SIGMET Help

Overview

US SIGMETs

A U.S. 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

If the total area affected during the forecast period is very large, only a small portion of this total area may 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.

US Convective SIGMETs

Convective SIGMETs are issued in the conterminous U.S. if these conditions are occurring or expected to occur:

  • Line of thunderstorms at least 60 miles long with thunderstorms affecting 40% of its length.
  • Area of thunderstorms covering at least 40% of the area concerned and exhibiting a very strong radar reflectivity or a significant satellite or lightning signature.
  • Embedded or severe thunderstorms expected to occur for more than 30 minutes.

Special issuance criteria include:

  • tornado
  • hail greater than or equal to 3/4 inches in diameter
  • wind gusts greater than or equal to 50 knots

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.

International SIGMETs

US SIGMETs cover the contiguous United States and follow US coding standards. Any SIGMET issued outside of the CONUS follows the international coding standard. The aviaiton hazards are similar to the US SIGMETs except convective SIGMETs are treated the same as other hazards. Here is the list of hazards:

  • Thunderstorms
  • Tropical cyclones
  • Turbulence
  • Icing
  • Volcanic ash
  • Dust and sand storms
  • Radiological cloud

International SIGMETs are defined within a specific FIR. Most FIRs are aligned with a country's airspace or a subset of that airspace. Ocean regions also have FIRs where SIGMETs are cwissuedcovered by adjacent countries. The US does issue international SIGMETs for Alaska and for oceanic areas off the east coast of the US, Gulf of Mexico and a large part of the central northern Pacific.

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

If the total area affected during the forecast period is very large, only a small portion of this total area may 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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