1. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Tropical Cyclones Characteristics and Forecasting Horace H. P. Burton and Selvin DeC. Burton Caribbean Institute for Meteorology and Hydrology

2. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Outline Introduction *Definitions *Characteristics Forecast Methods Forecast errors Probability forecasts

3. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Definitions Tropical cyclone An area of low pressure which develops over tropical or subtropical waters Tropical depression A weak tropical cyclone in which the maximum surface wind is 38 mph (62 km/h or 33 kt) or less

4. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Definitions Tropical storm A tropical cyclone in which the maximum surface wind ranges from 39 to 73 mph (63 to 118 km/h or 34 to 64 kt) Hurricane A tropical cyclone with highest sustained winds 74 mph (119 km/h or 65 kt) or more

5. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Saffir-simpson Scale

6. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Tropical Cyclone Characteristics Season - June to November Average - 9 tropical storms - 6 hurricanes Most active months - August and September Movement - west or west northwest - recurve to east generally in western and northwestern Atlantic

7. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Tropical Cyclone Characteristics Size - 300 to 1500 km in diameter Horizontal structure *Eye - 20 km *Eyewall - 30 to 50 km *Spiral bands

8. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Tropical Cyclone Characteristics Hurricane force winds generally extend out about 100 km (60 miles) from the centre Storm force winds may extend out as much as 500 km (300 miles) Maximum wind speeds between 12 and 50 km from centre

9. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Tropical Cyclone Forecasting Early and precise warnings (forecasts) do not necessarily remove the risk of damage or loss or life, but the effects may be significantly reduced Current inaccuracies in forecast dictate over warning False warnings can produce an attitude of scepticism

10. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Forecasting Methods Forecasting methods have evolve to become more complex *Pre 1960 - simple subjective methods cloud types and motion, swells, pressure *Post 1960 - more sophisticated objective methods complex statistical techniques and computer models

11. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Forecasting Methods A tropical cyclone forecast involves the prediction of several interrelated features, including *The track, winds, rainfall, storm surge, areas threatened NHC normally issues a forecast every 6 hours for period extending out to 72 hours Official forecast is based on the guidance obtained from a variety of subjective and objective models

12. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Forecast Errors Forecast errors arise from * A lack of a full understanding of the formation and growth of tropical cyclones *The limitations of the forecasting techniques Users of forecasts must be aware of the limitations of information in advisories

13. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Forecast Errors - characteristics Still substantially large although showing a slow and steady decrease Increase remarkably with increasing time Are approximately 30 % of the cyclone movement over the same time Large year-to-year variations

14. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Official Forecast Errors for Atlantic

15. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Initial forecast errors Average official initial forecast errors *26 km for position *3 kt for intensity Range of initial forecast errors *10 km for good eye to more 180 km for poorly defined centre *as much as 30 kt for intensity

16. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Strike probability forecasts One method use to objectively define uncertainty inherent in forecasts Derived from knowledge of past cyclones and forecast errors in the region of interest Provide valuable early guidance in estimating the risk of tropical cyclones affecting important or vulnerable facilities

17. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Use of strike probability forecasts To extend the usable length of forecasts To provide a quantitative assessment of the threat posed by a tropical cyclone To compare the relative threat To cause a consistent response As a tool in risk analysis

18. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Probability ellipses - Georges 1998

19. Caribbean Disaster Mitigation Project Caribbean Institute for Meteorology and Hydrology Strike probability forecasts Uncertainty increases as the forecast interval increases 72 hours probability less than that 12 hours Maximum probability at 72 hours is about 10% compared to about 70% at 12 hours Longer lead-time actions must be based on smaller probabilities