AIM
4/3/14
7−1−9
Meteorology
SIGMETs/AIRMET text (WA) products are issued
corresponding to the Area Forecast (FA) areas
described in FIG 7−1−5, FIG 7−1−6 and FIG 7−1−7.
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 considered “wide-
spread” 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.
c. SIGMET (WS)
1.
A SIGMET advises of non−convective
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 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.
3.
SIGMETs are identified by an alphabetic
designator from November through Yankee exclud-
ing Sierra and Tango. (Sierra, Tango, and Zulu are
reserved for AIRMET text [WA] products;
G−AIRMETS do not use the Sierra, Tango, or Zulu
designators.) The first issuance of a SIGMET will be
labeled as UWS (Urgent Weather SIGMET).
Subsequent issuances are at the forecaster’s discre-
tion. 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
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 area at 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.