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WHAT'S AHEAD – WEATHERWISE THE COLDEST DAY OF WINTER BY BOB MCDAVITT N ew Zealand usually gets its coldest days of the year in late July or early August. This year the selection was clinched for July 24-25 with the arrival of a river of air flowing straight from the Antarctic ice shelf to our shores. The composition of such a weather pattern requires a careful arrangement. There must be a high-pressure zone over Tasmania at the same time as a low-pressure system deepens over the Chathams area. The normal Southern Ocean weather pattern consists of a disturbed westerly air flow surrounding Antarctica – a weather pattern the sailors of old called the Roaring 40s. Frigid air sits like a dome over the Antarctic continent, held in place by this circumpolar vortex. However, a zone of high pressure sometimes forms on the ice shelf and this cracks the walls of the dam, allowing dense polar-chilled air to tumble outwards and northwards in a weather pattern known as the polar blast. If the polar blast had occurred a day or so earlier it would have produced more significant snowfall The air started off very dry and managed to pick up some moisture during its journey over the sea. When it reached New Zealand a day later it encountered some moisture left over from a recent period of rain. As it fed off the increasingly warmer sea it was travelling over, the air produced showers that brought snow to low levels. This event was unusual not so much for the temperatures or amount of snow produced, but for its penetration and spread. Many places received snow for the first time in 15 to 20 years. Surface weather map across Australia, New Zealand and the South Pacific for July 24 It almost did not happen this time, as the high part of the equation was interfered with by a low that formed in the Australian bight. However, as the weather map shows, the polar blast formed on July 24 and spread north toward New Zealand. TOTAL MARINE: • Wharves • Jetty and Marina Construction and Repair • Marine Towing • Pile Driving and Drilling • Salvage • Barge and Tug Hire TOTAL FLOATS: Design, Supply and Installation of all Floating Structures: Marinas – Commercial and Private Wharf Pontoons Phone 09 379 9752 • Fax 09 379 9751 www.totalmarineservices.co.nz 54 Professional Skipper September/October 2011 If the polar blast had occurred a day or so earlier it would have encountered ample rain clouds brought from the subtropics onto the North Island and produced more significant snowfall. This weather map shows a mismatch in timing of the low and the high. Ideally, the high and low on either side of this polar river need to stay in place and work together, something like the rotating arms of an eggbeater. The sea temperatures in the mid-equatorial Pacific continue to hover near their normal levels, so the ocean is now in neutral territory (neither La Nina nor El Nino). A zone of slightly warmer than normal sea observed near the Galapagos Archipelago last month has recently returned to more normal values as well. Most climate computer models pick this neutral state to continue into early 2012. The sea temperature around New Zealand also plays a role in determining our seasonal weather. A zone of warmer than normal sea in the South Pacific stretches from the Solomon Islands to east of the Chatham Islands, and this is helping to feed mild, humid air onto New Zealand when the area has a northeasterly airflow pattern. Atmospheric weather patterns are still reflecting some residual La Nina aspects, but this trend is decreasing. For comparative purposes, there was a similar fading of La Nina in mid-1999. The weather patterns for spring indicate a neutral balance between La Nina and El Nino may give extra variability and variety to our weather. We are now moving into the warming season, and this increasing energy usually feeds the Roaring 40s' weather pattern, bringing disturbed westerly winds and periods of squally showers in a pattern known as equinoctial gales. High-pressure systems may occasionally travel across northern districts and then build to east of the North Island. These may then feed moist nor'easterly winds onto the North Island, enhancing any approaching fronts. Be careful when you see a weather map showing a high moving over the North Island, as it is likely to be followed a few days later by an active front with wind and rain. The Tasman Sea should continue to be a breeding ground for low-pressure systems, but less so than during winter. VIP.S71