‘Weather’ describes seasonal changes in climate, temperature, rainfall, winds and storms. Extreme weather, such as heavy rainfall and high winds can threaten people, property and our environment. Heat, cold, rainfall, wind and storm events in a particular area vary, depending on:
Weather patterns are also influenced by other factors including:
The Waikato region, centred around 38 degrees south, covers most of the central North Island. It is exposed to prevailing west and south-west winds from the Tasman Sea bringing warm, humid summer conditions and mild winter conditions. Average annual rainfall is 1,250 mm. Coastal areas, flat floodplains, rolling hills, mountain ranges and steep volcanoes means weather can change rapidly.
Find out more about the Waikato region’s climate.
As in other parts of the world, our region may be affected by weather hazards such as:
Extreme weather can threaten ‘lifeline’ services - water, power, telecommunication and transportation networks. Find out more about managing Lifelines to deal with unexpected emergency or natural hazards.
Rainfall and flooding
The Waikato has more than 100 lakes, many rivers and 1,150 km of coastline, stretching from the rugged West Coast to the sparkling white sands of our East Coast. This, along with high rainfall in some areas, means we have regular flooding:
Flooding affects a wide cross section of the community, at times threatening people’s lives, property and agricultural land.
Our 0832 Infolines service provides information by telephone on readings collected at our various river level and rainfall recording sites. Information varies between sites but can include river level, rainfall, barometric pressure, wind speed, wind direction, air temperature and river flow.
Cyclones are strong depressions that form in the tropics. On average, one storm of tropical origin reaches Northland (at the top of the North Island) each year. New Zealand gets the strongest storms when we are between El Niño and La Niña weather patterns. They cause strong winds, high seas and heavy rain.
Check out the National Institute of Water and Atmospheric Research (NIWA(external link)) for information on how often tropical cyclones hit New Zealand.
An average of 20 tornadoes and waterspouts are reported throughout New Zealand each year, most of them small. Most are reported in the North Island from Waikato across to the Bay of Plenty and north. They move at random and can cause extensive damage to some structures, leaving others untouched.
Winter storm – the ‘Weather Bomb’
Winter storms are common in the region, with strong winds and heavy rain. They're not normally as destructive as the 'Weather Bomb' that hit the region on June 20-21, 2002, causing major flooding and wind damage, particularly on the Coromandel Peninsula.
Weather bombs form from the rapid deepening of a depression, where central pressure goes down by more than 24 hPa (hectoPascals) in 24 hours. These systems occur when the normal processes for developing depressions intensify. For example, when there are strong contrasts between air masses (cold or warm fronts) and when there's a release of energy when uplifting moist air forms clouds.
The deep and intense nature of 'weather bombs' leads to large areas of gale force winds and intense rain. This type of weather system can rapidly form over or near the North Island about once every two years, but few go on to develop into full-blown 'weather bombs'.
Climate change is expected to bring warmer weather and more floods and storms to New Zealand. Sea level rise could swamp low lying coastal areas. Salt from seawater coming inland may get into freshwater lakes, rivers and streams. It may also soak into the ground, affecting ground water supplies and leading to potential water shortages.
Find out more about how Climate Change affects our region’s natural hazards.
El Niño and La Niña are natural weather events - extremes of the ‘El Niño Southern Oscillation’ (ENSO) phenomenon. ENSO is the movement of air masses and ocean circulations, which cause small scale climate change lasting a maximum of two years.
During an El Niño period, trade winds weaken and nutrient rich currents off South America reduce. Fisheries become less productive. For New Zealand, El Niño means stronger and more frequent westerly winds in the summer. This may cause drought on the east coast and more rain in the west. In winter the winds will be more from the south, lowering temperatures.
During La Niña trade winds strengthen and blow westward across the Pacific Ocean, resulting in warm water in the Western Pacific around Indonesia. Meanwhile the cool nutrient rich currents rise up in the Eastern Pacific. For New Zealand this means more north-easterly winds, rain to the north-east of the North Island and lower rainfall in the south and south-west of the country. Central Otago and South Canterbury areas can get droughts in either situation.
Find out more about El Niño and La Niña(external link) in the National Institute of Water and Atmospheric Research’s education pages.
The Pacific Decadal Oscillation (PDO) (also called the Interdecadal Pacific Oscillation) (IPO), is a climate phenomena that is not fully understood. Unlike El Niño and La Niña that occur over a short time, PDO has an interval of 20-30 years.
Ocean temperatures change of a few degrees over a large area. When one large area of ocean drops in temperature while another increases, storms are more likely to come from slightly different directions than otherwise expected. Most studies have focused on how PDO has affected North America. However these changes could also be influencing our climate. PDO also appears to affect the productivity of ocean ecosystems.
For more information on the Pacific Decadal Oscillation, check out the University of Washington’s Department of Atmospheric Sciences(external link).
The Southern Oscillation Index is calculated from the monthly or seasonal fluctuations in air pressure between Tahiti and Darwin.
Sustained negative values of the SOI usually indicates an El Niño episode. For New Zealand, this means cooler temperatures (on average) due to stronger and more frequent south west winds. Rainfall generally increases in districts exposed to the south and west, and decreases in the north and east of the country. Tropical cyclone activity over New Zealand is less likely during El Niño events. The tropical cyclone season lasts from November through to April.
Sustained positive values of the SOI are usually associated with La Niña episodes. For New Zealand, this means warmer temperatures (on average) due to stronger and more frequent northeast winds. Increased rainfall occurs in regions exposed to the north and east, with decreased amounts in areas exposed to the south and west. Tropical cyclone activity is more likely near or over New Zealand during La Niña periods.
Find out more about the current value (state) of the SOI by visiting the NIWA website.(external link)
For policy information on natural hazards, see section 3.8 of our Regional Policy Statement.