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ADDS - AIRMETs Help Page (4 of 4)
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CONVECTIVE SIGMETs are issued in the conterminous U.S. for:
  • Severe surface weather including:
    • surface winds greater than or equal to 50 knots
    • hail at the surface greater than or equal to 3/4 inches in diameter
    • tornadoes
  • Embedded thunderstorms
  • Line of thunderstorms
  • 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. 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.