AIM
8/15/19
7
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1
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10
Meteorology
Hawaiian Islands and a large portion of the Pacific
Ocean. In Alaska, the Alaska Aviation Weather Unit
(AAWU) issues inflight aviation weather advisories
along with the Anchorage CWSU. All heights are
referenced MSL, except in the case of ceilings (CIG)
which indicate AGL.
2.
There are four types of inflight aviation
weather advisories: the SIGMET, the Convective
SIGMET, the AIRMET (text or graphical product),
and the Center Weather Advisory (CWA). All of these
advisories use the same location identifiers (either
VORs, airports, or well
−
known geographic areas) to
describe the hazardous weather areas.
3.
The Severe Weather Watch Bulletins (WWs),
(with associated Alert Messages) (AWW) supple-
ments these Inflight Aviation Weather Advisories.
b. SIGMET (WS)/AIRMET (WA or
G
−
AIRMET)
SIGMETs/AIRMET text (WA) products are issued
corresponding to the Area Forecast (FA) areas
described in FIG 7
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1
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4 and FIG 7
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1
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5. The
maximum forecast period is 4 hours for SIGMETs
and 6 hours for AIRMETs. The G
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AIRMET is issued
over the CONUS every 6 hours, valid at 3
−
hour
increments through 12 hours with optional forecasts
possible during the first 6 hours. The first 6 hours of
the G
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AIRMET correspond to the 6
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hour period of
the AIRMET. SIGMETs and AIRMETs are consid-
ered “widespread” because they must be either
affecting or be forecasted to affect an area of at least
3,000 square miles at any one time. However, 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.
1.
SIGMETs/AIRMET (or G
−
AIRMET) for the
conterminous U.S. (CONUS)
SIGMETs/AIRMET text products for the CONUS
are issued corresponding to the areas in FIG 7
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1
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4.
The maximum forecast period for a CONUS
SIGMET is 4 hours and 6 hours for CONUS
AIRMETs. The G
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AIRMET is issued over the
CONUS every 6 hours, valid at 3
−
hour increments
through 12 hours with optional forecasts possible
during the first 6 hours. The first 6 hours of the
G
−
AIRMET correspond to the 6
−
hour period of the
AIRMET. SIGMETs and AIRMETs are considered
“widespread” because they must be either affecting
or be forecasted to affect an area of at least 3,000
square miles at any one time. However, 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.
Only SIGMETs for the CONUS are for non-convec-
tive weather. The U.S. issues a special category of
SIGMETs for convective weather called Convective
SIGMETs.
2.
SIGMETs/AIRMETs for Alaska
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.