2011 – The Weather in Review

 

The temperature trend in New Zealand during the past year can be read from the graph below and tells a story. Last summer was significantly hotter than normal, and several tropical weather systems visited our northern parts in January.

Data taken from NIWA’s monthly climate summaries.

Autumn started normally enough in March and April.  Then blocking led to the dominance of north and northeasterly wind patterns, delaying the onset of winter (and the opening of the ski season) by about three weeks.  This all changed in July – click here for a more detailed explanation from our Chief Forecaster, Peter Kreft – when we experienced the coldest time of the year, with polar outbreaks on 25 July and 14 August to 16 August. Spring started a few weeks late but managed to finish on schedule.  And, of course, there was the deluge that struck Nelson in mid December.

Global temperature measurements currently score 2011 as the tenth highest on record. That data is from the global mean surface land-ocean temperature index. 2011 brought its fair share of global meteorological mayhem. In January Brisbane had its worst floods since 1974, and Brazilian landslides killed 900 people in the mountains just north of Rio de Janeiro. Rain in October eased the drought in the Horn of Africa that had been affecting around 12 million people, but the rain was so intense that it brought crop damage to Kenya. The Monsoon stalled over Thailand in October, taking the lives of 650 people in floods, notably around Bangkok.  On Sunday 18th December, Tropical Cyclone Washi became the deadliest storm of the year with landslides taking more than 1000 lives in the Philippines.

The World Meteorological Organization reports that 14 weather events, each with upwards of US $1 billion of damage have occured in North America during 2011:  heavy snow in February, the most active tornado season on record in April and May, a drought and wild fires in Texas, and the wettest year on record in the east along the Canadian/US Border, culminating in the late August with severe flooding from Hurricane Irene and Tropical Storm Lee.

 In New Zealand, 2011 started with an intense La Nina episode and large anticyclones over Chatham Islands. In the days between Christmas and New Year, an intense storm passed over the country and brought gale force winds to many areas and flooding to the Rakaia area, Pelorus Bridge camp site, Rai Valley, and Aorere River in Golden Bay.

In January there were many episodes of strong easterly winds as three lows of tropical origin brought torrential rain and gales. Former tropical cyclones Vania and Zelia produced heavy rain on the 18th on the West Coast, resulting in the Fox River bursting its banks. On 22nd/23rd, a low which formed near New Caledonia moved towards NZ producing rain that caused slips and road closures over much of the North Island, along with a significant storm surge for Auckland.  Insurance payouts for this flooding were $7NZ million. Also, Tropical Cyclone Wilma moved rapidly across the northeastern North Island on the 28th/29th, and brought severe flooding and slips to northeastern regions of the North Island. Insurance payouts for this were $20NZ million, making it the most expensive weather event in NZ in the past year (insurance payout figures from Insurance Council of NZ: http://www.icnz.org.nz/current/weather/).

MetService Weather Map showing Tropical Cyclone WILMA on 28 January 2011. This was our most damaging storm in the past year.

In February, weather conditions were generally settled.  There was a record-breaking heat wave on Waitangi Day, with Timaru Garden’s 41.3 degrees centigrade topping the list of available temperatures.  It was an extremely dry month in the North Island – the driest February in Dannevirke since records began there in 1951. By way of contrast, central Otago had twice its normal February rainfall.

March was very wet over the North Island and parts of the South. After crossing Raoul Island, Tropical Ccyclone Bune wound down as it passed east of East Cape at the end of the month.

April brought more southeast winds than usual, with wet conditions for eastern areas.  The flooding and slips in Hawke’s Bay on 26-27th April triggered evacuations and resulted in insurance payouts of $6.4million.

During May and most of June north or northeast flow patterns dominated, bringing record-breaking warmth.  Ash clouds from a South American volcano also challenged air travel. May will be long remembered for the tornado that struck Albany on the 3rd, taking the life of a worker.  Insurance payouts for this event were $6 million. On 18th June a landslide took the life of a teenager at Ohope. On 19th June tornadoes hit Taranaki, triggering an insurance payout of around $2 million.

