Chapter 26, Advisories 26-8
26.2.4.2.1 Convective SIGMET Routine Issuance Criteria
A Convective SIGMET will be issued when any of the following conditions are occurring , or, in the
judgment of the forecaster, are expected to occur:
• A line of thunderstorms at least 60 mi long with thunderstorms affecting at least 40 percent of its
length.
• An area of active thunderstorms judged to have a significant impact on the safety of aircraft
operations covering at least 40 percent of the area concerned and exhibiting a very strong radar
reflectivity intensity or a significant satellite or lightning signature.
• Embedded or severe thunderstorm(s) expected to occur for more than 30 minutes during the valid
period, regardless of the size of the area.
26.2.4.2.2 Convective SIGMET Special Issuance Criteria
A special Convective SIGMET may be issued when any of the following criteria are occurring or, in the
judgment of the forecaster, are expected to occur for more than 30 minutes of the valid period:
• Tornado, hail greater than or equal to ¾ in (at the surface), or wind gusts greater than or equal to
50 kt (at the surface) are reported.
• Indications of rapidly changing conditions if, in the forecaster’s judgment, they are not sufficiently
described in existing Convective SIGMETs.
Special issuance is not required for a valid Convective SIGMET.
26.2.4.2.3 Convective SIGMET Issuance Time and Valid Period
Convective SIGMET bulletins for the eastern, central, and western regions of the CONUS (see Figure 26-5)
are issued on a scheduled basis, hourly at 55 minutes past the hour. Each bulletin contains all valid
Convective SIGMETs within the region. Convective SIGMETs are valid for two hours or until superseded
by the next hourly issuance. A Convective SIGMET bulletin must be transmitted each hour for each region.
When conditions do not meet or are not expected to meet Convective SIGMET criteria within a region at
the scheduled time of issuance, a “CONVECTIVE SIGMET...NONE” message is transmitted.
Figure 26-5. AWC Convective SIGMET Areas of Responsibility
Chapter 26, Advisories 26-9
26.2.4.2.4 Convective SIGMET Format and Example
Each Convective SIGMET bulletin includes one or more individually numbered Convective SIGMETs for
the region. The content and order of each bulletin is as follows:
• Convective SIGMET series number and region letter (E, W, or C).
• Valid ending time (UTC).
• List of states affected by the phenomena.
• Location of phenomena delineated by high -altitude VOR coordinates covering the affected area
during the SIGMET valid time.
• Phenomena description (e.g., AREA SEV EMBD TS).
• Movement (e.g., MOV FROM 26030KT).
• Cloud top (e.g., TOPS ABV FL450).
• Remarks (e.g., TORNADOES...HAIL TO 2.5 IN...WIND GUSTS TO 70KT POSS).
Note: Tropical cyclone information will be added to the remarks section of the CONUS Convective
SIGMETs when appropriate.
Figure 26-6. Convective SIGMET—Example
Chapter 26, Advisories 26-10
Table 26-2. Decoding a Convective SIGMET for the CONUS
Line Content Description
1 MKC Issuing office (AWC)
C Region (East, Central, or West)
WST Product identifier
221855 Issuance date/time (DDHHMM)
2 CONVECTIVE SIGMET Product type
20 Issuance number
C Region (East, Central, or West)
3 VALID UNTIL 2055Z Valid ending time (UTC)
4 ND SD States/areas affected
5 FROM 90W MOT-GFK-ABR-90W MOT Phenomenon location (high-altitude
VOR coordinates)
6 INTSFYG AREA SEV TS MOVG FROM
24045KT. TOPS ABV FL450. WIND
GUSTS TO 60KTS RPRTD.
TORNADOES…HAIL TO 2 IN… WIND
GUSTS TO 65KTS POSS ND PTN
Phenomenon description,
movement, cloud top, remarks
The Convective SIGMET in Figure 26-6 is decoded as the following:
• (Line 1) Convective SIGMET issued for the central portion of the United States on the 22nd day
of the month at 1855Z.
• (Line 2) This is the 20th Convective SIGMET issued on the 22nd day of the month for the central
United States as indicated by “20C.”
• (Line 3) Valid until 2055Z.
• (Line 4) The affected states are North Dakota and South Dakota.
• (Line 5) From 90 NM west of Minot, ND; to Grand Forks, ND; to Aberdeen, SD; to 90 NM west
of Minot, ND.
• (Line 6) An intensifying area of severe thunderstorms moving from 240° at 45 kt (to the northeast).
Thunderstorm tops above FL450. Wind gusts to 60 kt reported. Tornadoes, hail with diameter of
two inches, and wind gusts to 65 kt possible in the North Dakota portion.
