Rural Monthly Outlook:

December 2018 - A brief spell of summer, then lows

Issued 4 December 2018

An extreme November

As expected, spring volatility was the name of the game during November. The month started out with Highs in charge, but a bitterly cold southerly on 18,19 November brought heavy snow to inland areas of Central Otago, and flooding rains to both Central and coastal Otago. Frequent Lows over the Tasman Sea then made a comeback for the remainder of the month,bringing unsettled weather.

The east coast of both islands continued to see significant easterly rainfall during the second half of the month. Canterbury and North Otago saw further rain on 20,21 November, and again on 24, 25 November. Between 24 and 26 November, the north and east of the North Island was affected by a significant Tasman Sea rainmaker. Over 100 mm was recorded in parts of southern Hawkes Bay on the 25th,producing flooding, with parts of the Wairarapa and Marlborough also seeing heavy falls. The last week of the month was characterised by scattered thunderstorms, some of which caused localised heavy rain. Kamo and Whangarei copped localised totals of 100mm in 3 hours on the 26th.

Monthly rainfall totals exceeded 120 percent of normal across the eastern North Island and exceeded 200 percent (double normal) along the east coast of the South Island. It was the wettest November on record at Dunedin Airport, and the fourth wettest November at Christchurch Airport, in 55 and 75 years of observations respectively. Tellingly, the January to November 11 month rainfall ranks as fourth highest on record at both locations, highlighting just how unusual 2018 has been. In contrast, it was a warmer and drier than usual November for the west and south of the South Island.

Temperatures bounced around during the month. The first few days of November were unusually cold, followed by extremely warm conditions in the second week (5-11 November). Temperatures peaked at 29 Celsius in Christchurch,and 28 Celsius in Napier, on 8 November. A bitter southerly during the third week of the month caused temperatures to plummet nationally, before swinging back humid and mild at the end of the month.

The Ocean

Sea temperatures around the New Zealand coastline also swung sharply during the month. By the end of November, sea temperatures exceeded 2 degrees above average about most of the South Island, and were around 1 degree above average around the North Island. In the tropical Pacific, ocean temperatures now exceed El Nino thresholds. However, not all atmospheric measures meet El Nino criteria - but things are very close. If El Nino does form before Christmas, it is likely to continue through summer. Impacts back here in New Zealand may not run true to typical El Nino form, however, given the likelihood of an 'opposing' Southern Ocean effect.

The December 2018 Outlook

Following a rough start to the month, High pressure and cooler southerlies prevail next week. Most regions are set to see relatively dry conditions next week, except for eastern regions of the North Island and possibly also the Marlborough coast. After that, low pressures return to New Zealand, bringing unsettled weather in the week before Christmas (17-23 December), and westerlies over the South Island for the last week of the month. Overall, monthly temperatures are forecast to run below average across the North Island, Nelson and Marlborough; near average temperatures are expected elsewhere. A wetter than usual December is expected for the southwest of the South Island, with near normal totals elsewhere. However, expect a high degree of rainfall volatility,with a wet first and third week, and a rather dry second week.

Bottom Line

A brief spell of summer next week, then low pressure returns. A cooler than usual December for the North Island, Nelson and Marlborough; near average elsewhere. Continued rainfall volatility from week to week.