MetService Blog

2024 Weather Recap

Met_Team — Wed, 08 Jan 2025 00:12:56 +0000

Compared to 2023, looking back on this past year may have felt like a relatively quiet year of weather for Aotearoa New Zealand. But digging into the details reveals many weather events that marked 2024. There were two Red Warnings issued, and a heavy rain event along the western South Island in April came remarkably close to being the third. Records were broken across the spectrum, spanning wet and dry, warm and cold.

January

The year began warm and humid, with a mostly settled weather regime keeping days hot and nights muggy. Between 19 and 23 January, Auckland experienced daytime temperatures of 27 to 30°C and overnight lows of 20 to 21°C. This unusual run of heat occurs roughly once per decade in the historical record and made for some uncomfortable nights. Elsewhere, Wellington Airport marked Anniversary Day (22 January) with its highest January temperature on record, 29.6°C. Meanwhile, MetService continued its summer trial of Heat Alerts.

Northwesterly winds that delivered the warm air also brought moisture to the western South Island. On the 18th, MetService issued the first Red Warning of the year for heavy rain in the West Coast Region, with 400 to 650 mm recorded over 48 hours in Westland ranges. Rivers rose to alert levels, with impacts felt even on Canterbury’s side of the divide.

In contrast, January was another dry month for eastern South Island areas after a parched December. Ashburton reported 40 mm of rainfall (January average 63.5 mm), while Blenheim tallied just 15 mm (January average 48 mm).

Wellington Airport reached 29.6°C on 22 January 2024, the warmest January temperature recorded there (records go back to 1972)

February

February’s active westerly pattern—characteristic of a waning El Niño summer—brought rain to the western South Island but left other regions dry. Many North Island locations recorded over 15 consecutive dry days (less than 1 mm of rain): Auckland Airport had a 21-day streak, and Gisborne went 23 days without significant rainfall. Similarly, Blenheim received its only notable rain at the start of the month.

Several northwesterly events also served to dry out soils and vegetation, and combined with the gusty winds, it was no wonder vegetation fires plagued the eastern South Island during this month. The most notable of these were the fires in the Lee Valley in Tasman and the Port Hill fires in Canterbury in the middle of the month.

Auckland Airport had a 21-day streak of dry weather (less than 1 mm of rain)

March

An unseasonable cold snap in early March brought early season snow and issued the year’s first Road Snowfall Warning for the Crown Range Road on the 5th—two weeks earlier than last year. Daytime temperatures in Southland and Dunedin resembled June averages, with frosts following on the 6th. The rest of March remained chilly, with major population centres including Auckland, Hamilton, Wellington, Christchurch, Timaru, and Dunedin recording temperatures around 1°C below normal. Auckland experienced its coldest March in 15 years, with an average 1.1°C below normal.

Temperatures in Dunedin at times were more reminiscent of June than March

April

The standout weather system of the month would turn out to be one of the more severe weather events of the year. Between the 9th and 12th, a slow-moving front laden with moisture stalled over Aotearoa, blocked by a high-pressure system to the east. The result was widespread heavy rain, gusty winds, and severe thunderstorms. The West Coast saw 500 to 800 mm in the ranges, triggering swollen rivers and slips that cut off roads. This event was what MetService meteorologists refer to in-house as a “Dark Orange” Warning, i.e. more than a normal Orange Warning and teetering on Red Warning status.

The rain had far-reaching impacts beyond the West Coast. Otago and Canterbury headwaters experienced significant rainfall, causing damage to bridges and infrastructure. Wānaka, Dunedin, Queenstown, and Blenheim all logged one of their top five wettest April days. Throughout the week, the weather map of Aotearoa was a patchwork of severe weather Warnings and Watches.

Following this turbulent spell, however, the remainder of April saw more settled weather apart from sporadic fast-moving fronts.

A severe weather event in the week of 8 to12 April came very close to having a Red Warning issued for Westland

May

If March started autumn on a cooler note, May echoed the cold weather sentiment to cap the season. Nationwide average temperatures were 1 to 2°C below normal, making it the coldest May since 2009. Southerly and southwesterly circulations brought severe frosts, with Christchurch dipping to -6.3°C on the 10th, giving May records a run for their money.

Overall, most of the country saw little wet weather. On the one hand, the eastern South Island continued its drier than average run, while on the other, aurora hunters around the country were able to take advantage of the clearer skies in the second weekend of the month to capture the rare phenomenon. However, a Tasman Sea low mid-month brought rain to the North Island and snow to the South. Auckland saw surface flooding, but rainfall helped replenish totals after a relatively dry start to the year.

Auroras were visible in many parts of Aotearoa New Zealand on 11 May 2024

June

Winter began mildly, with unseasonably warm conditions across the upper and eastern North Island, Otago, and southern Canterbury Plains. Many locations, including Kerikeri, Tauranga, and Timaru, recorded top-five warmest June days. Northerly winds driven by a low-pressure system to the west brought humid air and occasional fronts.

Later in June, rainfall increased in the eastern North Island. Gisborne Airport recorded its second-wettest day on the 25th with 125.7 mm, contributing to its third-wettest June on record (261.4 mm).

A sharp contrast in temperatures at 7am on 8 June where Kaitaia was 17°C while Pukaki was -5°C

July

One of the memorable weather features of July was a strong high pressure that settled over the country at the start of the second week. This strong high-pressure system saw barometric pressures exceeding 1040 hPa, a rare occurrence last seen in 2022.

This period of settled weather brought warmer than average temperatures to most of the country, but the rest of the month was starkly different. Orange Warnings for Heavy Snow covered the last few days of the month in Canterbury and Otago, with Watches for Southland.

High pressure of over 1040 hPa was recorded over Aotearoa New Zealand during July

August

August featured a mix of winter chill and dramatic weather. Frosty mornings began the month, with Christchurch recording -6.1°C on the 4th. Snowfall in elevated South Island areas closed roads like Lindis Pass in the second week and saw snow reach sea-level in the third week.

On the 12th, a memorable southerly blast swept up the east of the country, starting in Canterbury, reaching the capital around school pickup, and up the eastern North Island the rest of the day. This brought dramatic thunderstorms that turned day to night within minutes, hail, and a sudden plunge in temperature.

