Extremely low temperatures (between 2.0 and 2.5 °C lower than normal) were recorded over most of the South Island, lower parts of the North Island, King Country, Waikato, Auckland and parts of Northland.  Most other locations experienced well below average temperatures (between 1.2 and 2.0 °C lower than normal). The national average temperature of 9.0°C was 1.6°C below the long-term average for May.

 

Double the normal rainfall for May (about 200 percent of normal) occurred in the Wairarapa, Canterbury and Otago. Much of Northland, Auckland, Wellington and Southland received at least 150 percent of normal May rainfall. Only western Bay of Plenty (about 60 percent of normal) and Tasman District (between 20 and 60 percent of normal) received below normal rainfall for the month.

Well above normal sunshine totals for May (greater than 125 percent of normal) were recorded in Northland, Manawatu, the south-west of the North Island, and the West Coast. Below normal sunshine (between 75 and 90 percent of normal) was received along the Taranaki and eastern coasts of the North Island, and in Central Otago.

 

There were several extreme events during the month. Many locations received record low maximum and minimum temperatures, particularly between the 9^th and the 11^th and on the 21^st and 22^nd of May. Heavy rainfall resulted in flooding in south Canterbury on the 17^th, and in Otago on the 18^th. High winds buffeted Wellington on the 15^th, 17^th, 23^rd, 24^th, 30^th and 31^st.  Snow fell to low levels periodically during the month and tornadoes damaged property in Warkworth on the 11^th, and in Taranaki on the 17^th.

 

The average pressure pattern for May 2009 was dominated by the persistence of anticyclones (“highs”) over southern Australia and more depressions (“lows”) than normal passing over and to the east of New Zealand. The average May surface pressure over New Zealand was as much as 8 hPa lower than normal. The persistent anticyclones resulted in more southerly air flows than normal over New Zealand, which led to the low temperatures experienced during the month.