An International SIGMET (SIGnificant METeorlogical Information) advises of weather potentially hazardous to all aircraft. The purpose of this information is to advise pilots of the occurrence or expected occurrence of en-route weather phenomena which may affect the safety of aircraft operations. Criteria for Domestic and International SIGMETs are similiar, however the format, contractions, and wording used are different. International SIGMETs are issued for oceanic areas Pacific and Atlantic adjacent to the United States by a Meteorological Watch Office (MWO). The National Weather Service has MWOs at Anchorage, AK, Guam Island in the Pacific Ocean, Honolulu, HI, and the Aviation Weather Center in Kansas City, MO.

International SIGMET criteria:

• Thunderstorms
• Tornadoes
• Lines of thunderstorms
• Embedded thunderstorms
• Large areas of thunderstorms
• Large hail
• Tropical cyclone
• Severe icing
• Severe or extreme turbulence
• Duststorms and sandstorms lowering visibilities to less that three (3) miles
• Volcanic Ash

These SIGMET items are considered to be widespread because they must be affecting or be forecast to affect an area of at least 3000 square miles at any one time.

International SIGMETs are issued for 12 hour periods for volcanic ash events, 6 hours for hurricanes and tropical storms and 4 hours for all other criteria. If conditions persist beyond the forecast period, the SIGMET is updated and reissued.