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Warnings and watches are two levels of alert issued by national weather forecasting bodies to coastal areas threatened by the imminent approach of a tropical cyclone of tropical storm or hurricane intensity. They are notices to the local population and civil authorities to make appropriate preparation for the cyclone, including evacuation of vulnerable areas where necessary. It is important that interests throughout the area of an alert make preparations to protect life and property, and do not disregard it on the strength of the detailed forecast track. Tropical cyclones are not points, and forecasting their track remains an uncertain science.
The following terminology used by the US National Hurricane Center is the model for countries around the North Atlantic and in the Caribbean basin (except Cuba, see regional notes). This is also used for the Pacific coasts of Mexico, Central America, southern California, and Hawaii.
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 36 hours. 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 24 hours or less. 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 36 hours. 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 your home, 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 24 hours or less. 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.
The following alerts are issued for inland areas that may see tropical storm or hurricane force wind and/or rain conditions, but are not located along the coast. These started appearing in the 2000s, originally with the word "Wind" inserted before the "Watch" or "Warning", which was dropped for 2005. All appear to be issued with an Emergency Alert System event codes HWA and HWW, used for high wind watches and warnings, though they may now be under the same codes and regular tropical cyclone advisories. Previously, standard High Wind Warnings and Watches were issued (which imply tropical storm force or stronger). When they are inland, watches and/or warnings are posted for tropical storm or hurricane force winds today (as known in 2008 by local NWS offices in the USA). Below are the older watches and warnings.
Issued for inland areas when sustained winds of 39 to 73 mph (62 to 117 km/h) associated with a tropical storm are anticipated beyond the coastal areas though the actual occurrence, timing and location are still uncertain.
Issued for inland counties when tropical storm conditions are anticipated beyond the coastal areas in the next six to twelve hours.
Issued for inland counties that sustained winds of 74 mph (118 km/h) or greater associated with a hurricane are anticipated beyond the coastal areas though the actual occurrence, timing and location are still uncertain.
Issued for inland counties that sustained hurricane force winds are anticipated beyond the coastal areas in the next six to twelve hours.
Other advisories are also commonly issued in association with tropical cyclones, but are not specific to them. Tropical cyclones often produce tornadoes, prompting the issuing of severe thunderstorm and tornado watches and warnings. A wind advisory would be issued for inland wind conditions (either at the extreme edges of the storm or far inland) that are strong (at least 25 mph/40 km/h) but sustained below tropical storm force. Heavy rains associated with tropical systems often result in flood watches and warnings. Gale warnings are typically issued for coastal areas surrounding the storm but where it is unlikely to travel. Still lighter winds at the extreme fringes carry a small craft advisory.
Before the 2004 Atlantic hurricane season, these warnings were not issued in Canada (who simply issued standard wind and rain warnings, which are now issued alongside the NHC-standard warnings). That policy was changed when it appeared that the population did not realize the dangers from four storms in 2003 that affected different land and offshore areas of Canada, the worst of which was Hurricane Juan in Nova Scotia, even if regular warning bulletins where issued well in advance. The inland watches and warnings are not differentiated from the coastal watches and warnings in Canada; the hurricane and tropical storm watches and warnings can be issued for any forecast area.
In Cuba, warnings are issued by province, not by coastal location or breakpoints. There are no differentials between coastal and inland warnings there as they are automatically issued for both types of areas.
The Pearl River Delta uses a variety of warning systems to inform the public regarding the risks of tropical cyclones to the area. The Hong Kong Observatory issues typhoon signals to indicate the existence and approximate location of a tropical cyclone from Hong Kong. The Direcção dos Serviços Meteorológicos e Geofisicos in Macau uses a similar system.
A two-stage warning system was long-established in China for tropical cyclones of tropical storm intensity of above.
Nowadays, the use of this system is restricted to coastal waters only. Thus, similar to the US system, warnings may be discontinued even the cyclone is maintaining tropical storm intensity inland. However, color-coded alerts (mentioned below) may be in effect.
Guangdong introduced a color-coded tropical cyclone warning system for land use in 2000.(no longer in use)
Similar systems were developed in Fujian and Shanghai.
Later, China Meteorological Administration standardized the system for national use. This set is part of a larger warning system that covers other forms of severe weather conditions, such as extreme temperature, torrential rainfall, drought, etc.
Note that Guangdong maintained a white alert as in the old system.
The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) releases tropical cyclone warnings in the form of Public Storm Warning Signals (or just storm signals). An area having a storm signal may be under:
These storm signals are usually heightened when an area (in the Philippines only) is about to be hit by a tropical cyclone. Thus, as a tropical cyclone gains strength and/or gets nearer and nearer to an area having a storm signal, it may be heightened to another higher signal in that particular area. Whereas, as a tropical cyclone weakens and/or gets farther to an area, it may be downgraded to a lower signal or may be lifted (that is, an area will have no storm signal).
Warnings and watches are two levels of alert issued by national weather forecasting bodies to coastal areas threatened by the imminent approach of a tropical cyclone of tropical storm or hurricane strength. They are notices to the local population and civil authorities to make appropriate preparation for the cyclone, including evacuation of vulnerable areas where necessary. It is important that interests throughout the area of an alert make preparations to protect life and property, and do not disregard it on the strength of the detailed forecast track. Tropical cyclones are not points, and forecasting their track remains an uncertain science.