July 2007

A month of extremes and contrasts – severe floods; numerous damaging tornadoes and destructive winds in the north; ice and severe frost in the south

• Temperature: Below average in the lower South Island; above average throughout much of the North Island
• Rainfall: Well above normal in the north and east of the North Island, and coastal South Canterbury and Otago; below normal in the north and west of the South Island
• Sunshine: Above average in the west and south of the South Island; below average in the east of the North Island
• Wind: Easterly gales in the north, but quieter in the south

July 2007 was a month of extremes and contrasts with depressions (lows) often tracking over or to the north of the North Island. These produced floods and at least twice the normal July rainfall in parts of Northland (more than 400 mm in places), Hawke’s Bay, coastal South Canterbury and Otago, damaging windstorms to Northland, Auckland, and the Coromandel, and thunderstorms and tornadoes to Taranaki, Auckland, and Bay of Plenty. In contrast, calm conditions in the south, often with periods of freezing fog, produced dry, cold and very icy conditions during the first three weeks of July to the southern half of the South Island.

The national average temperature of 8.1 °C was 0.2 °C above normal. However this belies the north/south contrast with mean temperatures at least 1.0 °C below average over much of the southern half of the South Island. Freezing conditions existed for much of July in Central Otago and inland Southland. At Lauder (Central Otago), air temperatures were constantly below zero from July 12^th to 21^st, and there were 13 days from July 7 to 22 with minimum air temperatures below –10.0 °C. In contrast, temperatures were at least 0.5 °C above average throughout much of the North Island. Rainfall was double normal in Hawke’s Bay, parts of Northland, and the South Canterbury–Otago coast, but was 50 percent (half) or less of average in the north and west of the South Island. July was very sunny in inland South Island areas not affected by fog, as well as coastal Otago and north Westland, but rather cloudy in the east of the North Island.

These extremes and contrasting patterns were formed by much more frequent depressions (‘lows’) in the central Tasman Sea and over northern New Zealand, and anticyclones (‘highs’) in the southern Tasman Sea and over southern New Zealand. These produced more easterly winds over the South Island and more northeasterlies over much of the North Island. The mainly easterly anomaly over New Zealand as a whole was the strongest for July since 2001. Five depressions tracked across the North Island during the month (on the 6^th, 9–12^th, 15–18^th, 20–21^st, and 30–31^st).

Taranaki Tornadoes

A depression west of the North Island on 4 – 5 July with unstable northwesterlies produced thunderstorms and swarms of damaging tornados in Taranaki. Tornadoes producing severe damage in Taranaki occur on average once in four years, however, this is a statistical average and they occasionally occur in clusters. The episode of 4–5 July 2007 was the first time that a swarm of at least seven or more damaging tornadoes has been recorded in New Zealand, all within a 24 hour period. Another small tornado occurred on 31 July. The estimated damage is $7 million.

Northland Storms

Gale easterlies and heavy rainfall, due to a deep depression with an intense pressure gradient, tracked across the north of the North Island, producing widespread severe flooding, property, infrastructure damage and stock losses throughout much of Northland on the 10^th. Winds gusted as high as 180 km/h on the offshore island of Tititiri Matangi, with 254 mm of rain in 12 hours in Kaeo. On average, gusts this high from the east only occur about once every 20 years. The estimated damage is $60 million.

Hawke’s Bay Floods

Major flooding, due to moist easterly conditions, occurred in parts of Hawke’s Bay, particularly near Hastings on the 17^th, with further heavy rainfall resuming on the 18^th. As much as 300 mm of rain fell in a 48 hour period at a few locations.

South Canterbury–Otago Floods

More flooding, also due to moist easterly conditions, occurred in south Canterbury and Otago on the 30th, affecting roads and houses. Dunedin received more than a month’s rainfall within 24 hours. As much as 110 mm was reported at Morven, and over 100 mm in North and East Otago. High rainfall and slips also affected parts of Stewart Island.

Further Highlights

• The highest temperature during July 2007 was 20.4 °C recorded at Gisborne Airport on the 1^st and in Christchurch on the 26^th.
• The lowest air temperature during the month was -15.4 °C recorded at Lauder on the 18^th, the lowest there since July 1995. Many other inland South Island locations recorded minimum air temperatures below -10.0 °C during July, often accompanied by freezing fog and treacherous ice.
• Several damaging tornadoes affected parts of the north (Auckland and Tauranga) and west of the North Island. Damage was particularly severe in parts of Taranaki over 4–5 July. The first tornado affected New Plymouth’s central business district, lifting a large part of the roof off a major hardware store and destroying a wall. Other shops and houses were also damaged, along with cars. Trees were uprooted and signs destroyed. On the 5^th, multiple tornadoes affected Taranaki resulting in the declaration of a state of emergency. The township of Oakura was severely affected, with a substantial number of houses damaged. Other towns such Opunake, Motunui, Stratford, Hawera, Normanby, Oakiawa, Egmont Village, Inglewood, Waitara, Urenui, and Pungarehu were also affected. Another, although small, tornado occurred in Brixton, near New Plymouth, on the 31^st, tipping a truck and trailer unit on its side, and tearing roofing iron off and crumpling a building’s large roller doors.
• A state of emergency was declared on the 10^th in the far north as gale easterlies and heavy rainfall produced widespread severe flooding and landslips throughout much of Northland. Thousands of residents were without phones and electricity, and some had to evacuate. The town of Kaeo was worst hit, and Whangarei was completely blocked off by floodwaters and slips. The same weather system produced damaging winds in Northland, Auckland, and Coromandel which resulted in fallen trees, broken power lines, and other damage. More than 140,000 people were without electricity throughout Northland, Auckland, and the Coromandel. A wind gust as high as 180 km/h was recorded on the offshore island of Tititiri Matangi and 148 km/h at Mokohinau Island, north of Auckland, during this event.
• Major flooding also occurred in parts of Hawke’s Bay, particularly near Hastings on the 17^th, with further heavy rainfall resuming on the 18^th, and along the south Canterbury–Otago coast on the 30^th.
• Of the five main centres, Auckland was the warmest, and Christchurch was the coldest and driest. Dunedin was the sunniest of the main centres.

Temperature

Mean temperatures were more than 1.0 °C below average in north and Central Otago, and inland Southland, and also below average in south Canterbury, and areas along the Otago-Southland coast. In contrast, mean temperatures were at least 1.0 °C above average in parts of Northland, Thames-Coromandel, Waikato, and Gisborne, and also above average throughout most other North Island regions. The national average temperature was 8.1 °C (0.2°C above normal).

Rainfall

Rainfall totals were at least 200 percent (twice) of normal in parts of Northland, Hawke’s Bay, and coastal areas of South Canterbury and Otago, and also above normal throughout Thames-Coromandel. In contrast, rainfall was less than 50 percent (half) of normal in throughout much of the north and west of the South Island.

Sunshine

July sunshine hours and/or solar radiation were more than 120 percent of normal in north Westland, inland south Canterbury, and coastal Otago, and also above normal in other parts of Otago and Southland. In contrast, sunshine hours were below normal in the east from Gisborne to Marlborough.

Full report

• Full details of the July 2007 Climate Summary

For further information, please contact:

Dr Jim Salinger – Principal Scientist, Climate NIWA National Climate Centre – Auckland Phone +64 9 375 2053 [email protected]

Stuart Burgess – Climatologist NIWA National Climate Centre – Wellington Phone +64 4 386 0569 [email protected]

Geoff Baird – Communications Manager Phone +64 4 386 0543 [email protected]

Acknowledgement of NIWA as the source is required.