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winds, turbulence, flight visibility, icing, etc.,
between pilots and flight watch specialists. Pilots are
encouraged to report good weather as well as bad, and
to confirm expected conditions as well as unexpected
to EFAS facilities.
7−1−6. Inflight Aviation Weather Advisories
a. Background
1. Inflight Aviation Weather Advisories are
forecasts to advise en route aircraft of development of
potentially hazardous weather. All inflight aviation
weather advisories in the conterminous U.S. are
issued by the Aviation Weather Center (AWC) in
Kansas City, Missouri. The Weather Forecast
Office (WFO) in Honolulu issues advisories for the
Hawaiian Islands. In Alaska, the Alaska Aviation
Weather Unit (AAWU) issues inflight aviation
weather advisories. All heights are referenced MSL,
except in the case of ceilings (CIG) which indicate
AGL.
2. There are three types of inflight aviation
weather advisories: the Significant Meteorological
Information (SIGMET), the Convective SIGMET
and the Airmen’s Meteorological Information
(AIRMET). All of these advisories use the same
location identifiers (either VORs, airports, or
well−known geographic areas) to describe the
hazardous weather areas. See FIG 7−1−2 and
FIG 7−1−3. Graphics with improved clarity can be
found in Advisory Circular AC 00−45E, Aviation
Weather Services, which is available on the following
web site: http://www.faa.gov/avr/afs/afs400.
3. Two other weather products supplement
these Inflight Aviation Weather Advisories:
(a) The Severe Weather Watch Bulletins
(WWs), (with associated Alert Messages)
(AWW), and
(b) The Center Weather Advisories (CWAs).
8/5/04 AIM
Meteorology 7−1−9
FIG 7−1−2
Inflight Advisory Plotting Chart
AIM 8/5/04
7−1−10 Meteorology
FIG 7−1−3
Geographical Areas and Terrain Features
8/5/04 AIM
Meteorology 7−1−11
FIG 7−1−4
Aviation Area Forecasts
FA Locations − Contiguous United States
b. SIGMET (WS)/AIRMET (WA)
SIGMETs/AIRMETs are issued corresponding to the
Area Forecast (FA) areas described in FIG 7−1−4,
FIG 7−1−5 and FIG 7−1−6. The maximum forecast
period is 4 hours for SIGMETs and 6 hours for
AIRMETs. Both advisories 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.
AIM 8/5/04
7−1−12 Meteorology
FIG 7−1−5
Alaska Area Forecast Sectors
FIG 7−1−6
Hawaii Area Forecast Locations
8/5/04 AIM
Meteorology 7−1−13
c. SIGMET (WS)
1. A SIGMET advises of nonconvective
weather that is potentially hazardous to all aircraft.
SIGMETs are unscheduled products that are valid for
4 hours. However, conditions that are associated with
hurricanes are valid for 6 hours. Unscheduled updates
and corrections are issued as necessary. In the
conterminous U.S., 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) Dust storms or sandstorms lowering
surface or inflight 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 excluding
Sierra and Tango. (Sierra, Tango, and Zulu are
reserved for AIRMETs.) 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 phenomena across the
country can have the same alphabetic designator at
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