ADDS SIGMET Help

Overview

SIGMETs

A SIGMET advises of weather, other than convective activity, that is potentially hazardous to all aircraft. SIGMETs are issued (for the lower 48 states and adjacent coastal waters) for the following weather-impacted reasons:

  • Severe Icing
  • Severe or Extreme Turbulence
  • Dust storms and/or sand storms lowering visibilities to less than three (3) miles
  • 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.

Convective SIGMETs

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.

CWAs

CWAs are advisories issued by the Center Weather Service Units (CWSUs) that are for conditions just below severe criteria. CWAs are issued for:

  • Thunderstorms
  • Turbulence
  • Icing
  • Ceiling & Visibility (IFR)

AIRMETs

An AIRMET advises of weather potentially hazardous to all aircraft but that does not meet SIGMET criteria.

AIRMETs are issued by the National Weather Service's Aviation Weather Center (for the lower 48 states and adjacent coastal waters) for the following weather impacted reasons:

  • Instrument Flight Rules (IFR) or Mountain Obscuration -
    • Ceilings less than 1000 feet and/or visibility less than 3 miles affecting over 50% of the area at one time.
    • Extensive mountain obscuration
  • Turbulence
    • Moderate Turbulence
    • Sustained surface winds of greater than 30 knots at the surface
  • Icing
    • Moderate icing
    • Freezing levels

These AIRMET 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.

AIRMETs are routinely issued for 6 hour periods beginning at 0245 UTC. AIRMETS are also amended as necessary due to changing weather conditions or issuance cancellation of a SIGMET.


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