Before the 1987 Atlantic hurricane season, tropical storm alerts were described as Gale Watches / Warnings, not distinguished from non-tropical storms.
A tropical storm watch (TRA) is issued when tropical storm conditions, including winds from 39 to 73 mph (35 to 64 knots, 63 to 117 km/h) pose a possible threat to a specified coastal area within 48 hours (was 36 hours prior to 2010 hurricane season). Maritime flags indicate this with a single square red flag.
A tropical storm warning (TRW) is issued when tropical storm conditions (as above) are expected in a specified coastal area within 36 hours or less (was 24 hours or less prior to 2010 hurricane season). Maritime flags indicate this with two square red flags.
A hurricane watch (HWA) is issued for a specified area for which a hurricane or a hurricane-related hazard is a possible threat within 48 hours (was 36 hours prior to 2010 hurricane season). Maritime flags indicate this with a single square red flag with a black square in the middle.
The purpose of a hurricane watch is to inform families to obtain supplies, secure their homes, and be prepared to evacuate.
A Hurricane Warning
(HWW) is issued when a hurricane with sustained winds of 74 mph (65 knots, 118 km/h) or higher is expected in a specified coastal area in 36 hours or less (was 24 hours or less prior to 2010 hurricane season). Maritime flags indicate this with two square red flags with a black square in the middle of each.
A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continues, even though the winds may have subsided below hurricane intensity.
Where the intensity or track of a forecast cyclone are uncertain (such as a tropical storm bordering hurricane intensity or on the edge of a track), a Tropical Storm Warning and a Hurricane Watch are often in effect at the same time on parts of the coast. Maritime flags indicate this with two square red flags with a black square in the middle on only one of them.
GALE WARNING: To indicate winds within the range 34 to 47 knots (39-54mph) are forecast for the area.
STORM WARNING: To indicate winds 48 knots(55mph) and above, no matter how high the speed, are forecast for the area. However, if the winds are associated with a tropical cyclone (hurricane), the STORM WARNING indicates that winds within the range 48-63 knots (55-72mph) are forecast.
HURRICANE WARNING: Issued only in connection with a tropical cyclone (hurricane) to indicate that winds 64 knots (74mph) and above are forecast for the area.
NOTE: A "HURRICANE WATCH" is an announcement issued by the National Weather Service via press and television broadcasts whenever a tropical storm or hurricane becomes a threat to a coastal area. The "Hurricane Watch" announcement is not a warning, rather it indicates that the hurricane is near enough that everyone in the area covered by the "Watch" should listen to their radios for subsequent advisories and be ready to take precautionary action in case hurricane warnings are issued.
NOTE: A SPECIAL MARINE WARNING is issued whenever a severe local storm or strong wind of brief duration is imminent and is not covered by existing warnings or advisories. No visual displays will be used in connection with the Special Marine Warning Bulletin; boaters will be able to receive these special warnings by keeping tuned to a NOAA Weather Radio station or to Coast Guard and commercial radio stations that transmit marine weather information.
SMALL CRAFT ADVISORY: An advisory issued by coastal and Great Lakes Weather Forecast Offices (WFO) for areas included in the Coastal Waters Forecast or Nearshore Marine Forecast (NSH) products. Thresholds governing the issuance of small craft advisories are specific to geographic areas. A Small Craft Advisory may also be issued when sea or lake ice exists that could be hazardous to small boats. There is no precise definition of a small craft. Any vessel that may be adversely affected by Small Craft Advisory criteria should be considered a small craft. Other considerations include the experience of the vessel operator, and the type, overall size, and sea worthiness of the vessel. There is no legal definition of "small craft". The Small Craft Advisory is an advisory in Coastal Waters and Nearshore forecasts for sustained winds, frequent gusts, or sea/wave conditions, exceeding defined thresholds specific to geographic areas. A Small Craft Advisory may also be issued when sea or lake ice exists that could be hazardous to small boats.