Tropical cyclone warnings and watches
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Tropical cyclone 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.
Tropical cyclone warnings and watches
In conjunction with the National Hurricane Center, the National Meteorological and Hydrological Services of Central America and the Northern Atlantic Ocean, excluding mainland Africa and Europe, all issue tropical storm/hurricane watches and warnings. Tropical storm watches are issued when gale and storm force winds of between 34–63 knots (39–73 mph; 63–118 km/h) are possible, within 48 hours in a specified area in association with a tropical, subtropical or post-tropical cyclone. These watches are upgraded to tropical storm warnings, when gale and storm force winds become expected to occur somewhere in the warning area within 36 hours. Hurricane watches are issued when sustained winds of 64 knots (74 mph; 119 km/h) are possible, within 48 hours in a specified area in association with a tropical, subtropical or post-tropical cyclone. These warnings are upgraded to hurricane warnings, when hurricane force winds become expected to occur somewhere in the warning area within 36 hours.
Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane watch and warnings are issued in advance of the anticipated onset of tropical-storm-force winds, rather than in advance of the anticipated onset of hurricane-force winds. At times a Tropical Storm Warning and a Hurricane Watch can both be in effect due to uncertainties in the forecast. These watches and warnings are also issued by the Central Pacific Hurricane Center for the Hawaiian Islands and the Weather Forecast Office in Guam for parts of Micronesia but not for American Samoa due to an international agreement.
Within the United States an Extreme wind warning is issued by the National Weather Service for any land areas that are expected to be impacted by a category 3 or higher hurricane and by sustained surface winds greater than or equal to 100 knots (115 mph; 190 km/h). all landfalling hurricane is expected to bring winds of 100 knots (115 mph, 185 km/h, 51 m/s) to a specific location. The warning is issued just prior to when the strongest winds of the eyewall are expected to impact an area. The warning is to be issued for the smallest area possible, and be valid for times of two hours or less. It was developed in response to confusion resulting from the landfall of Hurricane Katrina. NWS offices in Jackson and New Orleans/Baton Rouge issued 11 tornado warnings for areas that would not experience an actual tornado, but would experience extreme wind speeds commonly associated with tornadoes. The extreme wind warning is now expected to be used in these situations.
West Pacific systems
A two-stage warning system was long-established in China for tropical cyclones of tropical storm intensity of above.
- Warning: Winds may reach Beaufort Force 8 or rainstorm may occur in 48 hours.
- Urgent Warning: Winds may reach Beaufort Force 8 or rainstorm may occur in 24 hours.
Nowadays, the use of this system is restricted to coastal waters only. Thus, 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.
- White alert: A tropical cyclone may affect the area in 48 hours.
- Green alert: Winds may reach Beaufort Force 6 in 24 hours or winds of Beaufort Force 6~7 are already blowing.
- Yellow alert: Winds may reach Beaufort Force 8 in 12 hours or winds of Beaufort Force 8~9 (gale force) are already blowing.
- Red alert: Winds may reach Beaufort Force 10 in 12 hours or winds of Beaufort Force 10~11 (storm force) are already blowing.
- Black alert: Winds may reach Beaufort Force 12 in 12 hours or winds of Beaufort Force 12 (hurricane force) are already blowing.
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.
- Blue alert: Winds may reach Beaufort Force 6 in 24 hours or winds of Beaufort Force 6~7 are already blowing.
- Yellow alert: Winds may reach Beaufort Force 8 in 24 hours or winds of Beaufort Force 8~9 (gale force) are already blowing.
- Orange alert: Winds may reach Beaufort Force 10 in 12 hours or winds of Beaufort Force 10~11 (storm force) are already blowing.
- Red alert: Winds may reach Beaufort Force 12 in 6 hours or winds of Beaufort Force 12 (hurricane force) are already blowing.
Note that Guangdong maintained a white alert as in the old system.
Hong Kong and Macau
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.