During July and August stormy west, southwest, and southerly flow patterns dominated; click here for a more detailed explanation from our Chief Forecaster, Peter Kreft.  This was the coldest time of the year, with polar outbreaks on 25 July and 14 August to 16 August.  Snow fell across the lower North Island, with flurries in locations well north in the country.

 

MetService Weather map at midnight start of Monday 15 Aug showed a blast of southerly air from Antarctica to New Zealand.

September was characterized by southwest flows and October by northeast flows, with winds less than normal and generally settled weather because of dominant anticyclones.  Most of the Rugby World Cup games were not directly affected by the weather.  It is interesting to note that on 11th September, just 36 hours after calm clear weather for the opening ceremony, a tornado hit within 10 km of Eden Park. Then on 13th September Wellington was coated with hail (MetService tending on Twitter, such was the intensity of the event), and on 14th September winds caused damage in Auckland, Waikato and Bay of Plenty.  On 18-19th October, Dunedin andChristchurch had their wettest day in 18 months.  On Labour Day (24 Oct) wind also caused damage in Southland and central Otago.

During November,anticyclones came south earlier than normal but the frontal systems of spring remained steadfast. As a result, isobars over central New Zealand were squashed closer together than normal, causing many bouts of wind.  This produced drier than normal conditions in the northeast, and cooler and wetter than normal conditions in the southwest. The moisture causing the rainfall that flooded the Grey River in Westland on 21st came from Australia.

In mid-December a major trough stalled for about two days between a low in the Tasman Sea and a large blocking high east of New Zealand.  This brought a deluge of rain to Nelson.

 

MetService weather map for 15th December 2011 showing a front stalled over Nelson.

The most significant falls of rain occurred in the hills behind Nelson and in the lowland areas of Takaka and Richmond. The rain gauge trace seen at Kotinga in the Takaka township measured 423 mm/24 hours, a new record for this site – the previous record of 256mm/24hr was measured in August 1990.  The New Zealand record for 24 hour rainfall is held by CroppRiver at its waterfall (inland from Hokitika), at 758 mm in 24 hours ending 0620 hours on 27 Dec 1989.

 

Rainfall accumulation graph for Takaka (TAN), Kotinga (TSR) and Nelson Airport (NSA). Accumulation of rain is in mm, and timestamps are in UTC (add 13 hours to get NZDT).

 
 

The total rainfall over the country from this trough can be seen in the rain map for the 7 days ending 18th Dec 2011.

MetService rainfall accumulation map for the 7 day period ending 9am on 18th Dec 2011.

The purple areas received more rain than desired, but other parts of the country received enough rain to be useful for filling up water tanks and replenishing pastures and gardens before the holiday break.

In summary, NZ’s weather in 2011 had its ups and downs, and produced some interesting events.

Everyone at MetService takes this opportunity to wish our blog readers a refreshing break at the end of the year, and we look forward to sharing with you our insights into the interesting weather events of 2012.

 

A Southerly ‘Buster’

On Monday 28th November, a south to southwest change swept its way northwards across Otago and Canterbury during the afternoon.   Temperatures soared to 28 C preceding this change then rapidly plummeted to around 16.  This was a good example of what is known in Australasia as a ‘buster’.

Temperature traces on 28 November in degrees Celcius. DNA=Dunedin, OUA=Oamaru, TUA=Timaru, CHA=Christchurch and KIA=Kaikoura. Timestamp is in UTC; 27 0000 is 1pm NZDT.

The weather map for 1pm Monday 28 November 2011 showed a typical trough moving across New Zealand. The last of a series of fronts within this trough was the one responsible for this dramatic drop in temperature.

Weather map for 1pm NZDT Monday 28 November

The reasons for temperatures soaring to between 26 and 28 C ahead of this southerly change are:
•    Northwest winds ahead of the trough warmed by around 5 to 10 degrees Celsius as they descended down the eastern slopes of the Southern Alps
•    Sunny conditions in the relatively clear skies over the Canterbury Plains – on a date less than one month ahead of the longest day.

