Active start for some and the big heat

By Daniel Corbett

The active weather that ushered out spring was a lingering trend in early summer.

A broad upper-level trough was partly responsible for December's unsettled first few days, when strong to severe gales buffeted many parts of the country, about 200mm of rain lashed the West Coast and Christchurch hit 30°C on the fifth.

Humid subtropical air lying across the far northeast of the North Island helped create a nasty line of thunderstorms on December 6. A small tornado also developed in the active line of thunderstorms across west Auckland around Hobsonville during the middle of the day.

There were active fronts for the rest of December but anticyclones brought some summer heat.

December was, on average, warmer than normal across New Zealand but that was nothing compared to the furnace-like heat in much of central and eastern Australia, with daytime highs well above 40°C.

This brought tinderbox dry conditions and lead to significant bush fires.

Warm dry conditions caused by the warm and dry norwesters usually occurring in front of active troughs were a common theme across eastern parts from Otago to Hawke's Bay.

The typical active trough spreads into the West Coast laden with moisture. The moisture barrels into the western slopes from Fiordland up to the northwest ranges of Nelson. Rainfall events in late December and early January brought more than 300-400mm to some places.

The tight pressure gradients associated with some of the active fronts brought strong to severe gales in some areas with 120-130km/h around Wellington on January 10 and briefly above 200km/h on Mt. Hutt.

There was a hint of El Nino in the overall neutral seasonal weather pattern. Perhaps this helped bring about recent weather conditions. In a full-blown El Nino, all the weather zones shift further north.

The Roaring Forties sit across southern New Zealand and the fine settled weather, with the large anticyclones, has shifted further north over the Tasman Sea and the northern half of New Zealand.

This has also been the cause of the extreme heat across Australia. The anticyclones have settled across the Tasman Sea, allowing the interior heat to build and the wind flow around the highs spread the heat down south and as far east over the Tasman to New Zealand.

The tropics have been active at times and will bubble to life as we proceed through the next couple of months. Decaying tropical lows could bring a break in the fairly persistent west to southwest average wind flow that should prevail.

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 began to relax. The equatorial Pacific Ocean is close to average but still warmer in the west than in the east, which has cooled slightly during recent weeks.

The conditions are still described as neutral, which is forecast to continue for the next several months.

Generally in a neutral scenario our weather patterns become more varied and it also widens the extremes of wind, rain and temperature. This also allows other factors to influence our day-to-day weather.

The atmosphere

Atmospheric weather patterns have fluctuated during recent months and have never really picked up on the early season El Nino signal. The Southern Oscillation index gives a snapshot of the weather patterns between Tahiti and Darwin. It has fluctuated around zero and is sitting just below.

The mostly neutral conditions across the Pacific bring an element of variety in our weather. The recent El Nino trend allowed the Roaring Forties to move slightly further north than normal, positioned across the southern half of New Zealand. The anticyclones have been positioned slightly further northwest as well. This will continue during the next few weeks.

Anticyclones are expected to build around Tasmania and spread across the Tasman Sea.

They are likely to bring extended periods of dry weather, especially in the north and east.

Troughs and fronts are likely to move on to New Zealand from the south, with lows forming in the troughs and deepening as they move off, east of us. This predominate pattern will bring frequent southwesterlies to many areas.

Passing lows in the Southern Ocean may bring brief bursts of northwesterly wind and the odd decaying systems from the tropics may bring a period of easterlies and outbreaks of rain, especially to northern and eastern areas.

- Hamilton News

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