26.2.4.2.5 Convective SIGMET Outlook
Each Convective SIGMET bulletin includes a two- to six-hour outlook at the end of the bulletin. The content
and order of each bulletin is as follows:
• Beginning and ending valid times.
• Location of expected Convective SIGMET issuances delineated by high-altitude VOR coordinates
for the outlook valid time.
Chapter 26, Advisories 26-11
26.2.4.2.6 Convective SIGMET Cancellations
Convective SIGMETs are not cancelled but are superseded by the next Convective SIGMET in the series.
26.2.4.2.7 Convective SIGMET Amendments
Amended Convective SIGMETs are not issued. Instead, a new Convective SIGMET is issued for that
region.
26.2.4.2.8 Convective SIGMET Corrections
Corrections to Convective SIGMETs are issued as necessary. The corrected Convective SIGMET is
identified by a “COR” located at the end of the first line after the issuance UTC date/time.
26.2.5 Outside the CONUS
26.2.5.1 SIGMET Issuance Criteria—Outside the CONUS
U.S. SIGMETs outside the CONUS are issued when any of the following conditions affect or, in the
judgment of the forecaster, are expected to affect an area judged to have a significant impact on the safety
of aircraft operations:
• Thunderstorm—of type below:*
o Obscured (OBSC TS).
o Embedded (EMBD TS).
o Widespread (WDSPR TS).
o Squall line (SQL TS).
o Isolated severe (ISOL SEV TS).
• Severe turbulence (SEV TURB).
• Severe icing (SEV ICE); with freezing rain (SEV ICE (FZRA)).
• Widespread dust storm (WDSPR DS).
• Widespread sandstorm (WDSPR SS).
• Volcanic ash (VA).
• Tropical cyclone (TC).
*Tornado (TDO), funnel cloud ( FC), waterspout ( WTSPT), and heavy hail ( HVY GR) may be used as
further descriptions of the thunderstorm, as necessary.
26.2.5.2 SIGMET Issuance Time and Valid Period—Outside the CONUS
A SIGMET is an unscheduled product issued any time conditions reaching SIGMET criteria are occurring
or expected to occur within a four-hour period. A SIGMET outside the CONUS can have a valid period up
to, but not exceeding, four hours, except for volcanic ash ( VA) and tropical cyclone ( TC), which can be
valid up to six hours. SIGMETs for continuing phenomena will be reissued at least every four (or six) hours
as long as SIGMET conditions continue to occur in the area of responsibility.
Chapter 26, Advisories 26-12
26.2.5.3 SIGMET Format and Example—Outside the CONUS
SIGMETs outside the CONUS contain the following information, related to the specific phenomena and in
the order indicated:
• Phenomenon and its description (e.g., SEV TURB).
• An indication whether the information is observed, using “OBS” and/or “FCST.” The time of
observation will be given in UTC.
• Location of the phenomenon referring to, where possible , the latitude and longitude and FLs
(altitude) covering the affected area during the SIGMET valid time. SIGMETs for volcanic ash
clouds and tropical cyclones contain the positions of the ash cloud, tropical cyclone center, and
radius of convection at the start of the valid time of the SIGMET.
• Movement toward or expected movement using 16 points of the compass, with speed in knots, or
stationary, if appropriate.
• Thunderstorm maximum height as FL.
• Changes in intensity, using, as appropriate, the abbreviations for intensifying (INTSF), weakening
(WKN), or no change (NC).
• Forecast position of the volcanic ash cloud or the center of the tropical cyclone at the end of the
validity period of the SIGMET message.
Figure 26-7. SIGMET Outside the CONUS—Example
Chapter 26, Advisories 26-13
Table 26-3. Decoding a SIGMET Outside of the CONUS
Line Content Description
1 WSPA07 ICAO communication header
PHFO Issuance MWO
010410 Issuance UTC date/time
2 SIGPAT NWS AWIPS communication
header
3 KZAK Area control center
SIGMET Product type
TANGO SIGMET series
2 Issuance number
VALID 010410/010800 Valid period UTC date/time
PHFO Issuance office
4 OAKLAND OCEANIC FIR FIR
FRQ TS OBS AND FCST WI 200NM
N3006 W14012 - N2012 W15016 CB
TOP FL400 MOV W 10KT WKN.
Phenomenon description
The SIGMET in Figure 26-7 is decoded as the following:
• (Line 1) The WMO product header is WSPA07. Issued by the Honolulu MWO on the first day of
the month at 0410 UTC.
• (Line 2) The NWS Advanced Weather Interactive Processing System (AWIPS) communication
header is SIGPAT.
• (Line 3) For the Oakland ARTCC (KZAK). This is the second issuance of SIGMET Tango series,
valid from the 1st day of the month at 0410 UTC until the 1st day of the month at 0800 UTC, issued
by the Honolulu MWO.