Late August saw frequent westerlies delivering heavy rain and strong winds. Pahiatua experienced significant flooding on the 18th, and Wellington saw slips and flooding on the 26th. Amid the storms, warm temperatures arrived in the east, with Napier Airport hitting 22.6°C on the 29th, its hottest August day on record. Volcanic eruptions at Whakaari/White Island added complexity, with ash disrupting flights in Whakatāne and Rotorua on the 22nd.

Whakaari/White Island saw ongoing eruptions from late July throughout August (image courtesy of GNS)

September

Spring westerlies prevailed in September, with several fast-moving fronts from the Tasman Sea. Four strong wind events marked the month’s first ten days, and a dry pattern persisted for central and northern regions. Whakatāne, Tauranga, Rotorua, and Kerikeri all recorded their driest Septembers.

In contrast, Milford Sound surpassed 1,000 mm of rain, which only occurs around 5% of the time, and with 202.2 mm of rain, Queenstown saw their wettest month since 1994.

Milford Sound recorded over 1,000 mm of rain during September 2024

October

October began with the year’s second Red Warning, the first ever for eastern Otago and Dunedin. Following a wet September, persistent heavy rain overwhelmed the region. Oamaru and Dunedin received over two months’ worth of rain in a couple of days. This prompted a state of emergency and widespread impacts such as road closures, damage to properties, evacuations, slips, and flooding.

Late October brought a significant snow event to the South Island, with Mount Cook Village seeing roughly 50 cm of snow. In stark contrast, on the same day (the 26th), Whanganui Airport saw its second-warmest October day at 24.1°C.

Over 50 cm of snow was measured at Mount Cook Village on 26 October 2024

November

November was marked by dryness. High pressure over the northern parts of the country kept weather settled, while systems weakened before reaching the north and east. Ashburton and Taumarunui recorded their third-driest Novembers, while Napier experienced its driest spring in 70 years. The month ended with a preview of summer, as temperatures climbed into the high 20s, with Napier Airport exceeding 30°C—its earliest since 2019. Meanwhile, Cantabrians welcomed the improved Canterbury Radar after being turned off for nine weeks while having 30-year old technology upgraded.

The Canterbury Radar went live on 28 November after an upgrade to replace 30-year-old technology

December

December began on a warm note, with the first heat alerts of the summer issued as the month got underway. Blenheim stood out as one of the hottest spots, reaching 32.5°C on the 5th—its second-warmest December day on record.

While colder bursts made occasional appearances, much of the country experienced above-average temperatures throughout the month. This warmth was bolstered by a return of northerly airflows in the latter half of December.

The second half of the month also brought a significant shift in rainfall for the east of the North Island. After a notably dry start, with parts of Central Hawke’s Bay reporting drought-like conditions, wet weather returned as rain flowed into the region from the east. Gisborne Airport recorded over 200 mm of rain, marking its wettest December since records began in 1937.

Temperature records broken in 2024

Rainfall records broken in 2024

As we move into 2025, the unpredictability of our climate reminds us to stay prepared and keep an eye on the skies. Our team of meteorologists will continue working hard to bring you the best, most accurate weather information, helping you stay informed and ready for whatever conditions lie ahead. Happy New Year - Ngā mihi o te Tau Hou!

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2024

wrap-up

Modelling the weather

Met_Team — Thu, 21 Dec 2023 01:02:33 +0000

Accurate weather forecasting requires three key ingredients – trustworthy observations, reliable computer models and experienced meteorologists.

Before the computer age and the satellite era, forecasters were reliant on weather observations from land stations, ships, and balloon soundings from which they could predict the movement of high and low-pressure systems and fronts.

Computer modelling has completely changed the forecasting game during the past three or four decades. Most of the MetService forecasts you read on the website or see elsewhere are the result of highly trained meteorologists studying a range of different models and deciding which ones, or which combination of them, are picking the most likely future weather outcomes.

These computer models have an endless appetite for crunching mind-blowing amounts of data.

(Above: One of the many equations used to help model the atmosphere)

How do they do it?

Observations are taken from as many sources as possible, right across the planet. All these figures are fed into the models, which use complex mathematical equations reflecting the physics of the atmosphere to produce simulations of its possible future evolution.

Each model has its own personality, its own strengths, and weaknesses in covering our part of the world. This is where the experience of the forecasters and their geographical knowledge comes in, determining which weather situation from which model is the most likely to come true and how it will be influenced by local conditions and features such as Cook Strait and the Southern Alps.

At MetService, we use several of the most-respected global weather models to aid our forecasting.

Probably the most consistent is the European Centre for Medium-Range Weather Forecasts (ECMWF) model, generally recognised as the best-performing of the global models. As well, we consider runs of the UK Met Office model, which is very good when it comes to predicting the development and movement of mid-latitude weather systems.

The GFS model, generated by the National Centers for Environmental Prediction (NCEP) in the United States, is also consulted, although this is perhaps not as steady and reliable in this part of the Pacific as the ECMWF model. On top of that, we get modelling from meteorological services in Canada and Japan, and from the Bureau of Meteorology in Australia.

We also run our own in-house models to simulate the turmoil of the atmosphere and can operate these at much higher resolutions to reflect the idiosyncrasies of Aotearoa New Zealand’s landscape. We can run these with data points just 200 metres apart, which takes into account the different terrain of an area and whether it is forested or urban, which can make a significant difference to temperature and winds across a short distance.

Are they any good though?

The models themselves are constantly being tweaked by their operators to reflect the latest research findings and to keep up with new technology. The users of the models are also changing what they require from them.

In recent months there has been some discussion about whether climate change may be affecting the running of the models and especially their outcomes. Is it possible that an increasing number of more extreme events are pushing the models beyond their comfort zones into unknown territory?

The jury is still out on this. One thing that is for certain is that the basic physics of the atmosphere, on which the models are constructed, has not changed.

What has changed, though, is how often the conditions that lead to extreme weather are now occurring in our warming world. So, the models which are really good at picking bog-standard weather are being asked to deal with potentially severe weather situations more often.

