Severe Weather and Storms

Severe weather and Storms

Auckland often experiences severe weather events that can affect broad areas and which can bring severe winds, heavy rain, flooding, thunderstorms, tornadoes, (ex)tropical cyclones, coastal inundation, and rough seas. These events may cause significant damage and disruption. Heavy rain associated with major storms is usually less intense, but much more widespread that it is with a thunderstorm. Therefore, rises in river levels and the onset of flooding brought about by widespread heavy rain tend to be slower but also long lasting

Severe weather may be:

  • Localised, or widespread
  • Short or long duration
  • Sudden or forecasted.

Major storms are almost invariably associated with atmospheric lows (depressions). While lows are common in the Tasman Sea - New Zealand - Southwest Pacific area, only the occasional one has the right characteristics to significantly affect New Zealand. In short, it has to be the right storm in the right place at the right time.

Impacts include:

  • Injuries to people from airborne debris
  • Uprooted trees damaging buildings and utility infrastructure such as power lines
  • Boats blown onto shorelines
  • Disruption to air travel
  • Land movement in the form of landslides, rockfalls and coastal erosion
  • Flood inundation of properties in floodplains and low-lying coastal areas.

In New Zealand, MetService defines broad-scale severe weather as widespread (i.e, over an area of 1000 km2 or more):

  • Rainfall greater than 50mm within six hours or 100mm within 24 hours; and/or
  • Snowfall below 1000m on the North Island or 500m on the South Island with a snow depth of 10cm within six hours or 25cm within 24 hours; and/or
  • Severe gales with a minimum mean speed of 90km/hr or frequent gusts exceeding 110km/hr.

Severe weather watches and warnings are issued by MetService.

 

Severe Wind

Severe winds are gusts of wind which are strong enough to be dangerous for people or cause significant damage to buildings and property. They are typically associated with storm events that can occur at any time of the year. The greatest danger during severe wind events is from flying debris and falling trees.

 

Heavy Rain

Heavy rainfall can be hazardous if it continues for a long time. It can cause landslides, floods, and streams and rivers to rise rapidly. The rain can also cause stormwater drains in urban areas to overflow which can lead to flooding.

 

Thunderstorm

Thunderstorms are short-lived, existing for no more than one or two hours. They may cause lightning (thunder is just the sound of lightning) along with very heavy rain and sometimes hail, localised severe winds and tornadoes. MetService is responsible for issuing severe weather warnings in New Zealand and may issue a “Severe Thunderstorm” warning if appropriate.

MetService defines “Severe Thunderstorms” as those which may produce:

  • Heavy rain (from thunderstorms): Rainfall of 25mm per hour, or more; and/or
  • Large hail: Hailstones 20mm in diameter, or larger; and/or
  • Strong wind gusts (from thunderstorms): Gusts of 110km per hour (60 knots) or stronger; and/or
  • Damaging tornadoes: Fujita F1 (wind speeds greater than 116km per hour (63 knots)) or stronger.

 

Tornado

A tornado is a narrow, violently rotating column of air extending downwards to the ground from the base of a thunderstorm. Only thunderstorms that have a particular sort of rotating air column produce tornadoes and it is only when this rotating air column touches down to the ground, or gets very close to the ground, does it become a hazard to land- (or sea-) based activity. A waterspout is simply a tornado that occurs over a body of water.

Tornadoes occur infrequently in Auckland and are much smaller than those that occur in the Midwest of the USA. They are usually around a few tens of metres wide, have tracks a few kilometres long and have a short duration. Like all tornadoes, their damage paths are extremely localised.

They are impossible to predict because of their small size, short lifespan and tendency to form offshore.

The greatest danger during severe wind events is from flying debris and falling trees which can cause injury, loss of life and damage property.

 

How to Prepare

What do I need to do to be ready?

Before

 

  • If you find your home or property lies within an areas that may be affected by severe wind secure, or move indoors, all items that could get blown about and cause harm in strong winds.
  • Check the Auckland's Hazards Viewer to find out it you live, work or play in an area prone to severe wind.
  • Pick a safe place in your home for household members to gather during a storm, away from windows, skylights and glass doors.
  • Prepare your property for high winds and surge waters. Secure loose items, protect windows and make sure your roof is secure.

During

 

  • Avoid tall structures, such as towers, tall trees, fences, and power lines.
  • Draw the blinds and curtains over windows to prevent glass being dispersed in the event of a breakage and close all interior doors. Remain on lower floors but above the basement.
  • If there is no building in sight, take shelter in a vehicle and keep the windows closed.
  • Watch out for flying debris. Use your arms to protect your head and neck.
  • Stay inside with your family and pets.
  • Watch out for fallen power lines or broken gas lines and stay out of damaged areas.
  • Be prepared for road closures or utility outages.
  • [For cyclones]: Be aware that 'eye of the storm' is calm and quiet. Maintain secure position and do not go outside as the winds will get stronger again.
  • [For tornadoes]: Do not try to outrun a tornado in a vehicle. If you are in a car or outdoors and cannot get to a building, cover your head and neck with your arms and cover your body with a coat or blanket, if possible.

After

 

  • After the event, help others if you can, continue to listen to the radio and follow instructions from emergency services.

tropical cyclone

Tropical cyclones typically form close to the equator in the southwest Pacific Ocean. Cyclones derive their energy from warm equatorial waters. Sometimes, a few cyclones migrate out of the tropics into the mid-latitudes, close to New Zealand. As they move south into cooler waters, their central core cools causing the cyclone to weaken. However, cyclones can re-intensify to become mid-latitude 'ex-tropical cyclones'.

Southwest Pacific cyclones tend to form between December and April with at least one ex-tropical cyclone passing within 500km of New Zealand most years. Each storm's severity depends on many factors including where the cyclone forms and the strength of the dominating La Niña or El Niño cycle.

The table below shows tropical storm classifications in the southwest Pacific.

Storm Type

Sustained wind (km/h)

Tropical depression

<63

Tropical cyclone category 1

63-125 (gales)

Tropical cyclone category 2

125-164 (destructive)

Tropical cyclone category 3

165-224 (very destructive)

Tropical cyclone category 4

225-279 (very destructive)

Tropical cyclone category 5

>280 (extremely destructive)

Coastal Inundation

Coastal inundation happens when the sea floods coastal land. This can also be intensified by rivers near the coast flooding. During severe storms Auckland’s low-lying coastline is prone to flooding.

Inundation is more likely to occur when high tides combine with strong onshore winds and low atmospheric pressure (storm surge), wave set-up and run-up and elevated water levels from heavy rainfall. During storm events, the likelihood and magnitude of coastal inundation is highly dependent on the particular occurrence or timing of high tides and their relative height, storm surge and accompanying wave/swell conditions.

Impacts include:

  • Damage and possible breaches or failures of coastal-defence structures, stopbanks or revetments
  • Potential damage to shoreline structures and facilities, (eg marinas, boat ramps, and car parks)
  • Coastal communities becoming isolated
  • Corrosion of electrical devices and other metal objects
  • Land erosion
  • Salinization of flooded land which can affect crops from changed soil chemistry
  • Secondary hazards, such as land instability and fire.

What do I need to do to be ready?

  • Stay informed and up-to-date on the risks faced by your community and your council’s local adaptation strategies to climate change.
  • Check the Auckland’s Hazards Viewer to find out if you live, work or play in an area prone to coastal inundation.

King tides

King Tide charts credited to Rob Bell from NIWA

Download king tide chart for Auckland - East Coast

Download king tide chart for Auckland - West Coast