Federated Farmers: Weather should settle into summer

A freak hailstorm hit parts of the country in November this year.

Spring is typically a windy season for New Zealand, characterised by mobile weather systems and changeable weather.

Usually things start to settle down and warm up, as we move into November. But last month was unusually cold and stormier than usual. Strong southwesterlies dominated, with lower pressures than usual across New Zealand and to the east.

The effect of these southwesterlies was very obvious -- November temperatures were below average almost everywhere, with several 'unusually late' cold outbreaks. Rainfall in the north and east of both Islands was generally well below normal, with some localised exceptions. In contrast, it was extremely wet for the west and south of the South Island. Rainfall was also above normal for the western coastline of the North Island. The month saw frequent extreme weather, including frosts, large hail and severe gales.

Unusually late frosts

Frosts were widespread early in November. Temperatures plummeted as well, as a cold outbreak washed over New Zealand. Air temperatures were below freezing for inland South Island locations on November 4, it fell to -2.6C at Tekapo and Mt Cook. It was also very cold for coastal locations that day, with air temperatures of 1.5C recorded at Christchurch, 1C for Dunedin, 1.7C for Milford Sound, and 2.8C observed at Blenheim. Te Puke recorded an air temperature of 4.4C on November 7, while Napier observed 2.5C on the 8th. Further frosts were experienced in November for inland parts of the South Island.

Damaging hail

November was characterised by frequent thunderstorms, with significant hail. Damaging hail occurred in parts of Canterbury and the Tasman district. Significant hail occurred in New Plymouth and large hail was experienced at Mount Maunganui and Papamoa.

Another cold outbreak mid-November resulted in localised large hail or significant accumulations in many regions. Dunedin experienced active thunderstorms while north Canterbury and Marlborough had localised, but significant, hail accumulations around mid-November.

Lightning and intense rainfall affected Napier and thereafter Northland. Canterbury once again was in the firing line for active thunderstorms later in the month.

Dry soils in the north and east South Island

It has been an extremely dry spring (September-November) for the northeast South Island, following a very dry August there. Nelson and Blenheim are on track for their fourth driest and second driest spring, respectively (both records start in 1941). Nelson recorded 98mm of spring rainfall (39 per cent of normal), while Blenheim observed 78mm (42 per cent). Soils are in significant deficit in Nelson, Marlborough, Canterbury and Central Otago, reflecting the high wind-run and low rainfall over the last four months.

What about El Nino?

The El Nino system has shown some renewed signs of life. During November, the Southern Oscillation Index (SOI) dipped into El Nino territory (30 day average of -1), while the Pacific Ocean has warmed further. Climate models indicate the El Nino-like conditions will continue or strengthen, with a 70 per cent chance of El Nino development in the next two to three months.

Regardless of whether El Nino fully develops or not, New Zealand has already shown some El Nino-like responses, as has Australia. (Southwesterlies and below average temperatures are typical spring impacts for New Zealand. Sea temperatures are now below normal around the coast.)

December outlook

The weather should settle during December, with higher pressures than usual expected for the month as a whole. A drier December is indicated for most regions, with the exception of the western South Island, Nelson, Taranaki and Northland (with closer to normal rainfall indicated). The signal for high pressures and drier conditions is strongest for the second half of December. Below average temperatures are expected for the first half, but look to return to near average mid-month.

The latest MetService four-week outlook is available at www.metservice.com/rural/seasonal-outlook.