What about Artificial Intelligence?

Like in any other industry, it is hard to predict just how machine-learning and artificial intelligence will affect weather forecasting.

AI-informed weather models have recently been found comparable to, or have even out-performed, physics-based models. But these AI-informed weather models aren’t coming in cold – they are first ‘trained’ on the physical models and then add a layer of prediction on top.

Then of course there remains the task of interpreting what the output of such a system might mean for the activities or safety of end-users.

At MetService we continue to work with the operators of modelling systems to ensure we get the best from them, while continuing to develop our in-house models. We have a keen eye on developments in the AI field and the opportunities it will bring. At the same time, we continually improve our forecasting expertise, which includes an understanding of the strengths and weaknesses of any modelling system.

By doing so, we will be in the best position to identify the risks future weather may bring, and how these are best communicated to those concerned.

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maths

A close eye on the weather

Met_Team — Thu, 21 Dec 2023 00:51:39 +0000

We all look out the window to see what the weather is up to. In a similar way, MetService meteorologists are always checking to make sure what they have predicted is actually happening.

The forecasters may not be examining the skies themselves every few minutes, but they are monitoring a network of instruments across Aotearoa New Zealand, including those sited on our sub-tropical and sub-Antarctic islands, which are constantly feeding back crucial information to the National Forecasting Centre.

Wind direction, wind speed, air temperature, humidity, rainfall, air pressure, cloud, and visibility, etc. – all these observations and data help build a picture of what is going on. But they can’t just be measured from anywhere.

For example, temperature readings taken above or next to a concrete or asphalt surface in a confined space will read several degrees higher than those taken in a grassy location which is better ventilated. While those exaggerated temperatures may be a true reflection of that super-heated spot, they do not represent those of the entire neighbourhood.

Accurate and reliable weather observations that are consistent with the standards of the World Meteorological Organization from around the motu are the bedrock on which good forecasts are based. MetService has a dedicated team who ensure that all our automatic weather stations operate to world standards, and that the observations from our observing network are quality controlled and delivered reliably to the forecasters.

Precise observations do more than tell meteorologists if the weather is unfolding as they had forecast in a certain situation. They are essential as an input to start the computer models which use complex mathematical equations to predict how weather systems will move, form and decay. In other words, the observations are critical to describe the weather now so that models can accurately step forward in time providing our forecasts and warnings. Most importantly the data keeps the computer weather models ‘true’ and in line with what is actually happening with the weather.

Thinking in 3D

We know the Earth’s atmosphere is not two-dimensional, which means observations of temperature, wind and air pressure are not just needed from the surface of the Earth but also right up through the lowest slice of the atmosphere, known as the troposphere, which contains nearly all of our weather.

At Aotearoa New Zealand’s latitude, the troposphere reaches an altitude of about 12km above the surface. Within this layer, large weather balloons carrying radiosonde equipment take regular measurements during their ascent. MetService operates four upper-air sounding stations, at Invercargill, Paraparaumu, and Auckland and one in the Kermadec Islands 1000km northeast of Auckland where these balloons are released once or twice a day.

Pilots flying aircraft at high altitude also pass on observations of wind, temperature, and turbulence to assist meteorologists with their predictions.

Even higher up, weather satellites provide important measurements of moisture and temperature in the atmosphere. MetService meteorologists have access to data from the Japanese geostationary satellite Himawari-8 which effectively provides a photo from space every 10 minutes, so they can see how weather systems develop and move.

On the ground and at sea

MetService has an extensive weather radar network of 10 radars, from Kerikeri in the north to Invercargill in the south. Radars are extremely useful tools for monitoring the movement and intensity of rain, hail, and snow, and are particularly valuable when lives are at risk from severe thunderstorms or for immediate information of rainfall rates in an area with no surface observation. Radars are also vital when tracking a severe thunderstorm cell when the heavy downpours from it could cause flooding.

To complement radar information, forecasters use observations from Transpower’s National Lightning Detection Network to pinpoint where lightning from the severe thunderstorms reaches the ground.

It’s not only the land and the sky which MetService has covered. Ships which travel around our coasts and to and from Aotearoa send through useful observations of the weather conditions out at sea. And a network of drifting buoys in the Tasman Sea provides vital data on developing systems before they reach our shores.

Our observations

Until a few decades ago, surface observations were made manually by people around the country, on farms, at airports, in lighthouses and the like. Some of MetService’s stations have been in operation for more than 100 years – the longer a consistent record the better from a climatological perspective.

Now, most observations come from more than 220 automatic weather stations (AWS) in all corners of Aotearoa New Zealand, which provide data every 60 seconds and are extremely efficient, operating more than 99% of the time. In addition to being used by weather forecasters, data from these stations also feed into the National Climate Database.

As well as operating its own AWS network MetService also partners with other organisations that have networks of stations. MetService also has more than 50 AWS units monitoring road conditions for Waka Kotahi on our state highways and provides Airways Corporation with airport weather conditions at airport control towers across the country.

As well as the observations keeping meteorologists up to date with what they are dealing with, they assess how well the global computer models are performing in the Aotearoa New Zealand region.

We need you!

There’s one other source of weather observations we haven’t covered yet – you! Public feedback and photos via social media and the

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observations

radar

The art of weather forecasting

Met_Team — Thu, 21 Dec 2023 00:42:04 +0000

Some may think it’s one of those jobs which anyone might turn their hand to, but the difficulties of weather forecasting are hugely underrated.

After all, there aren’t many jobs where you’re expected to predict the future. Economists are also required to make forecasts, but at least they have some control over the system they are foretelling. When it comes to weather, meteorologists have no such influence.

The best weather forecasting is a mixture of science, geography, and experience.

The science of meteorology is extremely complex, involving difficult atmospheric physics and chemistry, and convoluted mathematical equations. As well as having a higher degree in physics or maths, a meteorologist must know a great deal about the geography of Aotearoa New Zealand and how the prevailing westerly winds at our latitude push over and around mountains and through narrow sea straits, influencing our weather.

