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Tropical Cyclone Forecasting
1. What regions around the globe have tropical cyclones and who is responsible for forecasting there?
There are seven tropical cyclone "basins" where storms occur on a regular basis:
Atlantic basin (including the North Atlantic Ocean, the Gulf of Mexico, the Caribbean Sea)
Northeast Pacific basin (from Mexico to 140 degrees West Longitude)
North central Pacific Basin (from 140W to 180 degrees longitude)
Northwest Pacific basin (from 180 degrees longitude to Asia including the South China Sea)
North Indian basin (including the Bay of Bengal and the Arabian Sea)
Southwest Indian basin (from Africa to about 100E)
Southeast Indian/Australian basin (100E to 142E)
Australian/Southwest Pacific basin (142E to about 120W)
The 6 WMO recognized Regional Specialized Meteorological Centers (RSMC) for Hurricanes/Typhoons and 4 Tropical Cyclone Warning Centers (TCWC) provide the official tropical cyclone guidance track and intensity forecasts for their designated areas. These are:
- RSMC Miami. The National Hurricane Center in Miami, Florida, USA has responsibilities for monitoring and forecasting tropical cyclones in the Atlantic and Northeast Pacific basin east of 140W.
- RSMC Honolulu. The Central Pacific Hurricane Center in Honolulu has responsibilities for the remainder of the Northeast Pacific basin from 140W to 180 degrees longitude.
- RSMC Tokyo. The Japanese Meteorological Agency Typhoon Center has the responsibility for the northwest Pacific Ocean basin.
- RSMC New Delhi. The Indian Meteorological Agency Cyclone Center in New Delhi has responsibilities for the North Indian Ocean basin.
- RSMC La Reunion. The French Meteorological office in La Reunion has responsibilities for the Southwest Indian Ocean basin.
- RSMC Nadi. The Fijian Meteorological Service office in Nadi, Fiji has the responsibility for the southwest Pacific Ocean.
- TCWCs in Brisbane, Darwin, and Perth. The Australian Bureau of Meteorology (BoM) has responsibilities for the far south west Pacific Ocean basin and the southeast Indian Ocean basin surrounding Australia and Indonesia.
- TCWC Wellington. The New Zealand Meteorlogical Service office in Wellington has responsibility for the southwest Pacific Ocean basin south of 25 degrees south latitude.
- Note also that the US Joint Typhoon Warning Center (JTWC) issues warnings for tropical cyclones in the Northwest Pacific, the North Indian, the Southwest Indian, the Southeast Indian/Australian, and the Australian/Southwest Pacific basins, for U.S. A. interest only in those areas. JTWC is not a WMO recognized RSMC or TCWC.
In addition to the WMO recognized RSMCs and TCWCs, many national governments tailor the guidance forecast provided by recognized centers.
- For the Northwest Pacific basin, China, Thailand, Korea, Japan, the Philippines, Hong Kong, China, Macau, China among others provide tailored forecasts for their areas.
- For the North Indian Ocean basin, India, Thailand, Pakistan, Bangladesh, Burma, and Sri Lanka among others provide tailor forecasts.
- For the Southwest Indian Ocean basin, Madagascar, Mozambique, Mauritius, and Kenya among others provide tailored forecasts.
- For the southwest Pacific Ocean basin, Papua New Guinea, Vanuatu, and Samoa among others provide tailored forecasts.
On rare occasions, tropical cyclones (or tropical/subtropical systems appear to be similar in structure to tropical cyclones) can develop in the Mediterranean Sea. These have been noted in September 1947, September 1969, January 1982, September 1983, and, most recently, during 13 to 17 January, 1995. Some study of these storms has been reported on by Mayengon (1984) and Ernest and Matson (1983), though it has not been demonstrated fully that these storms are the same as those found over tropical waters. It may be that these Mediterranean tropical cyclones are more similar in nature to polar lows.
The recent hurricane that formed in the South Atlantic was handled by the Brazilian weather service. Since tropical cyclones are so rare in this region, the WMO has not designated a forecast center with responsibility for there.
