FM-12, or SYNOP, is the WMO (World Meteorological Organization) standard format for surface synoptic observations. These reports are typically issued at 3- or 6-hour intervals and are the primary observation format used at international weather stations.
While the US relies heavily on METARs, global models like the ECMWF rely on the FM-12 SYNOP network to fill in data gaps in remote regions and developing nations. These reports often include more detailed information on “Ground State” (e.g., is the ground frozen or flooded?) than standard aviation reports.
Because SYNOP reports often come only every 6 hours, they can miss the exact timing of a “Bust” in temperature or a sudden wind shift. Forecasters search for “SYNOP to METAR interpolation” tools to try and fill in the 5-hour gaps between international reports to get a better sense of a storm’s evolution.
Unlike METARs, which use abbreviations like “SN” for snow, SYNOP reports use five-digit numerical groups. This makes them language-independent but harder for humans to read without a decoder. Industry pros search for “SYNOP FM-12 decoders” to translate these international “Actuals” into their verification databases.