SIGMET

Definition

SIGMET is a message concerning the occurrence or expected occurrence of certain en-route weather phenomena (icing, cyclone, thunderstorm and turbulence) and other atmospheric phenomena (volcanic ash, radioactive cloud and volcano) that may affect the safety of flight operations, and the development of these phenomena in time and space.

SIGMET Types

By definition, there are three types of SIGMET:

• Volcanic Ash SIGMET (VA SIGMET or WV SIGMET): message concerning the occurrence or expected occurrence of volcanic ash;
• Tropical Cyclone SIGMET (TC SIGMET): message concerning the occurrence or expected occurrence of tropical cyclones;
• SIGMET for any other weather phenomenon (WS SIGMET) that may affect the safety of flight operations, except phenomena related to volcanic ash and tropical cyclones, namely: thunderstorm (TS), turbulence (TURB), icing (ICE), mountain waves (MTW), duststorm (DS), sandstorm (SS) or radioactive cloud (RDOACT CLD)

Structure of a SIGMET

WSBZ21 SBBS 051430
SBBS SIGMET 17 VALID 151930/152330 SBBS - SBBS BRASILIA FIR EMBD TS FCST WI S1724 W05407 - S1737 W05355 - S2044 W05102 - S1648 W04733 - S1557 W04333 - S1538 W04406 - S1320 W04534 - S1200 W04654 - S1020 W04719 - S1017 W04741 - S0944 W04758 - S0937 W04822 - S0950 W04852 - S1013 W04902 - S1031 W05105 - S1211 W05303 - S1258 W05330 - S1434 W05338 - S1643 W05306 - S1724 W05407 TOP FL480 STNR NC=

Header

WSBZ21 SBBS 051430
General SIGMET issued by Brazil, with bulletin number 21. Disseminated by Brasília FIR, on the 5th at 1430z.

The SIGMET header does not usually appear in SIGMET messages, but when present it contains the following information:

• Message Type: 2-letter code indicating the message type. For a general SIGMET, the code is "WS". For a volcanic ash SIGMET, the code is "VA" or "WV". For a tropical cyclone SIGMET, the code is "TC".
• Issuing Station Indicator: 2-letter code identifying the country or territory issuing the SIGMET. The list of country and territory codes can be found in the Glossary.
• Bulletin Identification Number: internal bulletin number. Not relevant for simulation.
• Disseminating FIR: code of the FIR disseminating the SIGMET.
• Date and Time of Issue: date and time at which the SIGMET was issued, in the format DDHHMMz, where DD is the day of the month, HH is the hour in 24-hour format and MM is the minutes.

Message Body

SBBS SIGMET 17 VALID 151930/152330 SBBS - SBBS BRASILIA FIR EMBD TS FCST WI S1724 W05407 - S1737 W05355 - S2044 W05102 - S1648 W04733 - S1557 W04333 - S1538 W04406 - S1320 W04534 - S1200 W04654 - S1020 W04719 - S1017 W04741 - S0944 W04758 - S0937 W04822 - S0950 W04852 - S1013 W04902 - S1031 W05105 - S1211 W05303 - S1258 W05330 - S1434 W05338 - S1643 W05306 - S1724 W05407 TOP FL480 STNR NC=
SIGMET number 17, valid from the 15th at 1930z to the 15th at 2330z, for Brasília FIR, concerning embedded thunderstorms (EMBD TS) forecast (FCST) within (WI) the area bounded by the coordinates given, with top (TOP) at FL480, stationary (STNR) and no significant change expected (NC=).

Location indicator of the reference FIR or CTA

SBBS
Brasília FIR

The first item of the SIGMET is the code of the FIR to which the SIGMET information refers, that is, the FIR that will be affected by the weather phenomenon. In the example, the FIR code is SBBS, which corresponds to Brasília FIR.

Message identification and sequence number

SIGMET 17
SIGMET number: 17

The SIGMET number is a sequence number that is incremented with each new SIGMET issued for the same FIR. In the example, the SIGMET number is 17, indicating that this is the 17th SIGMET issued for Brasília FIR.

Validity period

VALID 151930/152330
Valid from the 15th at 1930z to the 15th at 2330z.