• (Line 4) Concerning the Oakland Oceanic FIR, frequent thunderstorms observed and forecast
within 200 NM of 30° and 6 minutes north; 140° and 12 minutes west; to 20° and 12 minutes north,
150° and 16 minutes west, cumulonimbus tops to FL400 moving west at 10 kt, weakening.
26.2.5.4 SIGMETs for Volcanic Ash (VA)—Outside the CONUS
A SIGMET for a volcanic ash cloud is issued for volcanic eruptions. A volcanic eruption is any volcanic
activity, including the emission of volcanic ash, regardless of the eruption’s magnitude. Initial volcanic ash
SIGMETs may be issued based on credible PIREPs in the absence of a VAA but are updated once a VAA
is issued. Volcanic ash SIGMETs will continue to be issued until the ash cloud is no longer occurring or
expected to occur.
SIGMETs for a volcanic ash cloud are valid up to six hours and provide an observed or forecast location of
the ash cloud at the beginning of the SIGMET. A six-hour forecast position for the ash cloud, valid at the
end of the validity period of the SIGMET message, is also included.
Chapter 26, Advisories 26-14
26.2.5.5 SIGMETs for Tropical Cyclone (TC)—Outside the CONUS
SIGMETs for a tropical cyclone (which includes a hurricane, typhoon (western Pacific), and tropical storm)
may be issued for non-frontal synoptic-scale cyclones meeting the following criteria:
• Originates over tropical or subtropical waters with organized convection and definite cyclonic
surface wind circulation.
• Wind speeds reach 35 kt independent of the wind averaging time used by the TCAC.
SIGMETs for tropical cyclones will be valid up to six hours. SIGMETs for tropical cyclones will include
two positions. The first position will be the observed position of the center of the tropical cyclone, taken
from the TCA. The second position will be the forecast position of the center of the tropical cyclone valid
at the end of the SIGMET period, which will coincide with the six-hour forecast position in the TCA.
In addition to the two storm positions, SIGMETs will include associated convection when applicable.
SIGMETs will be reissued at least every six hours while the tropical cyclone winds remain or are expected
to remain above 34 kt.
26.2.5.6 SIGMET Cancellation—Outside the CONUS
SIGMETs are cancelled when the phenomenon is no longer occurring or expected to occur.
26.2.5.7 SIGMET Amendments—Outside the CONUS
SIGMET amendments will not be issued. Instead, the next SIGMET in the series is issued to accomplish
the update. The valid time of the new SIGMET is reset to reflect the new four-hour valid period (six-hour
period for volcanic ash and tropical cyclone SIGMETs).
26.2.5.8 SIGMET Corrections—Outside the CONUS
Corrections to SIGMETs are issued as necessary. This is done by issuing a new SIGMET in the series,
which advances the SIGMET number and cancels the previous SIGMET.
26.2.5.8.1 SIGMET for Volcanic Ash Example—Outside the CONUS
WVNT06 KKCI 082030
TJZS SIGMET FOXTROT 2 VALID 082030/090230 KKCI-
SAN JUAN FIR VA FROM SOUFRIERE HILLS LOC 1642N06210W
VA CLD OBS AT 2030Z WI N1730 W06400 - N1700 W06300 - N1650 W06300 - N1710 W06400
- N1730 W06400. SFC/060. MOV W 15KT. FCST 0230Z VA CLD APRX N1730 W06500 - N1700
W06300 - N1650 W06300 - N1710 W06500 - N1730 W06500.
The ICAO communication header for this product is WVNT06. It is a SIGMET issued by the AWC (KKCI)
in Kansas City, MO, on the eighth day of the month at 2030 UTC. This is the second issuance of SIGMET
series Foxtrot valid from the 8th day of the month at 2030 UTC until the 9th day of the month at 0230 UTC.
Within the San Juan Oceanic FIR, volcanic ash from the Soufriere Hills volcano located at 16°/42 minutes
north, 62 °/10 minutes west. Volcanic ash cloud observed at 2030 UTC within an area bounded by
17°/30 minutes north, 64°/00 minutes west to 17°/00 minutes north, 63°/00 minutes west to 16°/50 minutes
north, 63 °/00 minutes west to 17 °/10 minutes north, 64 °/00 minutes west to 17 °/30 minutes north,
64°/00 minutes west. From the surface to 6,000 ft MSL. Moving to the west at 15 kt. Forecast at 0230 UTC,
volcanic ash cloud located approximately at 17 °/30 minutes north, 65°/00 minutes west to 17°/00 minutes
north, 63 °/00 minutes west to 16 °/50 minutes north, 63 °/00 minutes west to 17 °/10 minutes north,
65°/00 minutes west to 17°/30 minutes north, 65°/00 minutes west.