AIM
4/20/23
b.
There are four types of inflight aviation weather advisories: the SIGMET, the Convective SIGMET, the
AIRMET, and the Center Weather Advisory (CWA). All of these advisories use VORs, airports, or well
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known
geographic areas to describe the hazardous weather areas.
c.
The Severe Weather Watch Bulletins (WWs), (with associated Alert Messages) (AWW) supplements these
Inflight Aviation Weather Advisories.
d. SIGMET.
A SIGMET is a concise description of the occurrence or expected occurrence of specified en
route weather phenomena which is expected to affect the safety of aircraft operations.
1.
SIGMETs:
(a)
Are intended for dissemination to all pilots in flight to enhance safety.
(b)
Are issued by the responsible MWO as soon as it is practical to alert operators and aircrews of
hazardous en route conditions.
(c)
Are unscheduled products that are valid for 4 hours; except SIGMETs associated with tropical
cyclones and volcanic ash clouds are valid for 6 hours. Unscheduled updates and corrections are issued as
necessary.
(d)
Use geographical points to describe the hazardous weather areas. These points can reference either
VORs, airports, or latitude
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longitude, depending on SIGMET location. If the total area to be affected during the
forecast period is very large, it could be that, in actuality, only a small portion of this total area would be affected
at any one time.
EXAMPLE
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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.
2.
SIGMETs over the contiguous U.S.:
(a)
Are issued corresponding to the areas described in FIG 7
5. and are only for non
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convective
weather. The U.S. issues a special category of SIGMETs for convective weather called Convective SIGMETs.
(b)
Are identified by an alphabetic designator from November through Yankee, excluding Sierra and
Tango. 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) area (reference FIG 7
5) for
phenomenon moving from the Salt Lake City (SLC) area will be SIGMET Papa 3, if the previous two issuances,
Papa 1 and Papa 2, had been in the SLC area. Note that no two different phenomena across the country can have
the same alphabetic designator at the same time.
(c)
Use location identifiers (either VORs or airports) to describe the hazardous weather areas.
(d)
Are issued when the following phenomena occur or are expected to occur:
(1)
Severe icing not associated with thunderstorms.
(2)
Severe or extreme turbulence or clear air turbulence (CAT) not associated with thunderstorms.
(3)
Widespread dust storms or sandstorms lowering surface visibilities to below 3 miles.
(4)
Volcanic ash.
3.
SIGMETs over Alaska:
(a)
Are issued for the Anchorage FIR including Alaska and nearby coastal waters corresponding to the
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convective weather. The U.S. issues a special category of
SIGMETs for convective weather called Convective SIGMETs.
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1
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Meteorology