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WEATHERWISE THE BIG FAT HIGH OF SUMMER BY DANIEL CORBETT, METEOROLOGIST T he summer of 2012-13 had an active start with persistent active troughs, however this eased back by the end of January to allow a big fat high to build across New Zealand. In the second half of the summer anticyclones became quite a dominate feature, but the big high of late January and early February certainly stood out. This high was unique, not just because of the size of the anticyclone, but also because of the length of time it lingered over New Zealand. The weather map from January 28 shows the large high in place with a central pressure of 1032mb. Whenever the pressure exceeds 1030mb it is a anticyclone of decent size for any time of year. the jetstream and how it behaves when the weather gets blocked. In a typical westerly flow the jet stream undulates across New Zealand in response to the various troughs and ridges that move along in the flow. In a blocking situation the upper wind flow cuts off in places and thus the mean flow is termed blocked or in other words it is stuck. This is a bit like a log jam in a fast flowing stream. The blocks can last several days or in some cases longer than a week and the type of weather received depends on where in the blocked weather pattern the observer happens to be. Back in January 2011 the weather became stuck or blocked, on this occasion the fine weather with the upper high was east of New Zealand. Most of New Zealand experienced a warm moist northerly flow and this resulted in bringing about the Nelson floods around the New Year 2011 and 2012. WEATHER MAP JANUARY 28 The large anticyclone brought a long spell of fine weather for most of New Zealand. Afternoon highs across much of the country climbed into the mid to upper 20s with a few inland places in the south of the South Island reaching 30 degrees Celsius or better. The most unique aspect of the long fine spell of weather was the weather across central New Zealand, most notably Wellington. In Wellington because of its location and the funnelling effect of the wind it does struggle to get more than a couple of fine days in a row, but five, six or more in a week, that is very rare indeed. A RARE SIGHT IN WELLINGTON – 10 FINE DAYS The reasoning behind the length of the fine spell for Wellington and the rest of New Zealand is all down to the upper level wind flow, 56 Professional Skipper May/June 2013 500 mb (18,000ft) upper level wind prognostic chart January 28 WHAT IS TO COME FOR THE REST OF THE SEASON? The neutral conditions across the Pacific will bring an element of variety in the weather we experience over the next several weeks. The dominant anticyclones of the past few weeks will still be a factor in our weather in the coming weeks but perhaps to a lesser degree. Anticyclones will continue to track across central New Zealand over several weeks bringing extended periods of dry weather, particularly if they linger for several days due to upper level blocking. The seasonal shift north of the anticyclonic ridge axis will cause them to track more across central regions and then perhaps to northern New Zealand in April-May. Between the anticyclones, fronts and troughs are expected to cross the country. During autumn we should see a slow transition to more active weather. In early autumn, low pressure systems are likely to move from the subtropics onto the north and east of the country, bringing wet and windy days. By the early to mid-autumn we should see a return of fronts moving across New Zealand from the Tasman Sea. In mid to late autumn a pattern change is likely with more active fronts moving from the Tasman Sea and/or Southern Ocean onto the South Island. The risk of a cyclone moving from the tropics onto New Zealand began to fade in April.