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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.
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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.
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[1] (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[1] or less (was 24 hours or less prior to 2010 hurricane season). Maritime flags indicate this with a single square red flag with a black square in the middle.
A hurricane watch (HWA) is issued when the onset of tropical storm conditions appear possible in the warning area within the next 48 hours[1] (was 36 hours prior to 2010 hurricane season). Maritime flags indicate this with two square red flags.
The purpose of a hurricane watch is to inform families to obtain supplies, secure their homes, and be prepared to evacuate.
A hurricane warning (HUW) is issued when a hurricane with sustained winds of 74 mph (65 knots, 118 km/h) or higher is expected. The National Hurricane Center will issue the HUW when tropical storm conditions are likely in the warned area within the next 36 hours. 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.
Extreme wind warnings are issued for any land areas that are in the path of a landfalling category 3 or higher (major) hurricane that are expected to see 115 mph or greater winds, usually associated with the eyewall. (This warning type was created after Hurricane Katrina when several tornado warnings were issued due to the extreme winds, but no actual tornadoes occurred or were forecast to occur.)
The following alerts were issued for inland areas that might have seen tropical storm or hurricane force wind and/or rain conditions, but were not located along the coast prior to 2011.[2][3] These started appearing in the 2000s, originally with the word "Wind" inserted before the "Watch" or "Warning", which was dropped for 2005. All appeared to be issued with Specific Area Message Encoding event codes HWA and HWW, used for high wind watches and warnings, though they may have later become be under the same codes and regular tropical cyclone advisories. Previously, standard High Wind Warnings and Watches were issued (which denote >=39 MPH winds or >=58 MPH gusts). When they were inland, watches and/or warnings were posted for tropical storm or hurricane force winds in the next 24 hours or so. In 2011, these alerts became deprecated in favor of issuing a unified coastal/inland warning.[4] The coastal strip is determined by the NHC, and the Local WFOs determine the inland portion of the warning. Below are the deprecated watches and warnings.
Formerly issued for inland areas where sustained winds of 39 to 73 mph (62 to 117 km/h) associated with a tropical storm were anticipated beyond the coastal areas though the actual occurrence, timing and location are still uncertain.
Formerly issued for inland counties where tropical storm conditions were anticipated beyond the coastal areas in the next six to twelve hours.
Formerly issued for inland counties where sustained winds of 74 mph (118 km/h) or greater associated with a hurricane were anticipated beyond the coastal areas though the actual occurrence, timing and location are still uncertain.
Formerly issued for inland counties where sustained hurricane force winds were 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.
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 were 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.
A similar system was implemented in 2011 in the United States. NHC-issued breakpoints are used as guidance and indicate the marine warnings in the area, and the local WFOs expand the warning to include affected inland areas. The expanded warning area is issued by counties, rather than areas as large as cuban provinces.
A two-stage warning system was long-established in China for tropical cyclones of tropical storm intensity of above.[5]
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.[6]
Similar systems were developed in Fujian and Shanghai.
Later, China Meteorological Administration standardized the system for national use.[7] 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 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 Macao Meteorological and Geophysical Bureau in Macau uses a similar system.
A multi-stage system called the Tropical Cyclone Condition of Readiness (TCCOR), is used for the United States military base and residents on Okinawa, Japan. Each stage relates to a level of preparation that is recommended to be undertaken before a storm arrives. Between June 1 and November 30, TCCOR 4 is constantly in effect. TCCOR 1 is broken into four different situation levels based on current conditions.
| Signal #1 winds of 30–60 km/h (20-35 mph) are expected to occur within 36 hours |
| Signal #2 winds of 60–100 km/h (40-65 mph) are expected to occur within 24 hours |
| Signal #3 winds of 100–185 km/h, (65-115 mph) are expected to occur within 18 hours. |
| Signal #4 winds of at least 185 km/h, (115 mph) are expected to occur within 12 hours. |
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).[8] 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).
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