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SIGMET (SIGnificant METeorlogical Information)
A SIGMET (SIGnificant METeorlogical Information) advises of weather
potentially hazadous to all aircraft other than convective activity.
SIGMETs are issued (for the lower 48 states and adjacent coastal waters)
for the following weather-impacted reasons:
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Severe Icing
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Severe or Extreme Turbulence
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Duststorms and sandstorms lowering visibilities to less that three (3)
miles
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Volcanic Ash
These SIGMET items are considered to be widespread because they must be
affecting or be forecast to affect an area of at least 3000 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 only a small portion of this total
area would be affected at any one time.
SIGMETs are issued for 6 hour periods for conditions associated with
hurricanes and 4 hours for all other events. If conditions persist beyond
the forecast period, the SIGMET is updated and reissued. Convective SIGMETS
are issued hourly for Thunderstorm-related aviation hazards.
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CONVECTIVE SIGMETs are issued in the conterminous U.S. for:
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Severe surface weather including:
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surface winds greater than or equal to 50 knots
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hail at the surface greater than or equal to 3/4 inches in diameter
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tornadoes
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Embedded thunderstorms
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Line of thunderstorms
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Thunderstorms greater than or equal to VIP level 4 affecting 40% or more
of an area at least 3000 square miles
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 which the forecaster feels is hazardous to all categories of
aircraft.
Convective SIGMET bulletins are issued for the Eastern (E), Central
(C), and Western (W) United States. The areas separate at 87 and 107 degrees
west longitude with sufficent overlap to cover most cases when the phenomenon
crosses the boundaries. Bulletins are issued hourly at Hour+55. The text
of the bulletin consists of either an observation and a forecast or just
a forecast. The forecast is valid for up to 2 hours.
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