The following RSMCs and TCWCs are responsible for issuing guidance advisories and/or warnings on tropical cyclones for their respective regional areas:
National Hurricane Center RSMC Miami
Central Pacific Hurricane Center RSMC Honolulu
Regional Specialized Meteorological Center Tokyo, Japan
Fiji Tropical Cyclone Warning Center RSMC Nadi
Regional Tropical Cyclone Advisory Centre - Reunion RSMC LaReunion
RSMC New Delhi Cyclone Warning Center
Brisbane Tropical Cyclone Warning Center TCWC Brisbane
Darwin Tropical Cyclone Warning Center TCWC Darwin
Perth Tropical Cyclone Warning Center TCWC Perth
New Zealand Meteorological Service TCWC Wellington
For U.S.A. interests only:
Naval Pacific Meteorological and Oceanographic Center Pearl Harbor Hawaii
Joint Typhoon Warning Center Pearl Harbor Hawaii
Other National Centers:
Hong Kong Observatory
Bangkok Tropical Cyclone Warning Center - Thailand
Port Moresby Tropical Cyclone Warning Center
Sub-Regional Tropical Cyclone Warning Center - Mauritius
Sub-Regional Tropical Cyclone Warning Center - Madagascar
Seoul, Republic of Korea
Colombo, Sri Lanka
Male, Maldive Islands
The World Meteorological Organization contains references on these various areas and addition information.
2. What are those track and intensity models that the forecasters are talking about in the hurricane and tropical storm discussions?
A variety of hurricane track forecast models are run operationally:
- The basic model that is used as a no-skill forecast to compare other models against is CLIPER (CLImatology and PERsistence), which is a multiple regression statistical model that best utilizes the persistence of the current motion and also incorporates climatological track information (Aberson 1998). Surprisingly, CLIPER was difficult to beat with numerical model forecasts until the 1980s.
- A statistical-dynamical model, NHC90 (McAdie 1991), uses geopotential height predictors from the Aviation model to produce a track forecast four times per day. The primary synoptic time NHC90 forecasts (00 and 12 UTC) are based upon 12 hour old Aviation runs. A special version of NHC90, NHC90-LATE, is run at primary synoptic times with the current Aviation run, and is available a number of hours after NHC90. Both versions of NHC90 have been run operationally since 1990. An update to this model, NHC98, was implemented in 1998.
- The Beta and Advection Model (BAM), follows a trajectory in the pressure-weighted vertically-averaged horizontal wind from the Aviation model beginning at the current storm location, with a correction that accounts for the beta effect (Marks 1992). Three versions of this model, one with a shallow-layer (BAMS), one with a medium-layer (BAMM), and one with a deep-layer (BAMD), are run. BAMS runs using the 850-700 mb layer,BAMM with the 850-400 mb layer, and BAMD with the 850-200 mb layer. The deep-layer version was run operationally for primary synoptic times in 1989; all three versions have been run four times per day since 1990.
- A nested barotropic hurricane track forecast model (VICBAR) has been run four times daily since 1989. The primary synoptic time runs are run from current NCEP analyses, the off-time runs are run from six hour old data (Aberson and DeMaria 1994). Another barotropic model, LBAR, for Limited-Area Barotropic Model, is also run operationally every 6 hours and performs slightly worse than VICBAR, but is available earlier for use by the forecasters.
- The NCEP GFS model, formerly the Aviation and MRF models (Lord 1993), has been used for track forecasting since the 1992 hurricane season. This is a global model.
- A triply-nested movable mesh primitive equation model developed at the Geophysical Fluid Dynamics Laboratory (Bender et al 1993), known as the GFDL model, has provided forecasts since the 1992 hurricane season.
- The United Kingdom Meteorological Office's global model (UKMET) is utilized for forecasting the track of tropical cyclones around the world (Radford 1994). The NWS starting receiving these operationally during 1996.
- The United States Navy Operational Global Atmospheric Prediction Systems (NOGAPS) is also a global numerical model that shows skill in forecasting tropical cyclone track (Fiorino et al. 1993). This model was also first received operationally during 1996.
Despite the variety of hurricane track forecast models, there are only a few models that forecast intensity change:
- Similar to the CLIPER track model, the SHIFOR (Statistical Hurricane Intensity Forecast model) is used as a "no-skill" intensity change forecast. It is a multiple regression statistical model that best utilizes the persistence of the intensity trends and also incorporates climatological intensity change information (Jarvinen and Neumann 1979). SHIFOR has been difficult to exceed until recent years.
