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Source: Small craft advisory

A small craft advisory is a type of wind warning issued by the National Weather Service in the United States. In Canada a similar warning is issued by Environment Canada. It is issued when winds have reached, or are expected to reach within 12 hours, a speed marginally less than gale force. A Small Craft Advisory may also be issued when sea or lake ice exists that could be hazardous to small boats.
The insignia that denotes a small craft advisory is one red, triangular flag (two such flags, one placed above the other, signify a gale warning).
Inland, this advisory is known as a wind advisory. A lake wind advisory is issued for winds just below this range, because unobstructed winds across the open waters of a lake are normally faster than across land.
The wind speed that triggers the advisory has changed over time. Until the late 1960s, the threshold was 32 to 38 miles per hour (or 28 to 33 knots). At some point, the lower limit was reduced to 23 miles per hour (20 knots). Today, however, most places have standardized on 25 to 38 miles per hour (22 to 33 knots), encompassing the combined ranges of forces 6 and 7 on the Beaufort scale. Winds strong enough to trigger a small craft advisory may be referred to as being advisory-force. Conversely, winds just above this intensity (39–57 miles per hour) are called gale-force, and have a separate associated warning.
Occasionally an informal lesser advisory, known as "small craft exercise caution", is issued for wind speeds lighter than those that call for a small craft advisory. Criteria for this vary in different localities: sometimes a range of 19 to 24 miles per hour (17 to 21 knots) is observed, or in some places 17 to 23 miles per hour (15 to 19 knots) may be used.
The National Weather Service does not specifically identify what constitutes a "small craft". The size of the boat is only part of what a person should consider when venturing out under such a warning. Weight, displacement and hull design are also important factors and an even more important consideration is not only the craft, but the experience of the captain.
The next step above these advisories is a gale warning, known as a high wind warning when issued for inland locations. If the winds are associated with a tropical cyclone, then the next level above a small craft or wind advisory is a tropical storm warning. Occasionally, such bulletins may also be issued for areas above a particular elevation, as wind speeds tend to increase with altitude in the mountains.