Tropical Cyclones Characteristics and Forecasting

1. Tropical CyclonesCharacteristics and Forecasting Horace H. P. Burton and Selvin DeC. Burton Caribbean Institute for Meteorology and Hydrology

2. Outline • Introduction • Definitions • Characteristics • Forecast Methods • Forecast errors • Probability forecasts

3. Definitions • Tropical cycloneAn area of low pressure which develops over tropical or subtropical waters • Tropical depressionA weak tropical cyclone in which the maximum surface wind is 38 mph (62 km/h or 33 kt) or less

4. 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) • HurricaneA tropical cyclone with highest sustained winds 74 mph (119 km/h or 65 kt) or more

5. Saffir-simpson Scale

6. 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. Tropical Cyclone Characteristics • Size - 300 to 1500 km in diameter • Horizontal structure • Eye - 20 km • Eyewall - 30 to 50 km • Spiral bands

8. 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. 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. 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. 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. 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. 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. Official Forecast Errors for Atlantic

15. 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. 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. 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. Probability ellipses - Georges 1998

19. 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