The SIGMET validity period is indicated by a time interval in the format DDHHMM/DDHHMM, where the first DDHHMM indicates the start of validity and the second DDHHMM indicates the end of validity. In the example, the validity period is from the 15th at 1930z to the 15th at 2330z.

Originating Meteorological Centre indicator

SBBS -
Meteorological centre: Brasília FIR

The originating Meteorological Centre is the FIR that made the observation or forecast of the weather phenomenon. In the example, the originating Meteorological Centre is that of Brasília FIR, indicated by the code SBBS.

Location indicator and name for which the information is issued

SBBS BRASILIA FIR
Reference FIR: Brasília FIR

The location indicator and name for which the information is issued is the reference FIR or CTA for which the SIGMET is issued. In the example, the SIGMET is issued for Brasília FIR, indicated by the code SBBS and the name "BRASILIA FIR".

Description of the weather phenomenon

EMBD TS
Weather phenomenon: embedded thunderstorm

Description of the weather phenomenon, using the abbreviations presented in the Glossary.

Type of Information

FCST
Type of information: forecast

Type of information provided. There are two options for this item:

• OBS: information based on observation;
• FCST: information based on forecast.

Location

WI S1724 W05407 - S1737 W05355 - S2044 W05102 - S1648 W04733 - S1557 W04333 - S1538 W04406 - S1320 W04534 - S1200 W04654 - S1020 W04719 - S1017 W04741 - S0944 W04758 - S0937 W04822 - S0950 W04852 - S1013 W04902 - S1031 W05105 - S1211 W05303 - S1258 W05330 - S1434 W05338 - S1643 W05306 - S1724 W05407
Coordinates bounding the area affected by the weather phenomenon.

Location of the weather phenomenon, expressed as a series of latitudes and longitudes in degrees and minutes.

The area affected by the weather phenomenon can be presented in several ways:

1. Point: the area is represented by a point, indicated by a single coordinate;

N2020 W07005
N48 E010
S60 W160
S0530 E16530

2. Reference Position: the area is represented by a reference relative to a point or a line, expressed in terms of cardinal position (N, S, E, W);

3. Polygon: the area is bounded by a series of coordinates forming a polygon;

WI N6030 E02550 – N6055 E02500 – N6050 E02630 – N6030 E02550

4. Line Width: the area is represented by a line and a width;

APRX 50KM WID LINE BTN N64 W017 – N60 W010 – N57 E010

5. FIR/UIR/CTA: the area is the entire referenced FIR, UIR or CTA;

ENTIRE FIR
ENTIRE UIR
ENTIRE FIR/UIR
ENTIRE CTA

6. Circular Area: the area is represented by a circle, indicated by a centre point and a radius.

WI 400KM OF TC CENTRE
WI 250NM OF TC CENTRE
WI 30KM OF N6030 E02550

Level or Altitude

Indication of the altitude of the weather phenomenon, expressed as an altitude or flight level. It can be given in several ways:

1. Altitude or Flight Level: indicates the altitude of the phenomenon or the band in which it occurs (base-top), given in feet or metres, or a flight level indicated by "FL" followed by three digits in hundreds of feet;
2. Top: indicates the altitude of the top of the weather phenomenon, indicated by "TOP" followed by a flight level or altitude;

Movement of the Phenomenon

Indication of the movement or expected movement of the weather phenomenon (direction and speed) in relation to the cardinal, intercardinal or secondary intercardinal points, or stationary.

MOV SE
MOV NNW
MOV E 40KMH
MOV E 20KT
MOV WSW 20KT
STNR

Changes in Intensity

Indication of changes in the intensity of the weather phenomenon, using the following abbreviations:

• INTSF: intensifying;
• WKN: weakening;
• NC: no significant change expected.

Forecast Time

Indication of the forecast time of occurrence of the phenomenon.

FCST AT 2200Z

Forecast Position of the Tropical Cyclone

Forecast position of the centre of the tropical cyclone, indicated by coordinates.

TC CENTRE PSN N1030 E16015
TC CENTRE PSN N1015 E15030 CB

Practical SIGMET Examples