When heading out onto coastal waters, large lakes, or other navigable waterways, understanding the hierarchy of marine weather alerts is paramount for safety. A routine marine forecast provides a long-range look at expected conditions, but sudden, localized threats require a more immediate response. The Special Marine Warning (SMW) is a specific type of urgent alert designed to protect mariners from rapidly developing short-term hazards.
Defining the Special Marine Warning
The Special Marine Warning is an urgent alert issued for hazardous conditions that pose an immediate threat over water. It is a geographically specific, short-fused product that signals conditions more dangerous than those addressed by a Small Craft Advisory. This warning is typically issued for severe local weather events not adequately covered by broader, longer-duration alerts like a Gale Warning. The SMW is the marine equivalent of a severe thunderstorm warning on land, tailored for the water environment. It is directed at hazards that can quickly capsize a smaller vessel, make navigation difficult, or endanger life on board. The warning focuses on sudden weather shifts, often originating from convective activity like rapidly moving thunderstorms or squall lines.
Specific Weather Criteria for Issuance
An SMW is issued when specific meteorological conditions are observed or expected to occur over water within a short period. The most common trigger involves strong winds, specifically sustained winds or frequent gusts reaching 34 knots (about 39 miles per hour) or greater. These wind speeds are often associated with the outflow boundaries or cores of sudden thunderstorms.
Another criterion for issuing an SMW is the presence or expectation of a waterspout, a rotating column of air over a body of water. Furthermore, the warning may be issued for large hail, typically three-quarters of an inch or greater in diameter, which can cause damage to a boat and injure occupants. The SMW is also used for intense, short-duration events like a strong cold front or gravity wave that suddenly generates hazardous winds and rough seas.
Duration and Geographic Coverage
A Special Marine Warning is designed to capture transient weather phenomena. These warnings are almost always valid for two hours or less, reflecting the rapid movement and short lifespan of the weather systems they track. This brief duration distinguishes the SMW from other warnings, which can be in effect for 12 hours or more.
The geographic coverage of an SMW is highly localized and focused on specific marine zones, such as coastal waters, nearshore areas, or particular sections of large inland lakes. The warning area is often defined using a polygon shape on a map, precisely targeting the area where the threat is expected to occur.
Required Safety Actions for Mariners
Mariners must take immediate, decisive action upon receiving a Special Marine Warning, as the conditions are considered life-threatening, especially for smaller vessels. The highest priority is to seek safe harbor or shelter immediately, moving toward a protected cove, a sturdy dock, or an inlet if possible. If reaching shelter is not feasible, boaters should prepare the vessel for severe conditions by reducing speed and facing the oncoming waves and wind at a slight angle.
All crew and passengers should don a properly fitted life jacket, and any loose gear on deck, such as anchors, fenders, or fishing equipment, must be secured or stowed below. Visibility can drop rapidly in heavy rain or squalls, so turning on navigation lights is important, even during the day. Monitoring a NOAA Weather Radio or a VHF marine radio for updates is essential to track the storm’s path and duration. If a waterspout is sighted, the vessel should be maneuvered perpendicular to its path to put the maximum distance between the boat and the rotating column of air.