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: 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

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
Extreme Winds
Extreme Rainfall
Storm Surge
Tornadoes
Honolulu Advertiser
SEMP

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.

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.

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!

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.

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.