NIWA May Climate Summary

Dry and sunny for eastern parts of the country.

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Overview

Rainfall

Temperature

Sunshine

May 2015 climate in the six main centres
Highlights and extreme events

Overview

May 2015 was a mixed bag across New Zealand as a whole, with bouts of heavy rainfall, severe frosts and snow occurring in addition to record or near-record dry and sunny conditions in eastern parts of Canterbury. Overall, the month was characterised by air pressures which were well below normal in the Tasman Sea, with below normal pressures extending over New Zealand. This circulation pattern was accompanied by a westerly flow anomaly over New Zealand. This flow anomaly was clearly reflected in the distribution of rainfall anomalies observed across New Zealand. Specifically, rainfall was below normal (50-79% of the May normal) or well below normal (< 50% of the May normal) for many eastern areas of New Zealand including North Otago, Canterbury, Marlborough, Wairarapa and Gisborne. It was especially dry in eastern parts of Canterbury, which was largely a result of the Southern Alps consistently drawing out most moisture from rain-bearing air masses as they progressed eastward. In contrast, rainfall was abundant for many western parts of New Zealand. Rainfall was either above normal (120-149% of May normal) or well above normal (> 149% of the May normal) for western parts of the South Island, the Southern Lakes, southwestern and western parts of the North Island, and eastern Bay of Plenty. Kapiti Coast was particularly hard hit by rain during the month, with parts of the region receiving in excess of 300% of normal May rainfall. A considerable proportion of Kapiti Coast’s rainfall fell during a single event, which resulted in significant flooding and record 1-day rainfall totals for May (see Highlights and extreme events for further information). Rainfall was typically near normal (80-119% of the May normal) for remaining areas of the country. The lack of rainfall for eastern parts of New Zealand has exacerbated concerns regarding soil moisture levels in parts of New Zealand, but particularly in North Canterbury. As of 1 June 2015, soils were notably drier than normal for East Cape, coastal Wairarapa, southern Marlborough and eastern parts of Canterbury. Elsewhere, soil moisture levels were nearer to normal for this time of year, with the exception of northwestern Southland, where soils were wetter than normal.

It was a sunny month for eastern parts of New Zealand, from eastern Bay of Plenty southwards to South Otago where May sunshine was above normal (110-125% of the May normal) or well above normal (> 125% of the May normal). Inland parts of the South Island, including the Southern Lakes, Central Otago and the Mackenzie Country also observed above normal or well above normal May sunshine hours. Remaining parts of the country typically received near normal sunshine (90-109% of the May normal).

Mean temperatures were much higher than average for much of the month across New Zealand. However, a polar outbreak in late-May brought snowfall and severe frosts to many parts of the country, and lowered mean temperatures throughout New Zealand for the month overall. Many parts of Southland, Otago, Canterbury, Wairarapa, Whanganui and Southern Taranaki recorded above average temperatures (0.51-1.20°C above the May average). Temperatures were typically near average (between -0.50°C to +0.50°C of the May average) for the remainder of the country. The exception was isolated parts of central Hawke’s Bay and western Waikato where temperatures were below average (-0.51°C to -1.20°C below the May average). The nation-wide average temperature in May 2015 was 11.2°C (0.4°C above the 1981-2010 May average from NIWA’s seven station temperature series which begins in 1909) .

Further Highlights:

• The highest temperature was 27.0°C, observed at Waiau on 6 May.
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• The lowest temperature was -9.0°C, observed at Hanmer Forest on 28 May.
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• The highest 1-day rainfall was 190 mm, recorded at North Egmont on 6 May.
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• The highest wind gust was 178 km/hr, observed at Cape Turnagain on 13 May.
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• Of the six main centres in May 2015, Auckland was the warmest, Christchurch was the coolest and driest, Wellington was the wettest, Tauranga was the sunniest and Dunedin was the cloudiest.
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• Of the available, regularly reporting sunshine observation sites, the sunniest four centres so far in 2015 (1 January to 31 May) are: Whakatane (1200 hours), Blenheim (1182 hours), Waipara West (1173 hours) and Appleby (1173 hours).
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For further information, please contact:

Mr Chris Brandolino

NIWA Forecaster – NIWA National Climate Centre

Tel. 09 375 6335, Mobile (027) 886 0014

Rainfall: Extremely dry in parts of Canterbury

May 2015 was a record or near-record dry month in seven New Zealand locations, with some parts of eastern mid-Canterbury receiving as little as 5% of normal May rainfall. In contrast, it was a particularly wet month in Paraparaumu, which observed its wettest May on record. In addition, Milford Sound recorded 896 mm of rainfall during May 2015; a remarkable contrast to the 2 mm of rainfall which was recorded in Timaru over the same period of time.