Tropical Cyclone Condition of Readiness
|5||96||This is set by military bases in the US, throughout the Atlantic hurricane season.|
|4||72||Guam is in TC-COR 4 throughout the year, while Japanese bases set this from June 1 - November 30.|
|3||48||Destructive winds are possible within 48 hours.|
|2||24||Destructive winds are now expected within 24 hours.|
|1||12||Destructive winds are now expected within 12 hours, but gale force winds are not yet occurring.|
|1C||12||Gale-force winds are occurring.|
|1E||0||Winds of above 50 kn (58 mph; 93 km/h) are occurring.|
|1R||Winds of above 50 kn (58 mph; 93 km/h) are no longer occurring, but gale-force winds are occurring.|
|Storm Watch||The system is moving away but the base is still feeling some effects.|
|All-Clear||Revert to seasonal TC-COR|
The United States Department of Defense uses a multi-stage system called the Tropical Cyclone Condition of Readiness (TC-CORs) otherwise known as the Hurricane Condition of Readiness (HURCONs), to prepare bases and evacuate assets and personnel in advance of adverse weather associated with tropical cyclones. TC-CORs are recommended by weather facilities either on base or by central sites like the National Hurricane Center or the Joint Typhoon Warning Center and are generally related to the timing and potential for destructive sustained windspeeds of above 50 kn (58 mph; 93 km/h). Recommendations are then considered by base or area commanders along with other subjective factors for setting the TC-CORs like assets, holidays or the bases experience in emergency preparedness. The bases prefer to set these TC-CORs sequentially, from TC-COR 5 with destructive winds expected within 96 hours, through TC-COR 4, 3, 2 and if needed to a series of four different TC-COR 1 conditions, however depending on the cyclone's movement or location some of these signals can be skipped. After a system passes and stops affecting the base, the authorities can decide to revert to the seasonal TC-COR or stay in a heightened approach as another tropical cyclone is approaching.
winds of 30–61 km/h (20-38 mph) are expected to occur within 36 hours
winds of 61–120 km/h (38–73 mph) are expected to occur within 24 hours
winds of 121–170 km/h, (74–105 mph) are expected to occur within 18 hours.
winds of 171–220 km/h, (106–137 mph) are expected to occur within 12 hours.
winds of at least 220 km/h, (137 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). An area having a storm signal may be under:
- PSWS #1 - Tropical cyclone winds of 30–61 km/h are expected within the next 36 hours. (Note: If a tropical cyclone forms very close to the area, then a shorter lead time is seen on the warning bulletin.)
- PSWS #2 - Tropical cyclone winds of 61–120 km/h are expected within the next 24 hours.
- PSWS #3 - Tropical cyclone winds of 121–170 km/h are expected within the next 18 hours.
- PSWS #4 - Tropical cyclone winds of 171-220 km/h are expected within 12 hours.
- PSWS #5 - Tropical cyclone winds greater than 220 km/h are expected within 12 hours.
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 closer 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 away from an area, it may be downgraded to a lower signal or may be lifted (that is, an area will have no storm signal).
Australia and the South Pacific
The Australian Bureau of Meteorology (BoM) will issue a cyclone watch for a specified part of Australia, when a tropical cyclone is expected to cause gale-force winds in excess of 62 km/h (40 mph) within 24-48 hours and subsequently make landfall. A cyclone warning is subsequently issued for a specified part of Australia when a tropical cyclone, is expected to cause or is causing gale-force winds in excess of 62 km/h (40 mph) within 24 hours and is subsequently expected to make landfall. The Fiji Meteorological Service (FMS) issues a tropical cyclone alert for the Cook Islands, Fiji, Kiribati, Nauru, Niue, Tokelau, Tonga and Tuvalu, when a tropical cyclone has a significant probability of causing gale-force winds or stronger winds within 24-48 hours. Gale, storm and hurricane force wind warnings are subsequently issued for the above areas by FMS, when a tropical cyclone is either causing or expected to cause either gale storm or hurricane force winds within 24 hours. Météo-France is responsible for the issuance of tropical cyclone watches and warnings for New Caledonia, Wallis and Futuna, French Polynesia and the Pitcairn Islands. The National Meteorological and Hydrological Services of the Solomon Islands, Samoa, Indonesia, Papua New Guinea, New Zealand, Timor Leste and American Samoa are responsible for their own watches and warnings.
- Gale warning
- iTyphoon – A mobile application used to provide information about typhoons in the Philippines.
- Severe weather terminology (disambiguation)
- Small craft advisory
- Storm warning
- Tropical cyclone
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