SIGMET

SIGMET (Significant Meteorological Information) is an unscheduled in-flight weather advisory issued by meteorological watch offices to warn all pilots of severe weather hazards, such as severe turbulence, severe icing, volcanic ash, and widespread dust storms that reduce visibility below three miles. 

Federal Aviation Administration (.gov) Types of SIGMETs

• Non-Convective SIGMET: Issued for widespread hazardous weather (affecting an area of at least 3,000 square miles) not related to thunderstorms. National Weather Service (.gov) 
• Convective SIGMET: Issued specifically for hazardous thunderstorm activity. In the contiguous United States, these are issued hourly and include conditions like tornadoes, heavy hail (over 3\4 inch), embedded thunderstorms, or wind speeds exceeding 50 knots