Record or near-record May rainfall totals were recorded at:

Temperature: Above average for eastern and southern parts of the South Island

Relatively few locations observed record or near-record high mean temperatures for May, however this doesn’t reveal the periods of record or near-record warmth and bitter cold that occurred from time to time (see Highlights and extreme events for further details). A small number of locations in eastern Canterbury observed record-or near record mean maximum air temperatures for May, which may be primarily attributed to the foehn effect associated with the westerly flow anomaly observed across New Zealand during the month. The nation-wide average temperature in May 2015 was 11.2°C (0.4°C above the 1981-2010 May average from NIWA’s seven station temperature series which begins in 1909).

Record or near-record mean air temperatures for May were recorded at:

Record or near-record mean maximum air temperatures for May were recorded at:

Record or near-record mean minimum air temperatures for May were recorded at:

Sunshine: Very sunny for eastern parts of the country

May sunshine was plentiful for eastern parts of both the North Island and South Island, as well as inland parts of the South Island. Blenheim and Cheviot observed their sunniest May on record, with five other South Island locations observing near-record sunshine hours for the month. Of the available, regularly reporting sunshine observation sites, the sunniest four centres so far in 2015 (1 January to 31 May) are: Whakatane (1200 hours), Blenheim (1182 hours), Waipara West (1173 hours) and Appleby (1173 hours). It was a relatively cloudy month in New Plymouth compared to other New Zealand centres. This has resulted in the city dropping from sunniest centre for the year to date at the end of April 2015, to fifth-sunniest centre for the year to date at the end of May 2015.

Record or near-record May sunshine hours were recorded at:

May climate in the six main centres

May temperatures were largely near average in the main centres for this time of year. The exception was Christchurch, where the mean temperature was above average. It was also very dry in Christchurch, with the city receiving just 29% of normal May rainfall. In contrast, rainfall was above normal in Hamilton and Wellington, with the latter suffering multiple flooding events during the month (see Highlights and extreme events section for further details). Of the six main centres in May 2015, Auckland was the warmest, Christchurch was the coolest and driest, Wellington was the wettest, Tauranga was the sunniest and Dunedin was the cloudiest.

May 2015 main centre climate statistics:

a Mangere b Tauranga Airport c Hamilton Airport d Kelburn e Christchurch Airport f Musselburgh g Ruakura

Highlights and extreme events

Rain and slips

On 6 May, torrential rain and flooding accompanied by strong northwest winds hit the West Coast of the South Island. State Highway (SH) 6 at Haast Pass was closed due to a slip. Caution was advised between Ross and Haast on SH 6 due to surface flooding and slips. SH 94 from Te Anau to Milford Sound was closed for a time due to flooding.

On 12 May, Wellington was affected by heavy rain, resulting in significant surface flooding in many parts of the city as drains couldn’t cope with the sudden downpour. The rain also forced the cancellation of Victoria University’s graduation parade.

On 14 May, torrential rain caused flooding in Kapiti, Porirua, and Lower Hutt areas. A slip blocked the road and rail link between the Kapiti Coast and Wellington, closing SH 1 between Paekakariki and Pukerua Bay. SH 2 was closed at Petone, and Paekakariki Hill Road was also closed. All commuter train services across the Wellington region were cancelled and Wellington Railway Station was closed. Thousands of Kapiti and Hutt Valley residents were trapped in Wellington city and forced to find alternative accommodation for the night. At least 20 homes were evacuated in Raumati Beach, and a number of schools were closed throughout the region. The Hutt River burst its banks and flooded High Street in Lower Hutt, threatening cars and stores. The Waikanae River also burst its banks, and floodwaters affected Tawa and Porirua. A man swept under a bridge in floodwaters in Paraparaumu was helped to safety by bystanders, and police rescued a cyclist who became trapped in floodwaters in the Waikanae River with a digger. An elderly man’s body was found near his car which was underwater in Petone. On 15 May, some roads around the greater Wellington region remained closed due to flooding, including Paekakariki Hill Road and Grays Road in Plimmerton. Surface flooding occurred in and around Levin, where some houses were flooded and SH 57 was closed for a time.

On 24 May, torrential rain caused power outages in Auckland. Power was lost to almost 3000 homes in West Auckland, northwest of Auckland, and Takapuna.

The highest 1-day rainfall was 190 mm, recorded at North Egmont on 6 May.

Record or near-record May extreme 1-day rainfall totals were recorded at:

Temperatures

On 6 May northwest winds prevailed over the South Island. The associated foehn effect resulted in high temperatures for this time of year throughout Canterbury. Daily maximum temperatures reached the mid-to-high-twenties in many locations. Pre-dawn temperatures were very high in some parts too: between 4 a.m. and 5 a.m. Christchurch (Riccarton) and Winchmore (near Ashburton) recorded a maximum temperature of 20.9°C and 21.1°C, respectively.

