Storms and Severe Weather

A storm is a weather system with strong winds and is likely to cause heavy rain. It can also bring hail, lightning, tornadoes, heavy swells, coastal inundation and storm surges.

Storms can make driving dangerous. They can also cause road damage, power supply disruption and damage to buildings. As a result, storms can isolate communities and cut off communications and power supply.

Storms can last for days or can be short-lived. Thunderstorms typically exist for no more than one or two hours but can cause significant damage.

A tornado is a narrow, violently rotating wind column, extending downwards to the ground, from the base of a thunderstorm. Every year, a few tornadoes are observed in New Zealand.

Compared with the tornadoes that occur over the Earth’s major continents, tornadoes in New Zealand are generally small, but they can still cause severe localised damage, injuries and death. They are usually around a few tens of metres wide, have tracks a few kilometres long, and exist for just a few minutes.

A tropical cyclone is a large storm that develops in the Tropics. They are also called hurricanes or typhoons in the northern hemisphere. A tropical cyclone has a sustained wind-speed of at least 63 kilometres per hour, but a severe tropical cyclone has winds of 120 kilometres per hour or more. Wind gusts can be much stronger, causing extensive damage.

Tropical cyclones usually change as they meet the cooler sea temperatures around New Zealand, so they are not classified as tropical cyclones by the time they reach our shores. These “ex-tropical cyclones” remain dangerous storms and can cause major damage affecting large parts of New Zealand.

Heavy rain can cause stream and river levels to rise, leading to dangerous, fast-flowing currents. Rain can lead to landslides and flooding. High river levels with strong currents can cause flooding and destroy bridges.

During storms, New Zealand’s low-lying coastal areas are particularly vulnerable, especially when high tides, storm surges and/or heavy swells/large waves occur at the same time.

Potential consequences of large waves include:

• Inundation/flooding.
• Overtopping, where waves spill over a seawall or breakwater.
• Hazardous driving conditions.
• Structural damage.
• Beach erosion.
• Rip currents.
• Stormwater and drainage networks overwhelmed.
• Higher salinity water up rivers and streams that can affect potable water supplies.
• Pasture damage for up to a year from salt burn.

Sea-level rise is likely to cause more frequent coastal inundation and wave damage.

Large hail can cause injuries to people, damage cars and roofs, and can break glass. Crops can be ruined and livestock killed. In large quantities, small hail can build up to centimetres deep on the ground, making driving dangerous.

Lightning can be fatal. There are more than 50,000 lightning strikes per year in New Zealand, with one death reported every five to ten years.

Strong winds and tornadoes can fell trees and poles, tear off roofs, and cause flying objects.

More often than not, the damage resulting from tornadoes in New Zealand is minor, because they only exist for a very short time. Once in a while, there is significant damage and threat to public safety, when one or more tornadoes passes through a built-up area