Lesson

WeatherXplore Graphical Airmet / Convective Sigmet

Graphical AIRMET

Graphical AIRMETs or G-AIRMETs are a graphical depiction of an AIRMET (Airman’s Meteorological Advisory).

AIRMETs describe hazardous weather for 6 hour periods.  G-AIRMETs are issued every 6 hours at 0300Z, 0900Z, 1500Z, and 2100Z.

The criteria that will be depicted by G-AIRMETs is:

  • Ceiling and Visibility (IFR)
  • Mountain Obscuration
  • Icing
  • Freezing Level
  • Turbulence
  • Low-Level Wind Shear
  • Strong Surface Winds (sustained winds greater than 30kts occurring or forecast to occur)

Convective SIGMET

Convective SIGMETs are hazardous to all aircraft.  They are issued when any other following conditions are occurring or are expected to occur.

  • A line of thunderstorms at least 60 miles long with thunderstorms affecting at least 40 percent of its length.
  • An area of active thunderstorms affecting at least 3,000 square miles covering least 40 percent of the area concerned and exhibiting a very strong radar reflectivity intensity or a significant satellite or lighting signature.
  • Embedded or severe thunderstorms expected to occur for more than 30 minutes during the valid period regardless of the size of the area. This may also be described as massive cloud layers obscuring the thunderstorms so you cannot see them before flying into them.

A special convective SIGMET may be issued for the following criteria:

  • Tornado, hail greater than or equal to 3/4″ at the surface, or wind gusts greater than or equal to 50kts at the surface are reported.
  • Indications of rapidly changing conditions, if the forecaster feels they are not already described in existing convective SIGMETs.

Convective SIGMETs are valid for 2 hours or until superseded by the next hourly issuance.   An example decoded SIGMET can be found in AC 00-45H or Weather Handbook.

The Scenario

You are expecting to fly from Miami to Jacksonville, FL tomorrow.  You hear tomorrow evening should be nice and cool as there is a front moving in from the northwest bring cooler air with it.  When you hear that, you think “It has been warm and humid for the last few days here in Miami, and with a cold front coming in there may be some weather along that front.”

You wake up the next morning and check weather to see a convective SIGMET has been issued as there is a line of thunderstorms stretching from offshore in the Gulf of Mexico to Atlanta.  Seeing this you notice that the forecast shows the weather improves by 2pm that day after the line moves through and you reschedule your flight for 5pm to give yourself safe margin behind the weather.