- A statistical-synoptic model, SHIPS (Statistical Hurricane Intensity Prediction Scheme ), has been available since the mid-1990s (DeMaria and Kaplan 1994). It takes current and forecasted information on the synoptic scale on the sea surface temperatures, vertical shear, moist stability, etc. with an optimal combination of the trends in the cyclone intensity.
- The GFDL model, described above in the track forecasting models, also issues forecasts of intensity change.
- A new statistical scheme for estimating the probability of rapid intensification has been developed (Kaplan and DeMaria 1999) and is now being used operationally. The RI scheme employs synoptic and persistence information from the SHIPS model to estimate the probability of rapid intensification (24 h increase in maximum wind of 35 mph or greater) every 6 hours.
3. What are the various forecasts that are being issued for seasonal tropical cyclone activity around the world?
There are a number of different seasonal forecasts currently being issued for various basins. Some of these are fairly new, while the oldest and most well known (Professor Bill Gray's forecast from Colorado State University) has been issued for almost two decades.
North Atlantic Basin:
North West Pacific:
South China Sea:
4. What is the official U.S. Government (NOAA) seasonal hurricane outlook for this year and what are the predictive factors?
NOAA outlook for the Atlantic Basin and a listing of the predictive factors used.
NOAA outlook for the East Pacific Basin and the predictive factors used.
NOAA outlook for the Central Pacific Basin
5. How has the official U.S. Government (NOAA) seasonal hurricane outlook done in previous years?
The NOAA Seasonal Outlook for Atlantic basin hurricane activity does not predict numbers of tropical storms, hurricanes and major hurricanes directly. Rather, the scheme is set up to forecast a range of expected values for the ACE index (Accumulated Cyclone Energy), a measure of overall activity. The ranges predicted for numbers of systems are obtained by looking at the years in the historical record which had observed values for ACE in the predicted range for the current year. Note that although the range for ACE might verify correctly for a given year (as it has so far for every year since the forecast began in 1998 -- see below), it is rare that the ranges for all three numbers (tropical storms, hurricanes and major hurricanes) will be correct. However, if ACE is correct, then usually at least two of the predicted ranges for numbers are correct as well. (Click here to see a chart of the observed values for ACE since 1950.)
Verification for the NOAA May Seasonal Outlook for the North Atlantic basin hurricane activity from 1999 - 2003
Verification for the NOAA August Seasonal Outlook for the North Atlantic basin hurricane activity from 1998 - 2003
6. How accurate are the forecasts from the Central Pacific Hurricane Center?
The following graphs show the average position and intensity errors in the forecasts of tropical cyclones by the Central Pacific Hurricane Center. The track forecasts have been steadily improving over the years, and forecast errors have been reduced by up to 50% since 1984. This means that although forecasts are still not perfect, the average error in location for a 48 hour forecast today is almost the same as that of a 24 hour forecast 20 years ago. The increase in track accuracy is a result of the numerous studies that have taken place in tropical cyclone motion, improved numerical model guidance and most importantly, forecaster knowledge.
Unfortunately the same can not be said for intensity forecasts. The inner structure and dynamic behavior of tropical systems is still not well understood. These elements play a large role in determining the actual strength of tropical cyclones. Also we can now more easily observe the fluctuating and sometimes rapid changes in intensity, forecasting these changes remain elusive. A large number of researchers globally are working to solve these problems, since all tropical cyclone forecast centers worldwide struggle to produce accurate intensity forecasts.
7. How accurate are the forecasts from the National Hurricane Center?
The National Hurricane Center (NHC) issues an official forecast, every six hours, of the center position, maximum one-minute surface (10 meter [33 ft] elevation) wind speed (intensity), and radii of the 34 knot (39 mph, 63 kph), 50 knot (58 mph, 92 kph), and 64 knot (74 mph, 117 kph) wind speeds in four quadrants (northeast, southeast, southwest, and northwest) surrounding the cyclone. The NHC has been issuing predictions for the forecast periods of 12, 24, 36, 48, and 72 hours since 1964. Forecasts for 12 and 24 hours were first issued in 1954. In 2003, the forecasts were extended and now include 96 and 120 hours. All official forecast are verified by comparison with the "best track", a set of six-hour center positions and maximum wind speed values, that represents the official NHC estimate of the location and intensity of a tropical cyclone. A best track is prepared for every tropical cyclone, after the fact, using all available data.
