Pilot's Tip of the Week

Convective SIGMETs

Featuring Scott Dennstaedt

Subscriber question:

 "I noticed the other day on a flight that I had to navigate around some thunderstorms, but I wasn't told about any convective SIGMETs during my weather brief. Did my briefer overlook something?" - Dave G.
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Convective Sigmets


"While it is critical for a pilot to avoid deep, moist convection, not all thunderstorms will fall within the confines of a convective SIGMET.

A single cell pulse thunderstorm is very easy to spot in the distance and maneuver around while in flight. However, when thunderstorms become embedded, severe or are dense in coverage within an area or along a line, the Aviation Weather Center (AWC) will protect this airspace by issuing one or more convective SIGMETs.

Consequently, convective SIGMETs represent more of a NOWcast rather than a forecast. Once thunderstorms have developed, a dedicated forecaster on each shift at the Aviation Weather Center has the responsibility to subjectively determine if the area or line of active thunderstorms represents a significant impact to aviation. This Aviation Weather Center forecaster must consider these minimum criteria:

  • 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 at least 40-percent of the area.
  • Embedded or severe thunderstorm(s) expected to occur more than 30 minutes during the valid period regardless of the size of the area.

Convective SIGMETs are issued on an hourly basis at 55 minutes past the hour and are valid for a two hour period. They are made available on the Aviation Digital Data Service (ADDS) website and are depicted as a red polygon. They may also be depicted as a small circle for an isolated embedded or severe thunderstorm on the Aviation Digital Data Service AIRMETs/SIGMETs Java tool.

In the end, convective SIGMETs will help you to identify areas of convection you should be sure to avoid."

Online source: http://www.aviationweather.gov/adds/data/airmets/airmets_CB.gif

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