There is no substitute for experience in forecasting and for having seen how a certain weather pattern has affected the country in the past. This base of knowledge and skills builds insight that comes naturally after years in the job and cannot easily be taught.

(Above: Metservice meteorologists working at the National Forecasting Centre)

Simplifying the complicated

MetService forecasters work every hour of every day of the year to interpret the complexities of the atmosphere above Aotearoa New Zealand and communicate this clearly and simply to different audiences.

Our highly trained staff work tirelessly to ensure New Zealanders are kept safe and that communities can be as resilient as possible in the face of severe weather. People need to be able to trust the weather information they receive to enable them to make the best decisions they can.

One of the biggest tasks for meteorologists these days is analysing which of the suite of computer models they receive from around the world is producing the most likely weather map on which to base their forecasts.

Technology may be crucial to our operation but the forecasts themselves are still the result of a great deal of human effort and acumen. Our meteorologists interpret the models and all the observations made around the country, and apply their knowledge of a region’s topography, aspect and broader physical characteristics before settling on a forecast.

Knowing when to trust the models

There are curve-ball occasions when the models appear to go rogue, predicting tropical cyclones or icy southerlies with snow right across the motu / land. This is when meteorologists have to be at the top of their game – knowledgeable and confident enough to ignore such over-the-top predictions. After all, the models are just a guide from which forecasts are made.

The MetService forecasting team meets for about half an hour every morning online, just before 10am, to discuss the weather situation and which of the global models seem to be going down the right track. Another briefing is held about 10.30pm for the night-shift forecasters.

The weather briefings were known as “nodding”, a tradition which started more than 60 years ago. In those days, the chief forecaster’s views were unchallengeable, and everyone else would just “nod” in agreement.

These days, the meetings in the National Forecasting Centre in Wellington are called “map discussion” and are much more democratic. One of the senior meteorologists will choose which model is telling the most likely story or, if there is no clear guidance, will consider which mix of models appear to work. This is where observations, backed up by radar and satellite imagery, are vital.

Discussions follow and, from those, teams of meteorologists then write forecasts for different customers.

Tailoring our forecasts

The aviation team provides forecasts for airlines, general pilots and airports around New Zealand, and also acts as the Volcanic Ash Advisory Centre, should a volcano erupt.

Marine forecasters warn of high winds and bad weather in our inshore areas and across a huge area of oceans around the country.

Specialised severe weather meteorologists focus on heavy rain and snow, high winds, and thunderstorms which could cause damage and threaten life and livelihoods. During the tropical cyclone season (November to March), they also monitor the development of such storms. These forecasters work closely with emergency management and local and regional council staff when the worst weather is on the way.

Meteorologists working on the public and weather communications sections make the day-to-day predictions and generate the forecasts and weather graphics most of us see on the MetService website, or on the app on our phone, or on the television and elsewhere in the media.

To be a MetService meteorologist, you need a degree in either maths or physics and then to have completed a post-graduate meteorology programme which aligns with the World Meteorological Organisation’s Basic Instruction Package for Meteorologists. This ensures all meteorologists around the globe are adequately skilled.

In Aotearoa New Zealand, the meteorology programme is run in conjunction with Te Herenga Waka Victoria University as a Master of Meteorology degree.

Next time there’s rain threatening, a storm is on the way, the wind is picking up, or it’s just a lovely sunny day, you can have confidence that highly trained MetService meteorologists are providing the best possible forecasts to keep you safe and in the picture.

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Forecasting

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trainee meteorologists

Tropical Cyclone Gabrielle – Event summary February 2023

l.ferris — Sun, 19 Feb 2023 01:20:39 +0000

Tropical Cyclone Gabrielle

Prepared by Lisa Murray, Head of Weather Communications at MetService

Tues 14 Feb image: A true colour satellite image with the centre of Cyclone Gabrielle north of the Bay of Plenty. The white is the cloud and note the shadow cast by the cloud on the western side of the image.

Few weather events are as terrifying or as powerful as a tropical cyclone. MetService have tropical cyclone specialists in its National Forecasting Centre who always have their eyes on the Tropics and issue appropriate and timely warnings based on expertise, weather observation network and weather modelling tools.

Head of Weather Communications Lisa Murray says, “Gabrielle is one of the worst storms to hit Aotearoa New Zealand in living history. Like Cyclone Bola in 1988, Giselle that caused the Wahine disaster in 1968 and the unnamed cyclone of 1936, Gabrielle caused shocking impacts to the North Island."

Between the 12th and 14th of February, parts of Aotearoa New Zealand recorded rainfall amounts of 300-400mm, wind gusts of 130-140km/h and waves were recorded as high as 11 metres along some of our coasts. It is no wonder that the North Island experienced the devastating impacts that have been seen from Cyclone Gabrielle.

By the numbers.

Rain:

o Whangārei had the largest 24-hour rainfall recorded for any weather station for this event. 183.8mm was recorded from 9am 12 February – 9am 13 February. However, power and communications outages affecting some of our weather stations mean that some places may have seen much more than this.

o Over 200 mm of rain was recorded around the Auckland region with Waitakere recording the most at 248mm.

o Gisborne Airport recorded 185.3mm until power and communications ceased at 2am Tuesday.

o Napier Airport recorded its wettest February day from 9am 13 February – 9am 14 February recording 175.8mm which was also its second wettest day of all time (since records began in 1950).

o Gabrielle has contributed to the wettest start to the year since records began for Whangārei, Auckland, Whitianga, Tauranga, Gisborne, and Napier airports.

o Whangārei, Auckland and Napier have had over five times more rain than their average for the first 46 days of the year. Gisborne has had four times more rain than their average.

Monday 13 Feb image: Rain swamps the North Island as seen by the blue and yellow radar imagery with a cluster of thunderstorms over the Coromandel Peninsula.

Wind:

o As Cyclone Gabrielle started impacting the North Island on 12 February, Cape Reinga weather station recorded a gust of 141km/h. This was an indication of things to come.

o As the system moved south and east, many areas were impacted by gale force constant wind as well as extreme gusts.

o Whangaparāoa recorded a gust of 132km/h on 13 February.

o Auckland Harbour Bridge recorded a gust of 115 km/h.

o New Plymouth Airport had a gust of 128km/h on 14 February, which was the fourth strongest gust in fifty years of records at that station.