Fig. 1 Yearly-average official track forecast errors for 24-, 48-, 72-, 96-, and 120-hours, Atlantic basin, excluding depressions. Straight lines are linear trend lines with all yearly-averages weighted equally. The official track error data includes all official forecasts issued since 1954.
NHC's official track errors have averaged in the last few years about 85 nmi (100 st. miles,160 km) at 24 hr, 140 nmi (160 st. miles,260 km) at 48 hr and 200 nmi (230 st. miles, 370 km) at 72 hr. One can see that NHC has even done better than these numbers during 2003. Forecasts are now also issued at 4 and 5 days lead time and these are likely to have an average error of about 250 nmi (290 st. miles,460 km) and 300 nmi (350 st. miles, 550 km), respectively. These are average errors so, of course, individual predictions may be substantially better or worse. It is to the National Hurricane Center's credit (and NOAA in general) that these predictions have gotten so much better in the last few decades, due to a combination of more accurate numerical models, more observations over the open ocean, and a better understanding of the physics of hurricane movement. Today a 3 day forecast is as accurate as those issued for a 2 day prediction in the late 1980s.
Figure 1 Fig. 1Yearly-average official track forecast errors for 24-, 48-, 72-, 96-, and 120-hours, Atlantic basin, excluding depressions. Straight lines are linear trend lines with all yearly-averages weighted equally. The official track error data includes all official forecasts issued since 1954.
NHC's official track errors have averaged in the last few years about 85 nmi (100 st. miles,160 km) at 24 hr, 140 nmi (160 st. miles,260 km) at 48 hr and 200 nmi (230 st. miles,370 km) at 72 hr. One can see that NHC has even done better than these numbers during 2003. Forecasts are now also issued at 4 and 5 days lead time and these are likely to have an average error of about 250 nmi (290 st. miles,460 km) and 300 nmi (350 st. miles, 550 km), respectively. These are average errors, individual predictions may be substantially better or worse. It is to the National Hurricane Center's credit (and NOAA in general) that these predictions have gotten so much better in the last few decades, due to a combination of more accurate numerical models, more observations over the open ocean, and a better understanding of the physics of hurricane movement. Today a 3 day forecast is as accurate as those issued for a 2 day prediction in the late 1980s.
Figure 2 Yearly-average official intensity forecast errors for 24-, 48-, 72-, 96-, and 120-hours, Atlantic basin, excluding depressions. Straight lines are linear trend lines with all yearly-averages weighted equally.
NHC's wind intensity errors have averaged recently about 9 kt (10 mph,17 kph) at a 24 hr forecast, 15 kt (17 mph,28 kph) at a 48 hr forecast, and 19 kt (22 mph,35 kph) at a 72 hr forecast. The 4 and 5 day predictions should average about 21 kt (24 mph,39 kph) and 22 kt (25 mph,41 kph). (One comparison of the ability of the long-range forecasts is to consider that a simple prediction of a constant value of 60 kt (70 mph,110 kph) gives an error of about 23 kt (26 mph,43 kph), so forecasts with errors close to this value have little to no skill.) One does see that the intensity forecasts have improved somewhat at 1 and 2 day predictions - 48 hr forecasts today have errors that are 20% smaller than they were in the mid-1970s.
Improvements in intensity are much slower than in the track predictions and the 3 day forecasts of intensity have not gotten substantially better at all. Much work still remains to better understand and predict wind intensity changes in tropical storms and hurricanes.
Tropical cyclone size (that is, the radius of high winds) has been forecasted by NHC for several years, though the first quantitative verifications have been provided just recently. These suggest that the errors in predicting the radius of gale force winds (34 kt, 39 mph, 63 kph) averages about 20 nmi (25 st miles, 35 km) at a 24 hr forecast, about 25 nmi (30 st miles, 45 km) at a 48 hr forecast, and about 30 nmi (35 st mi, 55 km) at a 72 hr forecast.
8. How is storm surge forecast ?
Storm surge, the abnormal rise of ocean water on land due primarily to strong onshore winds, is dependent on the strength and size of the tropical cyclone and the bathymetry near the area of landfall.
In the Pacific, particularly in Hawaii, the bathymetry of the islands prevents the storm surge from reaching the levels seen in the Gulf of Mexico and along the east coast of the US mainland. Storm surges can still be significant. In hurricane Iniki, the storm surge reached 3 to 4 feet with surf as high as 35 feet.
In the Atlantic Basin, storm surge is primarily forecast with the SLOSH computer model.