Climate Summary for May 2015

Dry and sunny for eastern parts of the country

Rainfall	Rainfall was well below normal (< 50%) or below normal (50-79%) for eastern parts of New Zealand from Gisborne all the way south to North Otago.  Rainfall was well above normal (> 149%) or above normal (120-149%) for western parts of the South Island, the Southern Lakes, southwestern and western parts of the North Island, and eastern Bay of Plenty.
Soil Moisture	As of 1 June 2015, soil moisture levels were below normal for this time of year for East Cape, coastal Wairarapa, southern Marlborough and eastern parts of Canterbury.  It was especially dry about North Canterbury where soils were considerably drier than normal for this time of year.
Sunshine	May sunshine was abundant for eastern parts of New Zealand including the eastern Bay of Plenty, Gisborne, Hawke’s Bay, Wairarapa, Marlborough, Canterbury and Otago where May sunshine was typically above normal (110-125%) or well above normal (> 125%).
Temperature	May temperatures were above average (+0.51°C to +1.20°C) for parts of Southland, Otago, Canterbury, Wairarapa, Whanganui and Southern Taranaki.  May temperatures were below average in isolated parts of central Hawke’s Bay and western Waikato (-0.51°C to -1.20°C).

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) (interim value).

Further Highlights:

• The highest temperature was 27.0°C, observed at Waiau on 6 May.
• The lowest temperature was -9.0°C, observed at Hanmer Forest on 28 May.
• The highest 1-day rainfall was 190 mm, recorded at North Egmont on 6 May.
• The highest wind gust was 178 km/hr, observed at Cape Turnagain on 13 May.
• 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.
• 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).

Download

Full Monthly Climate Summary - May 2015 [PDF 642KB]

Climate statistics for May 2015 [70 KB PDF]

For further information, please contact:

Mr Chris Brandolino
NIWA Forecaster – NIWA National Climate Centre
Tel. 09 375 6335, Mobile (027) 886 0014