On 28 and 29 May, low morning temperatures were experienced throughout the country, with severe frosts in some parts. Clear skies at night enabled considerable radiative cooling to occur, and a number of New Zealand locations observed record or near-record low temperatures for May.

The highest daily maximum temperature for the country was 27.0°C, observed at Waiau School on 6 May. This was followed by 26.8°C at Rangiora on 6 May, and 26.7°C at Christchurch (Riccarton) on 6 May.

The lowest daily minimum temperature for the country was -9.0°C, observed at Hanmer Forest on 28 May. This was followed by -8.6°C at Pukaki Aerodrome on 29 May, and -8.1°C at Ranfurly on 29 May.

Record or near-record daily maximum air temperatures for May were recorded at:

Record or near-record daily minimum air temperatures for May were recorded at:

Wind

On 6 May, extra caution was required by motorists travelling on the Desert Road (SH 1), SH 2 at Rimutaka Hill, and SH 73 near Arthurs Pass due to strong winds. Overnight on 6 May, over 1000 homes in Wellington’s northern suburbs lost power due to the wind. On 7 and 8 May, wind warnings remained in place for motorists travelling on SH 73 near Arthurs Pass, SH 7 at Lewis Pass, and SH 2 at Rimutaka Hill.

In the early morning of 12 May, a mini tornado caused damage in Urenui, north of New Plymouth. The roof of a petrol station was damaged as well as fences, trees, and guttering.

On 14 May, a tornado ripped through Mt Maunganui, damaging homes and buildings, pulling down fences and sending trampolines flying. About 20 homes had roofs lifted and 11 had significant damage. Part of the roof and grandstand of Baypark Stadium was severely damaged. Earlier, a reported tornado ripped the roof off a house and damaged at least three more properties at Coopers Beach in the Far North.

On 15 May, a Cessna plane flipped on the runway at Tauranga airport due to high winds. Strong winds lasting about five minutes battered Tauranga Girls’ College, damaging shade sails and table umbrellas, as well as scattering outdoor furniture.

On 20 and 21 May, strong wind warnings were present for SH 2 at Rimutaka Hill, with caution advised for motorists travelling there.

On 24 and 25 May, strong southwest winds affected much of the country, especially in the lower South Island. Numerous ferry services on Cook Strait were cancelled or delayed due to strong winds and associated large swells. Interislander sailings remained affected by the rough conditions until 27 May.

The highest wind gust was 178 km/hr, observed at Cape Turnagain on 13 May.

Record or near-record May extreme wind gusts were recorded at:

Snow and ice

On 12 May, light snow was observed in the area around Naseby, Central Otago.

On 14 May, a road snowfall warning was in place for SH 94 from Te Anau to Milford Sound, with snow falling near the Homer Tunnel. All vehicles were required to carry chains and no towing was allowed.

On 15 May, SH 94 from Te Anau to Milford Sound was closed due to snow.

On 21 and 22 May, a road snowfall warning was in place for SH 94 from Te Anau to Milford Sound, and vehicles were required to carry chains.

On 24 and 25 May, a cold southerly blast hit the country and caused snowfall throughout most of Otago, Southland, as well as the mountain passes and relatively high-elevation settlements in Canterbury. Up to 30 cm of snow was reported in Arrowtown, with 10-25 cm reported in Queenstown, Cromwell and Wanaka. Snow briefly fell to sea level in Dunedin but didn’t settle at that elevation, however up to 10 cm was reported in the hill suburbs. Snow fell down to 300 m above sea level in Hawkes Bay, and heavier falls were reported on the Napier-Taupo Road. Roads closed by snow included: SH 75 from Little River to Barrys Bay, SH 58 from Alexandra to Ranfurly, SH 6 from Cromwell to Frankton, SH 1 from Pine Hill to Waitati, SH 93 from Clinton to Mataura, SH 85 from Kyeburn to Palmerston, and SH 87 from Kyeburn to Outram. Many roads required chains to be worn and towing was prohibited. Caution was advised at SH 2 on Rimutaka Hill due to snow and ice. All schools in Wanaka were closed, and many schools around Dunedin started later in the day. Flights were delayed in Queenstown as crews cleared the runway of snow.

On 26 and 27 May, many roads in Otago and Southland were still affected by snow and ice, and caution was advised to motorists travelling in these areas.

Lightning and hail

On 12 May, thousands of lightning strikes were recorded over the country (mainly in the west of both Islands). The thunderstorms were generated by atmospheric instability associated with fronts within a disturbed northwesterly flow.

On 25 May, a lightning strike caused power to be knocked out to some homes in Dunedin.

Cloud and fog

On 4 May, hundreds of people were stranded due to fog cancelling and delaying flights into and out of Christchurch airport.

On 11 May, heavy fog blanketed the Waikato with motorists urged to take extra caution on the roads.

ENDS

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