Garmin GXM 40 - Manual (Page 13)

GXM 40 Owner’s Manual 
XM WX Weather Features
SIGMET
A SIGMET (Significant METeorological Forecast) advises
of weather that is potentially hazardous to all aircraft. In the
contiguous United States, the following items are covered:
severe icing, severe or extreme turbulence, volcanic ash,
dust storms, and sandstorms that lower visibility to less than
three statute miles.
A Convective SIGMET is issued for the following
conditions: thunderstorms, isolated severe thunderstorms,
embedded thunderstorms, hail at the surface, and tornadoes.
A SIGMET is widespread and must affect or be forecast to
affect an area of at least 3,000 square miles. SIGMETs are
displayed as a yellow-dashed line. To view SIGMET data,
select the SIGMET using the map pointer.
SIGMET Detailed Information
SIGMET Selected With Map Pointer
SIGMET Detailed Information
SIGMET Selected With Map Pointer