Waves:

o The biggest wave recorded during the event was 10.9m in the Bay of Islands.

o Storm surge is the abnormal rise of water generated by a storm, over and above the predicted tides. Storm surge of over 0.5 metres was experienced in places along the northern and eastern coasts of the North Island as well as parts of the South Island as the system moved away to the east.

o Coastal inundation and flooding were amplified due to storm surge associated with the cyclone – either by directly inundating the coastal area or by reducing the efficiency of the rivers draining into the sea.

Cyclone track map showing the forecast path of Gabrielle as at 13 February 7am. Noting the sharp deviation in the track near Great Barrier Island.

Why was Tropical Cyclone Gabrielle so impactful?

Well before this event, when meteorologists at MetService were asked “What would be the worst weather scenario to impact Aotearoa New Zealand?” A tropical cyclone on the track that Gabrielle took was what was described! One that hit a large number of regions including Auckland. The sheer number of people and urbanization exacerbate any extensive weather event.

The difference between this cyclone and others is the track Gabrielle took, along with warmer sea surface temperatures and an atmospheric environment that encouraged tropical cyclone growth.

Tropical cyclones only form over sea surface temperatures of greater than 26°C. Tropical Cyclone Gabrielle was named by on 8 February 2023 in the Coral Sea and at that time the ocean sea surface temperature was warmer than normal along the corridor from the Coral Sea to Aotearoa New Zealand.

This is likely to have contributed to Gabrielle maintaining its intensity a little longer than normal, and likely contributed to the system holding onto slightly more moisture than normal. Travelling through this warmer environment helps the tropical cyclone gather moisture which fuels more rain.

Days ahead of this event the weather models aligned their projections of the forecast track of Gabrielle towards Aotearoa New Zealand. This gave MetService meteorologists confidence that Gabrielle would impact Aotearoa New Zealand.

The key to how impactful Gabrielle was going to be, lay in the curve of the track just north of Aotearoa New Zealand. At this point, higher up in the atmosphere a passing weather feature (trough) lined up to give extra energy to the cyclone and this helped steer it towards the top of the Coromandel Peninsula.

On this more southern direction the center of the cyclone moved closer to Great Barrier Island, where it sat at midnight Monday 13 February, driving wind and rain into Northland, Auckland and the Coromandel Peninsula. After this, the track changed southeasterly and headed towards the East Cape above Gisborne, driving strong winds and torrential rain into the east coast of the North Island, before moving east on a track far north of the Chatham Islands.

The forecast accuracy of Gabrielle’s track was impressive given the difficulty of predicting where tropical cyclones may go. This particular track, combined with how expansive the cyclone was and how deep the central pressure of the cyclone was – is why Gabrielle was so destructive.

As Cyclone Gabrielle passed Aotearoa New Zealand it had an extremely low central pressure of 966.6hPa as recorded at Great Mercury Island. The lower the pressure, the more intense the winds and therefore the greater the impacts of the storm.

The very low pressure of Gabrielle combined with the size/width of the cyclone (covering hundreds of kilometres), spread strong winds across a huge part of the North Island all at once (for example there were gales in Northland and Wellington at the same time).

This extremely low central pressure also influenced the oceans around Aotearoa New Zealand. It raised the level of the ocean at our coast and combined with energetic swell, storm surge, onshore gales and large accumulations of rain literally caused inundation and impeded the rivers outflow to the seas.

The impact of Gabrielle was exacerbated by extensive rainfall on the 27 January hitting the northern areas (in particular Auckland, Northland and Coromandel Peninsula). This left these regions in a vulnerable state so the rainfall and strong winds from Gabrielle had a more extensive impact due to the sodden ground.

When did we first know Gabrielle was threat?

Ten days ahead of its impacts being felt in Aotearoa New Zealand, MetService’s tropical cyclone specialists first raised the potential of a tropical cyclone developing in the Coral Sea. From that point it was monitored more closely to see if it would form. As it developed MetService shared this information with the public and authorities.

The cyclone formed over open waters south of the Solomon Islands and was officially named by Australia’s Bureau of Meteorology (who are the Tropical Cyclone Warning Centre with responsibility for naming cyclones in the Coral Sea) on 8 February. As the system moved closer to Aotearoa New Zealand, and in the early hours of 11 February, the responsibility for forecasting the track of the tropical cyclone was handed to Tropical Cyclone Warning Centre Wellington which is run by MetService.

MetService began issuing Severe Weather Watches for Heavy Rain and Strong Wind from 9 February, followed on the 11 February by a number of the Watch areas being upgraded to Orange Warnings. The first of a swathe of MetService Red Warnings for wind and rain associated with Cyclone Gabrielle were issued that afternoon.

A total of five MetService Red Warnings for Rain (Northland, Auckland, Coromandel, Gisborne, Hawkes Bay) and four MetService Red Warnings for Wind (Northland, Auckland, Coromandel, and Taranaki) were issued over the course of the intensive three-day event.

How it formed.

Cyclone Gabrielle originated as a small low north of Fiji in early February. Although initially weak, it moved slowly westwards into the Coral Sea (south of the Solomon Islands) during Waitangi weekend where conditions were very favourable for the low to intensify into a tropical cyclone.

Three of the key ingredients that helped the tropical cyclone’s formation were:

1. very warm sea surface temperatures in the Coral Sea of around 30 °C

2. abundant moisture

3. light winds higher up in the atmosphere.

Even at this early stage, there were indications that it might track towards the upper North Island after becoming a tropical cyclone, although the exact path was uncertain.

Can we expect more events like this one?

While climate scientists believe that the number of ex-tropical cyclones affecting Aotearoa New Zealand might remain about the same or perhaps even decline, it is expected they will become more severe as the planet continues to warm. That will mean tropical cyclones will be of a higher category bringing stronger winds, heavier rain, and more problems from wind damage, flooding and storm surge.

