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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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  • $\begingroup$ I looked too, even fed it into an automated decoder and couldn't find anything. I want to say it's west thirteen, south three, but I do not know what they are referencing. $\endgroup$
    – RetiredATC
    Aug 29, 2021 at 0:01

2 Answers 2

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From metar-taf.com:

Information about the sea

Weather stations located at sea can report the temperature of the sea water and the height of the waves.
W12/S8 means that it is sea water 54ºF [12ºC] with heavy waves. The height of the waves is indicated from 0 (light) to 9 (heavy).

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  • $\begingroup$ I saw that page and must've missed it, thanks for pointing it out! $\endgroup$
    – Sameer Puri
    Aug 29, 2021 at 0:07
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    $\begingroup$ It should be noted that ENQA is a natural gas platform off the coast of Norway, where such information is quite important. $\endgroup$
    – Bianfable
    Aug 29, 2021 at 9:35
  • $\begingroup$ It's amusing to me that W12 means 54F[12C] and not 12C[54F]. $\endgroup$
    – J...
    Aug 30, 2021 at 11:45
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    $\begingroup$ @J: it may be different elsewhere of course, but in the US our ASOS machines measure the temperature in °F to the degree. They convert this to °C with one decimal place of precision, then round to the nearest °C for the actual METAR/SPECI. And then the pilots take that rounded number and convert back to a now-inaccurate °F figure for the passengers... $\endgroup$
    – randomhead
    Aug 30, 2021 at 12:16
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    $\begingroup$ @randomhead Yes, we understand why people convert units - the point was that 12 means 12, but can also be interpreted as 54 (following conversion) so it seems to make more sense to say W12 = 12C[54F] and not the other way around, ie W12 = 54F[12C]. $\endgroup$
    – J...
    Aug 30, 2021 at 16:25
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These values are related to sea state, and originate from a helideck. W13/S3 means:

  • Sea surface temperature = 13± 0.5°C
  • Waves are slight, that is between 0.5m and 1.25m

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

Offshore platform helideck, source

Aviation agencies recommendations

In EASA Technical requirements and operational procedures for the provision of meteorological services

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.

Weather agencies

From WMO Aerodrome Reports and Forecasts - A Users’ Handbook to the Codes

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.

Example:

  • Sea-surface temperature: 18.7 °C
  • State of the sea: Moderate
  • W19/S4

Table 3700 is found in the WMO Manual on Codes:

WMO Table 3700

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