This is issued by the National Weather Service to alert small boats to sustained (more than 2 hours) hazardous weather or sea conditions. These conditions may be either present or forecasted. The threshold conditions for it are usually sustained winds of 18 knots (21 mph) (less than 18 knots in some dangerous waters) to 33 knots (38 mph) inclusive or hazardous wave conditions (such as 4 feet or greater). In the Great Lakes, this advisory relates to conditions within 5 nautical miles of shore. As a result, these will be only issued in the Nearshore Forecast. Along the coastal regions of the East Coast, Gulf of Mexico, and West Coast, this advisory relates to conditions out to as much as 100 nautical miles of shore (coastal waters). As a result, these will be only issued in the Coastal Marine Forecast. Mariners learning of this advisory are urged to determine immediately the reason by turning their radios to the latest marine broadcast. Decisions as to the degree of the hazard will be left to the boater, based on experience and size and type of boat. There is no legal definition for a "small craft".

"Hieroglyphics? Let me be Pacific
I wanna go down in your South Seas
But I got this notion
That the motion of your ocean means
Small Craft Advisory"

- The Bloodhound Gang, The Bad Touch

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