The Southern Hemisphere season for tropical cyclones is normally 1 November to 30 April. The South Pacific can expect on average about nine tropical cyclones a year but there are exceptions. Of this nine, on average usually one affects Aotearoa New Zealand. There have been years when we have seen more than that – as has happened in the last two months with Cyclone Hale and now Gabrielle.

We are currently in a La Niña phase, which is conducive to tropical cyclones forming in and around the Coral Sea near Vanuatu and New Caledonia. This is a breeding ground for many of the cyclones that have impacted Aotearoa New Zealand historically.

There is a likely regime change from La Niña to a neutral climate state over the next month. There won’t be a tangible change to our weather maps through the rest of this summer whilst the atmosphere catches up.

MetService meteorologists are working 24/7, 365 days of the years monitoring the weather around Aotearoa New Zealand and in the Tropics to ensure New Zealanders have timely, useful, and accurate information to best prepare for these impactful events.

MetService National Forecasting Centre in Wellington

Details per region

Many areas of the country were hit hard by this extreme weather event, here are some numbers illustrating just how much rain and wind was experienced across the North Island and the top of the South Island (from 12am Sunday to 7am Wednesday):

Northland:

250-400 mm of rain was recorded across the region with the largest amount recorded at Glenbervie Forest (408.7mm). Whangārei Airport received 320.4mm. Wind gusts over 130 km/h were recorded in the most exposed stations with 141 km/h recorded at Cape Reinga. Whangārei Airport recorded a gust of 102 km/h.

Auckland:

Over 200 mm of rain was recorded around the region with Waitakere recording the most (248mm). Wind gusts of up to 130-150 km/h were recorded at the most exposed sites. Auckland Harbour Bridge recorded 115 km/h and the airport recorded 78 km/h.

Coromandel Peninsula:

The Pinnacles station in the ranges recorded 447mm of rain and other elevated stations received 220-270 mm. Whitianga recorded 170.6 mm. Golden Valley (near Waihi) experienced wind gusts of up to 109 km/h.

Bay of Plenty:

The highest rainfall recorded in the region was at Queenshead (163.5mm). Tauranga recorded 122.7 mm, Rotorua 86.1 mm, Whakatāne 82.8 mm, and Te Puke 78.6 mm. Wind gusts recorded in the area were between 80-90 km/h.

Tairāwhiti / Gisborne:

The largest rainfall accumulation was recorded in Hikuwai where 488 mm of rain fell. Several other stations (mainly in the ranges) recorded over 400 mm. Gisborne Airport received 185.3 mm (although due to power cuts, it stopped recording after 2am Tuesday). Wind gusts were recorded across the region up to 93 km/h.

Hawke's Bay:

Glengarry received nearly 540mm, and Pukeorapa receiving over 400mm of rain. Napier Airport recorded 203.8 mm and Hastings recorded 143.8 mm. Wind gusts were recorded up to 90 km/h with exposed stations near the coast reaching higher such as Cape Kidnappers (131 km/h).

Wairarapa:

The most rainfall was recorded at elevated stations in the ranges, between 180-280 mm. Masterton Airport received 62 mm. Exposed stations near the coast recorded high wind gusts - Cape Turnagain recorded 146 km/h and Castlepoint recorded 113 km/h.

Taranaki:

Rainfall was moderate with 30-45 mm recorded across the region, but the wind was very strong - New Plymouth Airport recorded gusts of up to 128 km/h.

Whanganui:

Rainfall of 60-90 mm was recorded around the region and gusts of 98 km/h were experienced at Whanganui Airport.

Wellington:

Highest accumulations were recorded in northern and eastern parts of the Hutt Valley (100-160 mm). Mt Kaukau (an elevated site) recorded the highest gust of 146 km/h, around the city gusts of up to 100 km/h were experienced.

Marlborough:

Snowflake Ridge (a very elevated site) recorded 230 mm of rain. Other stations around the region recorded up to 100-130 mm. Kaikoura received 59.8 mm and Picton received 59.5 mm. Cape Campbell recorded wind gusts of up to 93 km/hr and Kaikoura recorded 65 km/h.

Tags:

#Gabrielle #Tropicalcyclone

What's in a name?

Met_Team — Wed, 18 Jan 2023 21:48:49 +0000

What do Irene, Osai, Tam and Xavier all have in common?

The answer is these are all feature on the list of names for upcoming South Pacific forecasts. But who gets to pick the names and more importantly why do we name Tropical Cyclones?

Giving Tropical Cyclones a name helps meteorologists track a system as it moves across large areas of ocean. These systems can bring severe weather to large areas and impact several countries so keeping track of where they are is really important. The name means that the many different weather agencies in different countries know they are all looking at the same weather system.

A name also makes communicating the risks associated with the tropical cyclone much easier to get across to the public and other forecast users (like pilots and sailors), especially when there is more than one tropical cyclone happening at the same time.

In the past, cyclones (or their northern hemisphere cousins Typhoons and Hurricanes) have been named after things like the saint’s day that they arrived, the latitude and longitude and date they formed or even political figures. Imagine being on a boat in a huge waves and gales trying to listen to the forecast; “Tropical Cyclone Hale” is a lot easier to hear than “Tropical Cyclone 17.5S 160.5E”

Nowadays, the names are agreed in advance by the various meteorological agencies around the globe that have the responsibility for monitoring different parts of the tropics and issuing forecasts for anything that form. Find out more about these different areas at bit.ly/TropicalCycloneInfo

Tropical Cyclone centres around the world have several lists of names which they work through alphabetically - starting with an A-name, then the next Tropical Cyclone gets a B-name, and so on. The most recent TC named by the Fiji MetService was Irene, meaning the next storm to develop will get a name that starts with J. Eventually we’ll get to the end of the lists and start back from the beginning. However, if a Tropical Cyclone has been especially powerful or has caused a lot of damage the name will be retired from the list and a new name picked in its place.

Tags:

tropical cyclone

severe weather

Understanding Tropical Cyclone and Cyclone Track Maps

Met_Team — Tue, 10 Jan 2023 02:00:40 +0000

Understanding Tropical Cyclone/Cyclone Track Maps

Tropical Cyclone (TC) season runs between November 1st to April 30th for New Zealand, and Cyclone Hale is the first ex-TC to affect us this season. MetService’s duties during TC season include publishing track maps, which detail where a Tropical Cyclone has been in the past, and where we expect it to be in the future.

