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weatherwise The winter blast of early July BY DANIEL CORBETT, METEOROLOGIST T he winter blast of early July was quite a severe and intense event because many of the key ingredients came together in the right place and in the right amount. The first big piece of the puzzle for the polar blast was abundant cold air that began to arrive across the far south of the South Island around Wednesday June 19. Snow began to fall across inland Southland and Otago by the evening with snow falling above 200 metres across inland Canterbury by the morning of Thursday June 20. Accumulations easily reached a few centimetres. But some places had in excess of 30cm. STORM HIGHLIGHTS Snow >1 m in South Island High Country (1.2 m reported at Mt Hutt on 20 June) Rain 156 mm in Wainuiomata (Wellington region) 70 mm in Ashburton Wind Wellington: 105 km/h gusting 140 km/h at 7pm on 20 June Kaikoura: 100 km/h gusting 135 km/h at 4pm on 20 June Wanganui: 80 km/h gusting 125 km/h at 12am on 21 June Hauraki Gulf: 90 km/h gusting 110 km/h at 7am on 21 June Lightning Strikes 3457 strikes between midnight Tues 18 June and midday Friday 21 June. Mostly in the west from Northland to Taranaki Cold temperatures (minimums) Tekapo: -6 degrees C Large waves 10-15m waves observed in Cook Strait Gore: -2 degrees C Waiouru: -1 degrees C OUTLOOK Midnight Thursday June 20 And then the fuel injected southerly. The important factor that helped bring this storm to life and make it an intense beast was the sharp upper level trough that moved across central New Zealand during the day on Thursday June 23. This helped develop a deep low to the east of central New Zealand and in turn feed a strong southerly and associated moisture into the southern half of the country. Strong to severe gales ripped their way up the east coast of the South Island during the afternoon of Thursday June 23. The strong southerly reached wellington during the early evening and Seven day rainfall valid to 9am June 24, 2013 reached 140km/h at the airport in the evening and over 200km/h on some of the surrounding hills. Waves reached 10-15m around Cook Strait with many southern facing coasts of the lower North Island bearing the full fury of the intense storm. The strong winds were also felt as far north as the west coast of Northland where gust exceeded 100km/h. Heavy persistent rain spread in on the wrap around area of moisture behind the area of low pressure. This fed in 50-100mm of rain to coast and lower elevations but heavy snow fell above 200-300m. This accumulated to as much as three metres by the time storm eased over the weekend. www.skipper.co.nz The neutral El Niño Southern Oscillation signal from the Pacific Ocean will continue to bring an element of variety in the weather patterns over the next few weeks. Other local factors will play more of a role in the make-up of our daily weather patterns. The slightly warmer seas to the west of New Zealand along with a projected signal for lower pressure in the Tasman Sea will keep Tasman lows as a main feature in our weather during August and September. These lows can mix moisture laden winds from the subtropics with cooling winds from the south, sometimes resulting in several days of strong wind and heavy rain. The point of origin in the Tasman Sea can also play a part in how much moisture these features ultimately bring. We can also expect the typical late winter/early spring pattern of disturbed westerly winds (sometimes called the "roaring forties") covering the whole country. Within the disturbed westerlies comes an endless and rather chaotic procession of cold fronts, usually with the more active fronts crossing New Zealand at five to seven day intervals, and the occasional blast of chilling polar air is still possible. The strength of the westerlies may ease from time to time, especially when anti-cyclones build down from the west of northwest across New Zealand. These features will bring mostly fine days but cool and perhaps frosty nights. We are still vulnerable to cold southerly blasts especially when lows deepen off to the southeast of New Zealand. These can sometimes bring some last season snow, which can be great for the skiers but troublesome for farmers during lambing. This could be prolonged if any blocking takes place. The rainfall over the coming weeks should be just above normal across most of the North Island and the north of the South Island. Elsewhere across the country rainfall will be close to normal. Temperatures over the next few weeks will generally be about to just above normal across most of the country. September/October 2013 Professional Skipper 63