SIGMET reporting constructs as defined in ICAO Annex 3.<br /></p><p><br /></p><p>SIGMETs report the occurrence and/or expected occurrence of specified en-route weather phenomena which may affect the safety of aircraft operations, and of the development of those phenomena in time and space.  These weather phenomena are reported as impacted regions of airspace.  Only a single SIGMET of a given type (e.g., Thunderstorm) shall be in force within a specific FIR at any given time.<br /></p>