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6/17/21 

AIM 

Alaska SIGMETs are valid for up to 4 hours, except 
for Volcanic Ash Cloud SIGMETs which are valid for 
up to 6 hours. Alaska AIRMETs are valid for up to 
8 hours. 

3. 

SIGMETs/AIRMETs for Hawaii and U.S. 

FIRs in the Gulf of Mexico, Caribbean, Western 
Atlantic and Eastern and Central Pacific Oceans 

These SIGMETs are valid for up to 4 hours, except 
SIGMETs for Tropical Cyclones and Volcanic Ash 
Clouds, which are valid for up to 6 hours. AIRMETs 
are issued for the Hawaiian Islands and are valid for 
up to 6 hours. No AIRMETs are issued for U.S. FIRs 
in the the Gulf of Mexico, Caribbean, Western 
Atlantic and Pacific Oceans. 

c.  SIGMET 

A SIGMET advises of weather that is potentially 
hazardous to all aircraft. SIGMETs are unscheduled 
products that are valid for 4 hours. However, 
SIGMETs associated with tropical cyclones and 
volcanic ash clouds are valid for 6 hours. 
Unscheduled updates and corrections are issued as 
necessary. 

1. 

In the CONUS, SIGMETs are issued when 

the following phenomena occur or are expected to 
occur: 

(a) 

Severe icing not associated with thunder-

storms. 

(b) 

Severe or extreme turbulence or clear air 

turbulence (CAT) not associated with thunderstorms. 

(c) 

Widespread dust storms or sandstorms 

lowering surface visibilities to below 3 miles. 

(d) 

Volcanic ash. 

2. 

In Alaska and Hawaii, SIGMETs are also 

issued for: 

(a) 

Tornadoes. 

(b) 

Lines of thunderstorms. 

(c) 

Embedded thunderstorms. 

(d) 

Hail greater than or equal to 

3

/

4

 inch in 

diameter. 

3. 

SIGMETs are identified by an alphabetic 

designator from November through Yankee exclud-
ing Sierra and Tango. (Sierra, Tango, and Zulu are 
reserved for AIRMET text [WA] products; 
G

AIRMETS do not use the Sierra, Tango, or Zulu 

designators.) The first issuance of a SIGMET will be 
labeled as UWS (Urgent Weather SIGMET). 
Subsequent issuances are at the forecaster’s discre-
tion. Issuance for the same phenomenon will be 
sequentially numbered, using the original designator 
until the phenomenon ends. For example, the first 
issuance in the Chicago (CHI) FA area for 
phenomenon moving from the Salt Lake City (SLC) 
FA area will be SIGMET Papa 3, if the previous two 
issuances, Papa 1 and Papa 2, had been in the SLC FA 
area. Note that no two different phenomena across the 
country can have the same alphabetic designator at 
the same time. 

EXAMPLE

 

Example of a SIGMET: 

BOSR WS 050600 

SIGMET ROMEO 2 VALID UNTIL 051000 

ME NH VT 

FROM CAR TO YSJ TO CON TO MPV TO CAR 

OCNL SEV TURB BLW 080 EXP DUE TO STG NWLY 

FLOW. CONDS CONTG BYD 1000Z. 

d.  Convective SIGMET (WST) 

1. 

Convective SIGMETs are issued in the 

conterminous U.S. for any of the following: 

(a) 

Severe thunderstorm due to: 

(1) 

Surface winds greater than or equal to 

50 knots. 

(2) 

Hail at the surface greater than or equal 

to 

3

/

4

 inches in diameter. 

(3) 

Tornadoes. 

(b) 

Embedded thunderstorms. 

(c) 

A line of thunderstorms. 

(d) 

Thunderstorms producing precipitation 

greater than or equal to heavy precipitation affecting 
40 percent or more of an area at least 3,000 square 
miles. 

2. 

Any convective SIGMET implies severe or 

greater turbulence, severe icing, and low

level wind 

shear. A convective SIGMET may be issued for any 
convective situation that the forecaster feels is 
hazardous to all categories of aircraft. 

3. 

Convective SIGMET bulletins are issued for 

the western (W), central (C), and eastern (E) United 
States. (Convective SIGMETs are not issued for 
Alaska or Hawaii.) The areas are separated at 87 and 
107 degrees west longitude with sufficient overlap to 
cover most cases when the phenomenon crosses the 

Meteorology 

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