7110.65R

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 thunderstorms.
(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 Alasknd 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 excluding Sierrnd Tango. (Sierra, Tango, and Zulu are
reserved for AIRMET text [WA] products;
G-AIRMETS do not use the Sierra, Tango, or Zulu

7-1-12

designators.) The first issuance of a SIGMET will be
labeled as UWS (Urgent Weather SIGMET).
Subsequent issuances are at the forecaster"s discretion. 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 phenomencross 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 aret 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