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:
Seven tropical cyclone basin map
WMO

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:

In addition to the WMO recognized RSMCs and TCWCs, many national governments tailor the guidance forecast provided by recognized centers.

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
Jakarta, Indonesia
Sub-Regional Tropical Cyclone Warning Center - Mauritius
Sub-Regional Tropical Cyclone Warning Center - Madagascar
Nairobi, Kenya
Maputo, Mozambique
Manila, Philippines
Seoul, Republic of Korea
Hanoi, Vietnam
Dhaka, Bangladesh
Rangoon, Myanmar
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:

Despite the variety of hurricane track forecast models, there are only a few models that forecast intensity change:


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:

Australian Basin:

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.

CPHC Tropical Cyclone track forecsat five year running mean error

CPHC Tropical Cyclone intensity forecast error verification statistics five year running mean


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. Tropical Prediction Center Performance Measures

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.

Tropical Prediction Center Performance Measures

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.


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