specified period but do not specifically include the severe weather details covered by SIGMETs.

Understanding these distinctions helps pilots navigate and respond to weather-related challenges during flight safely.