At sea, a storm warning is a warning issued by the National Weather Service of the United States when winds between 48 knots (89 km/h, 55 mph) and 63 knots (117 km/h, 73 mph) are occurring or predicted to occur soon. The winds must not be associated with a tropical cyclone. If the winds are associated with a tropical cyclone, a tropical storm warning will be substituted for the storm warning and less severe gale warning.
In US maritime warning flag systems, a red square flag with a black square taking up the middle ninth of the flag is used to indicate a storm warning (the use of two such flags denotes a hurricane force wind warning or a hurricane warning). The same flag as a storm warning is used to indicate a tropical storm warning.
Example
The following is an example of a storm warning issued by the National Weather Service office in Seattle, Washington.
246 WHUS76 KSEW 180451 MWWSEW URGENT - MARINE WEATHER MESSAGE NATIONAL WEATHER SERVICE SEATTLE WA 851 PM PST SUN JAN 17 2010 PZZ150-153-156-170-173-176-181300- /O.UPG.KSEW.GL.W.0087.000000T0000Z-100118T2300Z/ /O.NEW.KSEW.SR.W.0003.100118T0451Z-100118T2000Z/ COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND OUT 10 NM- COASTAL WATERS FROM JAMES ISLAND TO POINT GRENVILLE OUT 10 NM- COASTAL WATERS FROM POINT GRENVILLE TO CAPE SHOALWATER OUT 10 NM- COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND 10 TO 60 NM- COASTAL WATERS FROM JAMES ISLAND TO POINT GRENVILLE 10 TO 60 NM- COASTAL WATERS FROM POINT GRENVILLE TO CAPE SHOALWATER 10 TO 60 NM- 851 PM PST SUN JAN 17 2010 ...STORM WARNING IN EFFECT UNTIL NOON PST MONDAY... THE NATIONAL WEATHER SERVICE IN SEATTLE HAS ISSUED A STORM WARNING...WHICH IS IN EFFECT UNTIL NOON PST MONDAY. THE GALE WARNING IS NO LONGER IN EFFECT. PRECAUTIONARY/PREPAREDNESS ACTIONS... A STORM WARNING MEANS WINDS OF 48 TO 63 KNOTS ARE IMMINENT OR OCCURRING. RECREATIONAL BOATERS SHOULD REMAIN IN PORT OR TAKE SHELTER UNTIL THE WINDS AND WAVES SUBSIDE. COMMERCIAL VESSELS SHOULD PREPARE FOR VERY STRONG WINDS AND DANGEROUS SEA CONDITIONS...AND CONSIDER REMAINING IN PORT OR TAKING SHELTER IN PORT UNTIL WINDS AND WAVES SUBSIDE. && $$