National Weather Service United States Department of Commerce

Strong to Severe Thunderstorms May Bring Excessive Rainfall to the Southern U.S. Saturday

Marginally severe thunderstorms capable of strong wind gusts and hail will be possible Saturday across much of central Texas, and during the afternoon across parts of the Southeast U.S. Heavy rain from thunderstorms may bring isolated flash and urban flooding, along with new and renewed rises on rivers and streams throughout East Texas and the lower Mississippi Valley. Read More >


WMO Message Structure

2000 Paraphrased Version

Message Components

The format of meteorological messages is:

  • A starting line
  • An abbreviated heading line
  • Text (body of bulletin)
  • An end-of-message signal

Basic Rules and Definitions

  1. The starting line through end-of-message signal is defined as a WMO message.
  2. The abbreviated heading line and text is defined as a bulletin.
  3. There shall be only one bulletin per message.
  4. A "non-scheduled" message shall have the format of an addressed message and follows different rules of form.
  5. The starting line, abbreviated heading and end-of-message signals shall be in alphanumeric form.

This discussion will use International Alphabet No. 5(ASCII). International Telegraph Alphabet No. 2 (BAUDOT) is no longer a common form for data transmission in North America. The BAUDOT was a code form designed for relatively low-speed circuits where perforated paper tape was the common form of information input using 5 or 7 holes (levels) to form characters. These tapes were read by crossing the paper tape over a hole reader for transmission.


The procedures outlined below shall apply to transmission of routine messages on the GTS and most U.S. national circuits.

A.   The starting line of the meteorological message has the form:

[ SOH ] [ cr ] [cr ] [ lf ] nnn


For purposes of this document the following ASCII character notations are provided.

  1. Transmission control characters and coded representations
  2. Start of heading - [ SOH ] (01 - hexadecimal)
  3. End of text - [ ETX ] (03 - hexadecimal)
  4. Format effector characters for meteorological messages
  5. Carriage return [ cr ] (0D - hexadecimal)
  6. Line feed [ lf ] (0A - hexadecimal)
  7. Message sequence number nnn - A queue number for the message, assigned at the time the message was readied for transmission by the switching system.

B.   Abbreviated heading line

The WMO message structure contains a WMO Abbreviated Heading line which provides product identification for purposes of transmission and communication handling. This page explains the components of the abbreviated heading and its place within the overall WMO message.

The format of the WMO Abbreviated Heading Line is:   T1T2A1A2ii   CCCC   YYGGgg   (BBB)

The WMO abbreviated heading is always proceeded by the "format effectors" [ cr ] [ cr ] [ lf ], and constitutes the complete WMO abbreviated heading line. The T1T2A1A2 ii characters are defined by WMO Manual 386. The CCCC is the location identifier of origin of the bulletin. The ( ) signifies an optional group in the heading, as defined below.

The WMO abbreviated heading has two forms. There is the routine WMO message form and the non-routine WMO message form. The WMO routine form is discussed here. The non-routine WMO message form is called an addressed message and is discussed elsewhere.

The WMO abbreviated heading line is sometimes called the "product ID" or more often the "heading". The heading can be decoded by taking each character and, referring to a table in WMO manual 386, to find the definition of what it represents. If the product contains encoded data, most definitions of the first character or a combination of the first two characters will define the WMO code type. The "content" of the WMO bulletin is normally called the text of the message. The WMO "bulletin" enveloped with a starting line and end-of-message signal constitutes the WMO message.

Heading Line Components

[ cr ] [ cr ] [ lf ] T1T2A1A2ii   CCCC   YYGGgg   (BBB)

    3. ii     Used to differentiate two or more bulletins which contain data in the same code, originate from the same geographical area, and have the same originating center. It shall be a number with two digits. The following sets of "ii" numbers shall be used for indicating the bulletins for global, interregional, regional and national distribution. The national distribution may have further definition as in the case of the U.S. National Weather Service.
    4. ii = 01-19 inclusive for global distribution
    5. ii = 20-39 inclusive for regional and interregional distribution
    6. ii = 40-89 inclusive for national and bilaterally agreed distribution
    7. ii = 90-99 reserved
  • CCCC     International four-letter location indicator of the station originating or compiling the bulletin, as agreed bilaterally or multilaterally, and published in WMO Publication No. 9, Volume C, Chapter I, Catalogue of Meteorological Bulletins. Once the bulletin has been originated or compiled, the CCCC must not be changed even if (because of inadequate reception, or for any other reason) the bulletin in question has to be edited at another center. The United States National Weather Service also has some special CCCC practices which are used to further define sources of products within the National Centers for Environmental Prediction (NCEP).
  • YYGGgg     International date-time group
    YY - day of month
    GGgg -:
    • For bulletins containing meteorological reports intended for standard times of observation, the time shall be the standard time of observation in UTC.
    • For aerodrome, route and area (aeronautical) forecasts: the full hour in UTC (the last two digits shall be 00 ) preceding the transmission time.
    • For other forecasts and analyses: standard time of observation in UTC on which forecast or analysis is based
    • For other messages the time shall be the time of compilation in UTC.
  • BBB     An abbreviated heading defined by T1T2 A1A2ii   CCCC YYGGgg shall be used only once. Consequently, if this abbreviated heading has to be used again which contains text for an addition, a correction or an amendment, it is mandatory to add an appropriate BBB indicator, identified by a three-letter indicator which shall be added after the date-time group as: T1T2 A1A2ii   CCCC YYGGgg BBB
        Where; The indicator BBB shall have the following forms:
    • RRX for delayed routine meteorological reports;
    • CCX for corrections to previously relayed reports;
    • AAX for amendments to processed information;
    • Pxx for segmenting a large set of information into several bulletin; where X is an alphabetic character of A through X;
    NOTE: The BBB group all follow the rules outlined in the Guidelines on the Use of the Indicator BBB discussion.

C.   Text

The text of meteorological bulletin by definition

  1. If encoded, the text shall be in one code form
  2. The text of a bulletin shall not contain both "essential" and "additional" data as defined in Resolution 40 (Cg-XII)
  3. The text of a bulletin shall be in alphanumeric or binary representation. It shall start with the following sequence:

    [ cr ] [ cr ] [ lf ]

  4. Each individual meteorological report shall start with [ cr ] [cr ] [ lf ]
  5. Signal No. 22 (3D- Hexadecimal) represented as an equal sign = shall be used as an "end-of-report indicator" signal. This signal shall follow the last figure of each report, with no intervening space.

D.   End-of-message signals

The end-of-message signals shall be in the following form

      [ cr ] [ cr ] [ lf ] [ETX]



[ cr ] [ cr ] [ lf ] 345
[ cr ] [ cr ] [ lf ] SMYG10 [space] LYBM [space] 280000
[ cr ] [ cr ] [ lf ] AAXX [space] 28001
[ cr ] [ cr ] [ lf ] 13131 [space] ..... [space] ..... [space] ..... [space] etc...=
[ cr ] [ cr ] [ lf ] 13272 [space] ..... [space] ..... [space] ..... [space] etc...=
[ cr ] [ cr ] [ lf ] 13333 [space] ..... [space] ..... [space] ..... [space] etc...=
[ cr ] [ cr ] [ lf ] 13462 [space] ..... [space] ..... [space] ..... [space] etc...=
[ cr ] [ cr ] [ lf ] 13586 [space] NIL =
[ cr ] [ cr ] [ lf ] [ETX]