Special marine warning

A special marine warning (SAME code SMW) is a warning issued by the U.S. National Weather Service for potentially hazardous marine weather conditions usually of short duration (up to 2 hours) producing sustained marine thunderstorm winds or associated gusts of 34 knots or greater; or hail 3/4 inch or more in diameter; or waterspouts affecting areas included in a coastal waters forecast, a nearshore marine forecast, or a Great Lakes open lakes forecast that is not adequately covered by existing marine warnings. It is also used for short duration mesoscale events such as a strong cold front, gravity wave, squall line, etc., lasting less than 2 hours and producing winds or gusts of 34 knots or greater.^[1]

The following is an example of a Special Marine Warning issued by the National Weather Service on September 4, 2025:

SPECIAL MARINE WARNING
National Weather Service Alexandria VA
315 PM EDT Thu Sep 4 2025

The National Weather Service has issued a

* Special Marine Warning for...
  Coastal waters in and around Alexandria and the Potomac River.

* Until 415 PM EDT.

* At 247 PM EDT, strong thunderstorms were located along a line
  extending from near Gunston Hall to 11 nautical miles west of
  Leesylvania State Park, moving northeast at 20 knots.

  HAZARD…Wind gusts 34 knots or greater.
  SOURCE…Radar indicated.
  IMPACT…Boaters in small craft could be thrown overboard by
          sudden higher winds and waves, capsizing their vessel.

* Locations impacted include…Fort Washington, Tidal Basin,
  Reagan National Airport, Gunston Hall, Mason Neck State Park,
  Woodrow Wilson Bridge, Key Bridge, Occoquan Bay, Fort Hunt,
  National Harbor, Indian Head, and Alexandria.

PRECAUTIONARY/PREPAREDNESS ACTIONS…
Boaters should seek safe harbor immediately until these storms pass.
Wind gusts 34 knots or greater, high waves, dangerous lightning,
and heavy rain are possible with these storms.