Looking at a recent METAR report for ENQA, I see
W13/S3 at the end:
282050Z AUTO 36017KT 9999NDV BKN013/// 13/10 Q1026 W13/S3
Does anyone know what it means? I couldn't find a description of it online.
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Information about the sea
Weather stations located at sea can report the temperature of the sea water and the height of the waves.
W12/S8means that it is sea water 54ºF [12ºC] with heavy waves. The height of the waves is indicated from 0 (light) to 9 (heavy).
These values are related to sea state, and originate from a helideck. W13/S3 means:
METAR from helidecks, like offshore vessels and fixed platforms, are commonly generated by a helideck monitoring station, using the auto METAR feature. In that case the mention "AUTO" appears in the METAR message, like in your example.
This METAR originates from Troll Alpha, an offshore plateform in North Sea. Helicopters have limits like the vertical motion of a vessel (heave) or offshore platform where they are landing (the helideck operator may define operational limits too).
Aviation agencies recommend providing the sea status in weather reports, including METARs.
Weather agencies have defined how such reporting is done.
Offshore platform helideck, source
Aviation agencies recommendations
AMC2 MET.TR.215(a) Forecasts and other information SPECIFIC INFORMATION FOR HELICOPTER OPERATIONS
(b) Particular mention should be made of the expected surface visibility, [...], sea state and sea-surface temperature, mean sea-level pressure, and the occurrence and expected occurrence of turbulence and icing.
Other information only in accordance with regional air navigation agreements, including:
- Sea-surface temperature and the state of the sea or significant wave height;
- State of the runway.
and at section Sea-surface temperature and the state of the sea or significant wave height
Information on sea-surface temperature and the state of the sea or significant wave height shall be given using the following group: (WTsTs/SS’) or (WTsTs/HHsHsHs) where:
- W is a letter indicator for the temperature of the sea surface;
- TsTs is the sea-surface temperature coded or decoded as indicated in Part A, section 7 (page 21);
- SS’ is the state of the sea as indicated in Code table 3700 where S is the letter designator and S’ is the state of the water surface;
- HHsHsHs is the significant wave height where H is the letter indicator and HsHsHsis the significant wave height in decimetres.
- Sea-surface temperature: 18.7 °C
- State of the sea: Moderate
Table 3700 is found in the WMO Manual on Codes: