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AIM

10/12/17

7−1−10

Meteorology

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 7−1−4 and FIG 7−1−5. The

maximum forecast period is 4 hours for SIGMETs

and 6 hours for AIRMETs. The G−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 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 7−1−4.

The maximum forecast period for a CONUS

SIGMET is 4 hours and 6 hours for CONUS

AIRMETs. The G−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.
(b) Lines of thunderstorms.
(c) Embedded thunderstorms.
(d) Hail greater than or equal to 

3

/

4

 inch in

diameter.