Tropical Cyclone Hazards in the Pacific

1. Tropical Cyclone Hazards in the Pacific Robert A. Ballard Central Pacific Hurricane Center Honolulu, HI

2. Briefing Outline • What is a tropical cyclone? • Tropical cyclone ingredients • Tropical cyclone climatology • Tropical cyclone impacts

3. What is a Tropical Cyclone? • A low pressure area that is warmer on the inside than on the outside – i.e., warm core • Can be dozens to hundreds of miles wide, and last for days • No fronts attached; i.e., no fronts through low center • Forms over warm tropical or subtropical oceans • Produces organized thunderstorm activity • Has a closed surface wind circulation around a well-defined center • Classified by maximum sustained surface wind speed • Tropical depression: < 39 mph • Tropical storm: 39-73 mph • Hurricane/Typhoon: 74 mph or greater

4. Tropical Cyclone Ingredients • Sea Surface Temperatures usually above 26.5°C (80°F) over a large area • Low level disturbance in the wind field • Deep moisture • Instability • Weak vertical wind shear (winds slowly changing with height) • Distance from the equator www.weather.gov/cphc

5. Sea surface temperature climatology

6. Tropical Cyclone climatology average: 31 average: 4-5 average: 6 average: 28 • Western North Pacific is the most active Tropical Cyclone basin in the world • Can’t be too focused on total number or forecast number for the season! • “It only takes one.”

7. Central Pacific Tropical Cyclones 1970-2011

8. Tropical Cyclone Impacts Honolulu Advertiser SEMP Extreme Winds Extreme Rainfall Storm Surge Tornadoes www.weather.gov/cphc

9. Tropical Cyclone Impacts • Extreme Winds • Can last from an hour or two, to a day or more, depending on how fast the tropical cyclone is moving. • Can affect a relatively small area, or a relatively large swath, depending on the size of the tropical cyclone. • Possible to build structures to withstand the winds, but flying debris can still cause damage & kill. • Terrain effects can locally accelerate winds. www.weather.gov/cphc

10. Tropical Cyclone Impacts • Intense Rainfall • Results in catastrophic flash flooding. • Several months’ worth of rainfall can fall in a 24 hour period. • Is not dependent on the strength of the tropical cyclone. • Even weak or dissipated tropical cyclones can bring catastrophic rainfall. • Any slow moving system brings the largest threat. • Can result in numerous rockslides & landslides. • Blocks roads • Failure of structures built on hillsides. • Rain often can’t escape to the ocean because of storm surge flooding. www.weather.gov/cphc

11. Tropical Cyclone Impacts • Storm surge • Abnormal rise of water level generated by a storm, well above the predicted astronomical tide. • Temporary rise in sea level effectively moves the coast well inland. • Structures near the shore are inundated. • Storm surge + wave runup + astronomical tide + freshwater flooding = “total water level” • Greatest potential killer in a tropical cyclone! www.weather.gov/cphc

12. Tropical Cyclone Impacts • Tornadoes • Friction of extreme winds interacting with land can cause sudden tornado spinups. • Most common to the right of the track of the tropical cyclone. • Usually fast moving, brief, and hard to issue warnings for. • Can cause areas of enhanced damage above & beyond that expected from the tropical cyclone itself. • Can occur even well away from the center in the outer rain bands. www.weather.gov/cphc

13. Conclusion Tropical cyclones present a complex variety of weather & water hazards. Tropical cyclones are often not well understood. They require a considerable amount of education before & during an event. Preparation & planning can help mitigate losses. www.weather.gov/cphc