Here’s an example of a track map for Cyclone Hale, issued at 8:36am Jan 10th 2023.

There’s a lot of information packed into this one image. Here’s how to understand what’s going on.

Part One: The Past

This section of the track map shows the past. The black line on the map shows the path that the centre of the tropical system has taken, with the points on the line showing the position of the centre every six hours. Times are marked in local New Zealand time, which is the (UTC+13) timezone, and are given as twenty four hour times.

Points labelled with a number indicate the category of the system at that point. Find out more about TC categories . Remember to stay up to date with the forecasts; more watches and warnings may be issued as the system moves closer to New Zealand.

Why don’t you always have a track map when there is a Tropical Cyclone?

The meteorologists at MetService only issue track maps when the system moves into New Zealand's area of responsibility. You can find more information about the other Tropical Cyclone Warning Centres

2022 is one for the record books

l.ferris — Fri, 30 Dec 2022 00:16:11 +0000

With 2022 coming to an end, we thought it’d be timely to look back at the year of weather. Two words come to mind, wet and warm. Low pressure systems from the northern Tasman Sea were frequent around Aotearoa and brought a rise in both the temperature and the amount of rain that fell. Read on to find out more about MetService’s notable weather of 2022.

Severe Weather
Firstly, we had 53 Severe Weather Events this year and issued 182 Severe Weather Warnings, that is about an average amount, but we had several events which were far from average. We introduced our colour coded warning system in May 2019 and have issued eight Red Warnings since, four of those were this year.
The beginning of February saw incredibly heavy rain impact Westland, Buller, and Taranaki. 500-800mm of rain fell in 72 hours around parts of Westland. Evacuations were required in Buller and a state of emergency was declared. When the rain made it to Taranaki it was very intense with a station near Cape Egmont receiving a month’s worth of rain in just four hours.
In the last week of March, the flooding rain took aim at Gisborne with extensive damage sustained across the region including the Tokomaru Bay bridge being washed out, isolating communities. Gisborne Airport recorded a month’s worth of rain in seven hours and parts of the region ticked up 200-300mm over the event. A state of emergency was declared for the Tairawhiti region.
A few weeks later Cyclone Fili approached New Zealand and we issued another Red Warning for Gisborne. While this event may not have been a Red Warning on its own – for a region in recovery mode we knew significant impacts would come around more readily. 150-200mm of rain fell in 24 hours and there were power outages, toppled trees, flooding, and more communities isolated.
The middle of August saw the extreme rain move back west, with another flooding event for Westland, Buller, and Nelson. Some stations in the ranges of Tasman recorded over 1,000mm of rain. A state of emergency was declared in Nelson, Westland, and Marlborough. Our station at Nelson Airport ticked up four months of rain in three days and August came in as the second wettest month in 80 years of records at that station.

High and dry to start
Those were the four Red Warnings but, the year started out on a much more settled note. High pressure was situated around our shores in January and prolonged dry conditions spread from Auckland to Whanganui. Our Whanganui station received 0.2mm, making it the driest month in over 50 years of records. Our New Plymouth Airport station had 9.4mm, the driest January there since records began in 1944.

Flip the script February
With a dry January in the bag, Mother Nature decided we needed a bit of rain. Cyclone Dovi arrived from the north and brought record-breaking rainfall to Taupō with our station clocking up 118.8mm in a day, the wettest 9am-9am there in almost 50 years of records. New Plymouth Airport recorded their 4th wettest day. The rain made it to the South Island also with Westport Airport setting its 1st and 5th wettest days but also smashing its record wettest month in almost 80 years. 609.8mm up from 453.4mm from April 1957.
It wasn’t just rain records that fell in February, our tropical visitors also brought some steamy temperatures. Whanganui Airport got up to 32.7°C on the 10th of Feb setting a new maximum temperature for that station in the 44 years of records. The heat remained overnight from Northland to Queenstown Lakes with Whangārei Airport only dropping to 23.8°C overnight, setting a new record there. Kerikeri, Auckland, Rotorua, Ashburton, Wanaka and Queenstown Airports all broke their overnight records.

Winter thunderstorms
Getting closer to the middle of the year the temperatures dropped off, but we saw an increase in thunderstorm activity. A tornado ripped through parts of Levin on the 20th of May and other tornadic systems were seen around southwestern parts of the North Island. Thunderstorms were frequent around that time and into June with the lightning detection network we use picking up over 111,000 strikes in the seven days up to the 13th of June.
MetService meteorologist Lewis Ferris says, “I can’t remember being woken by lightning so much in the last five years living in Wellington.”

A winter for the record books
Moving on into July and we were back into record rainfall territory. Our station at Christchurch Airport hasn’t exceeded 200mm of rain in a month since it was installed in 1943. This July clocked up 309.8mm – five times its average July rainfall. Blenheim and Timaru also had their record wettest month.
After the August Red Warning it may come as no surprise that our station at Nelson Airport decimated its previous winter record of 570mm set in 1970. They ticked up 701mm and exceeded their winter average rainfall in four consecutive days in August alone (17th – 20th). Other central areas also broke winter records: Blenheim Airport, Wellington Airport, Paraparaumu Airport, Levin and our station in Kelburn (which has observations back to 1928). Moving north; Palmerston North, New Plymouth, Taupō and Rotorua airports all hit their wettest winters on record. It wasn’t just northern regions of the South Island that were unusually wet; Christchurch, Timaru, Wanaka, and Hokitika airports all set new winter records, with Hokitika recording a whopping 1142mm – more than 400mm above their winter average.
Ok, so it was a wet winter, that’s a step towards making a good snow season right? Well, yes that was true, just not everywhere. A lot of our big rain makers came from the north this winter. This means they brought heaps of moisture which is good for snow making but they also brought warmer temperatures which was bad for the skifields that it hung around. This is exactly what happened for the North Island fields, they got all the moisture but also the warm air. The South Island fields were lucky enough to get some good southerly outbreaks meeting the warm air from the north so managed to receive a few decent dumps of snow.
Staying on the snow tune, we had an Antarctic outbreak early in October which saw snow settle on beaches from Stewart Island to Christchurch with flakes captured by a few of our meteorologists here in Wellington.

