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WEATHERWISE THE SOU'WESTERS BY DANIEL CORBETT, METEOROLOGIST T he anticyclones that were such a feature during the first half of the autumn took a back seat during most of June. This southwest flow also helped generate some very highs seas during the month with several occasions when waves exceeded six metres in the waters around New Zealand. The month began with a high but that was soon replaced with an active weather system from the Tasman Sea, loaded with moisture which collided with polar air sliding up from the southern ocean across the northern half of the South island. This brought some significant snow across parts of Canterbury and Marlborough on June 6. The southern ocean was again breathing its cold icy breath when a bone chilling southerly blew up the east coast of the New Zealand for over a week in mid-June. A blocked upper level flow kept all the weather systems stuck in place and the cold sou'westers continued to spread north with even some wintry showers mixed in that fell to fairly low levels across southern and eastern parts of New Zealand. This southwest flow persisted for over a week generating seas of over six metres along the east coast of New Zealand by the latter half of the week. The big southwest surge of mid-June brought six to eight metre waves around the coasts and these exceeded nine metres in the core of the sou'westers wrapping around the low. The image below is an image showing the significant wave height. Notice the reddish brown shaded area to the east of the South Island denoting the eight metre seas. Weather map June 27 Steep Head Wave height significant waves reached 10 metres. There was also some coastal flooding at high tide during this strong south-westerly across coastal parts of the lower North Island. What was the cause of persistent sou'westerlys for a good part of the month? Forecast seas maps for June 12 In the latter half of June the Southern Ocean was the source region for further active weather systems. A broad upper level trough kept steering them across New Zealand. A deepening depression and its associated front brought spells of rain, wind and snow to much of New Zealand in the last days of the month. The first part of the storm system was an active front with heavy rain and thunderstorms that spread up the length of the country. In the polar chilled air behind the weather front the snow levels dropped down to near sea level across the far south of the South Island. The persistent sou'westers on the back side of this low also brought some high seas around the coast of New Zealand. Take a look at the image below showing the wave data from Steep Head, Banks peninsula. Notice on the 27th the 56 Professional Skipper September/October 2012 The southern ocean at this time of year, as many know, it is a great spawning grown for active lows, that in tandem with a broad upper level long wave, prevailed for most of the month keeping a sou'west flow in place for much of the time. There were also lows to the east of New Zealand that were blocked (stuck) at times and this kept the flow on their back side, sou'westerly. The outlook for spring: We are on the brink of El Nino but the weather over the next several weeks may still have some hints of variability before we ease into a more El Nino-like pattern in the months to follow. The peak intensity of the El Nino will also then determine the flavour of our weather patterns through the late spring and summer. In a typical period of transition into El Nino-like weather patterns, there should be enhanced westerly winds but this may not be the case with this El Nino. Settled periods, with clear sunny days and frosty or foggy mornings, associated with passing anticyclones, are expected. Fronts and troughs rolling in from the Tasman Sea may be followed by episodes of cool southwest winds across the whole country lasting several days. Occasionally a low pressure centre may move onto the country from the north, preceded by an easterly flow with some heavy rain for north-eastern areas. Keep an eye out for a blocking pattern over the next several weeks where the weather patterns get stuck and anomalies can sometimes occur as a result.