These warm temperatures combined with falling air pressure to produce a zone of relatively low density. Higher density air in the cooler southerly flow that followed this cold front accelerated into this zone of low density air producing a squally “gust front” with the wind change. This “gust front” built in size and intensity during the afternoon as can be seen from the tweets sent from @metservice during the afternoon

  • Southwest change arrived Dunedin Airport around 11:50am. Temp dropped from 22 to 14 C , gusts to 50 kph , and its on its way north. ^BM
  • Southerly change got to Oamaru about 1:30 pm, temp. dropped from 22 to 14 C, initial gusts were 54 kph . South Canterbury’s next ^BM
  • Southwest change arrives in #Timaru just before School’s out, Temperature drops from 28 C at 2:30 pm to 16 C by 3 pm , Gusts to 70 kph ^BM
  • Southerly change reached #Ashburton between 3:30 and 3:45 pm, temp. dropped from 26 to 16 C, gusts 70 kph see http://t.co/YhuAEZ4L ^BM
  • Southwest reached #Christchurch at 5 pm in time for evening commute, temps 28 C to 17 C in 20 minutes. with wind gusts 75 kph , ^BM
  • Hi #Christchurch be quick and look at wind blown dust of that southerly change on MetService radar at http://t.co/lmw4BcYA past hour ^BM

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September to November is the season for the strongest of these southerly busters (but they can occur at any time of the year).  Spring brings the strongest westerly winds of the year to South Island and it is also a time of relative cold offshore sea temperatures.  The temperature difference between the heated air over the Canterbury Plains and seas in the Canterbury Bight is what feeds the wind gusts of a buster. The coldest sea temperatures of the year occur in early spring, and they only just start rising in November.   You can find the latest reading from metservice.com by clicking on ‘marine’ and then ‘beach’ and selecting a suitable site.  The one shown below is Jack’s Point near Timaru (timestamp is 10am Wed, 30 Nov 2011).

Jack's Point on the metservice.com Marine & Surf section. Wind, Wave ands Sea conditions are now available on metservice.com

 

The showers with this buster occurred mainly along the coast and at sea.   In the drier air over the Canterbury Plains the southwest wind change picked up dust and dirt, especially over the Rakaia River, and lifted and blew these as a “dust storm” into Christchurch.  This can clearly be seen on the animation below, taken from the high frequency Christchurch rain radar site at metservice.com.

The science behind the new Auckland forecasts

We’ve added a lot more detail to the forecasts for Auckland on metservice.com.

In addition to the existing Auckland forecast on the Towns & Cities page, we’ve divided the greater Auckland region into five sub-regions – each with its own characteristic  rainfall, temperature and wind patterns:

To provide hourly predictions of air temperature, wind speed and rainfall for these sub-regions, we’re using

  • Data from our own localised-for-New-Zealand weather modelling and statistical processing systems (for more about this, see the blog on MetService’s Investment in Forecasting)
  • Observations of temperature and wind speed from representative weather stations within each of the sub-regions (see next point), which we’re blending with  the modelled data for the first few hours of the forecast.
  • The weather stations we’re using are Whangaparaoa (for North Shore), Whenuapai (for Waitakere), Auckland Airport (for Manukau), and Ardmore (for Hunua). For Auckland City we’ve created a “virtual weather station” near the Newton Interchange; this will do the job for now, but we want to replace it with a real station within Auckland City soon.

 Auckland Central forecast page screenshot

The index map at the bottom left of the map area links back to the Auckland Towns & Cities page you’re already familiar with, containing the overall Auckland urban forecast and max/min temperatures for the next 10 days.

This initiative was partly motivated by the great feedback we received about the ‘dust graphs’ of wind speed and rainfall added to the Christchurch pages in February, to alert people to the potential dust nuisance in the areas affected by liquefaction and, more recently, demolition of large buildings.

As always, we’re looking forward to hearing what you think! Tweet @MetService or drop us an email at enquiries@metservice.com