Northern rain November
November was another unsettled month with more low pressure systems out of the northern Tasman Sea providing us with humid conditions and plenty of rain. Whangārei airport recorded 367.0mm of rain, adding over 100mm to the previous wettest November there in almost 80 years of records.
December has seen us issue Severe Thunderstorm Watches almost every other day with plenty of downpours popping up from Northland to Southland. Over 140,000 strikes have been detected on the network this December.

What about 2022 as a whole?
Our station here in Wellington looks to have had its wettest year since records began in 1928. Our stations in Taupō and Blenheim also appear to have made the mark. Taking the sum of rainfall across our stations in the six main centres also puts 2022 as the wettest in the last 30 years. Hamilton, Tauranga, Wellington, and Christchurch were the wettest in the last 30 years, Auckland came in fifth, but Dunedin bucked the trend and was below average.
How about the temperatures? The average mean temperature across the six main centres also came in as the warmest of the last 30 years. 2022 was the warmest over the last 30 years for our stations in Auckland, Hamilton, Tauranga, and Wellington. Christchurch and Dunedin were a wee way down the pack but still above their 30-year average.
Maybe you’ve been paying attention to the weather this year and none of that comes as a surprise. Well, we’ve kept one card up our sleeve. Our station at Milford Sound Airport came in below average rainfall! Maybe the one instance where we can say a place ‘only’ recorded around 5,300mm.

If there's one thing we can guarantee for 2023 is that New Zealand will see some interesting weather and the team here at MetService will be with you 24 hours a day, seven days a week, 365 days of the year keeping you updated with all the latest information and forecasts, so you can make informed decisions and remain safe.

Happy New Year - Ngā mihi o te Tau Hou!

Swell Period

l.ferris — Wed, 10 Mar 2021 01:25:52 +0000

What is swell period and why does it matter?

From late September 2020, MetService’s coastal forecasts have included information about swell period. What’s all the fuss about you may ask? Isn’t the height of the waves the most important piece of swell information? Although wave height is important, period is too. This blog post explains what swell period is and why it matters.

How does swell develop?

When wind blows over bodies of water it generates waves. However, although the initial impulse for a wave is the wind, it is the restoring force of gravity on the water that allows the wave to propagate, like ripples on a pond, even if the wind later dies out.

Marine forecasters distinguish swell waves and sea waves. Roughly, the distinction is that swell is generated by a region of strong winds some distance away, whereas sea waves are a product of the local wind environment. When the wind eases, sea waves die down quickly.

If you want to know more about swell and sea waves you can read about them here.

What is swell period?

The period of a wave is the time it takes (in seconds) for successive wave crests (or troughs) to pass an object (a boat, say) which is stationary on the ocean surface. Swell waves have long periods, whereas sea waves have shorter periods.

Why does swell period matter?

Many passenger and cargo vessels have lengths which make them prone to resonant motions set up by long-period swells. These motions can cause the vessel to roll or pitch in a way that can become uncomfortable for those on board, can cause loss of power and, in extreme cases, endanger the vessel. When steaming into such swells at speed, resonance can be set up quickly until the bow dips into the ocean.

Sea waves, with their much shorter period, are a greater hazard to smaller vessels like recreational or fishing vessels. Not only might resonant motions be encountered, but because they are shorter in period, sea waves are typically steeper. These make smaller vessels pitch and roll, potentially causing passengers to lose their balance, or even capsizing a poorly loaded vessel. What’s more, they may be sufficiently steep to break. When waves break over a ship, damage may be considerable, especially if large quantities of water enter the hull.

Wave energy is related not just to wave height, but also to period. As period increases, so too does the energy the waves carry. Because long-period waves carry more energy, when they come into shallow water – this process is known as “shoaling” – they typically grow larger, and break more heavily, than a shorter period swell of a comparable wave height. As such, long-period swell can make for more hazardous bar crossings or beach surf conditions.

Figure 1: Examples of “clean”, long-period swell waves about Wairarapa Coast

Figure 2 – Example of “untidy”, mixed-period sea waves about Wellington coast

How will swell period be described in the forecasts?

A descriptor for swell period will be used from

Examples of old vs New

OLD

NEW

You can find the coastal forecasts produced by MetService meteorologists at:

At Home Meteorological Experiments

l.ferris — Mon, 06 Apr 2020 03:28:00 +0000

If you are trying to entertain the kids at home or are just looking for a meteorological experiment then you've come to the right place!

First up we have the Cloud in a Bottle experiment:

Equipment:
• 1 large clear-plastic bottle with a screw-on top (e.g. a 3-litre juice bottle)
• 1 match
• 1 cup of water

Instructions:
Step 1. Pour the water into the bottle, put the top on and shake it around. Leave it for a few hours so that the air in the bottle gets very humid. (You can do the word find below while you wait)

Step 2. Remove the top of the bottle

Step 3. Light the match (children need adult supervision), let it burn a few seconds, blow it out, then quickly drop the match into the bottle. Screw the top tightly back on the bottle.

Step 4. Now squeeze the bottle firmly and hold for about 30 seconds

Step 5. Now look closely at the air in the bottle as you suddenly release the pressure on the bottle. The temperature of the saturated air will cool, and there are so many condensation nuclei in there, that tiny droplets should form. It’s a cloud!
Step 6. If you squeeze the bottle again, the cloud droplets will evaporate again and the cloud disappears

Technical explanations:
Step 1: When air is very humid air, it is called being “saturated”
Step 3: The smoke from the match will act as tiny ‘condensation nuclei’ on which cloud droplets can form
Step 4: Squeezing it increases the pressure of the air in the bottle, warming it slightly. Continuing to squeeze the bottle allows it to cool back to room temperature.

Meteorological Word Find.

Create a rain gauge

Measuring how much rain has fallen needs a rain gauge. We can make a simple rain gauge by using a straight sided bottle, some stones, scissors, a marker and a ruler: