Tropical Cyclone Hazards in the Pacific

Robert A. BallardCentral Pacific Hurricane Center

Honolulu, HI

Briefing Outline

• What is a tropical cyclone?• Tropical cyclone ingredients• Tropical cyclone climatology• Tropical cyclone impacts

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

Tropical Cyclone Ingredients

www.weather.gov/cphc

• 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

Sea surface temperature climatology

Tropical Cyclone climatology

• 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.”

average: 31average:

4-5

average: 28

average: 6

Central Pacific Tropical Cyclones 1970-2011Central Pacific Tropical Cyclones 1970-2011

Tropical Cyclone ImpactsTropical Cyclone Impacts

www.weather.gov/cphc

• Extreme Winds• Extreme Rainfall• Storm Surge• Tornadoes

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Tropical Cyclone ImpactsTropical Cyclone Impacts

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• 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.

Tropical Cyclone ImpactsTropical Cyclone Impacts

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• 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.

Tropical Cyclone ImpactsTropical Cyclone Impacts

www.weather.gov/cphc

• 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!

Tropical Cyclone ImpactsTropical Cyclone Impacts

www.weather.gov/cphc

• 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.

ConclusionConclusion

www.weather.gov/cphc

• 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.