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WEATHERWISE AN ACTIVE START TO SUMMER AND THE BIG HEAT BY DANIEL CORBETT, METEOROLOGIST T he active weather that ushered out spring was a lingering trend in the early part of the summer. A broad upper level trough was partly responsible for the very active and unsettled first few days of December. The first few days saw strong to severe gales buffet many parts of the country, heavy rain of up to around 200mm lashed the west coast of the South Island, Christchurch hit 30 degrees Celsius on the Wednesday January 5 in the warm and dry nor-westers, and the humid subtropical air that lay across the far northeast of the North Island helped give rise to a nasty line of thunderstorms that barrelled across the north island on Thursday December 6. OUTLOOK FOR THE REST OF THE SUMMER The Ocean: Several months ago the waters across the equatorial Pacific began to warm across eastern areas hinting at a developing El Nino. This area spread slowly west but soon after, it began to relax. The waters across the equatorial Pacific are now close to average across most areas. It is still warmer in the west than in the east and has actually cooled slightly in the east over the past few weeks. Conditions in Pacific waters are still described as neutral and this trend is forecast to continue for the next several months. It should be worth mentioning that there may still be some hints of El Nino across the Pacific. This can be noted by the increased westerlies we have experienced across the southern half of New Zealand over the past few weeks, the roaring forties being slightly further north than normal. Generally in a neutral scenario, our weather patterns become more varied and the extremes of wind, rain, and the temperature range, also widen. This also allows other factors to come into play to influence our day-to-day weather. The Atmosphere: Atmospheric weather patterns have fluctuated over the past few months and have never really picked up on the early season El Nino signal. This can be seen in the Southern Oscillation index, which gives a snapshot of the weather patterns between Tahiti and Darwin. It has fluctuated around zero and is currently just below zero, perhaps a small hint of the very weak El Nino flavour, and an overall neutral scenario. Weather map, Thursday December 6, 2012 A small tornado developed in the active line of thunderstorms across west Auckland around Hobsonville during the middle of the day. There were further active fronts for the remainder of December but anticyclones brought some fine weather along with some summer heat too. December ended up being on average warmer than normal across New Zealand, but that was nothing to the furnace-like heat that developed across Australia. Much of central and eastern Oz had daytime highs well into the 30���s and 40���s. This in turn brought tinderbox-dry conditions which lead to significant bush fires. The seasonal neutral weather pattern has had hints of El Nino which tends to keep the large anticyclones situated further north and over eastern Australia. They in turn sit in place allowing the interior heat to build, and then the wind to flow around the highs, spreading the heat down south and even east over the Tasman to New Zealand. Dry conditions have also been a feature along the east coast of New Zealand because of the warm and dry norwesters. The tropics have been active at times and will again bubble to life as we proceed through the next couple of months. These decaying tropical lows will bring a break in the fairly persistent west to southwest average wind flow that should prevail. Still neutral but a weak El Nino flavour: Neutral conditions across the Pacific will bring an element of variety in the weather we experience over the next several weeks. The recent El Nino flavour has also perhaps allowed the roaring forties to be slightly further north than normal, positioned across the southern half of New Zealand. The anticyclones have also been positioned slightly further northwest as well. Expect this trend in our weather patterns to continue over the next few weeks. Anticyclones are likely to build in the Tasmania area, and then These anticyclones are expected to bring extended periods of dry weather, especially in the north and east. 56 Professional Skipper March/April 2013 fade away as they move north-eastwards across the Tasman Sea and over Northland, bringing periods of settled weather. These anticyclones are expected to bring extended periods of dry weather, especially in the north and east. Troughs and fronts are likely to move onto New Zealand from the south, with lows forming in these troughs and deepening as they move off to east of New Zealand and deepen near the Chatham Islands. This type of predominant pattern will bring frequent south-westerlies to much of New Zealand. Passing lows in the southern Ocean may bring brief bursts of north-westerly wind to New Zealand.