TownsRuralMarineMtnsMaps

Rural Monthly Outlook:

Issued 2 October 2018

October to run on the drier side for most regions

September 2018

Higher than normal pressure favoured the South Island during September, while south easterly winds intermittently affected the North Island. This set up produced a drier than usual September for most regions, with between 35% and 60% of September normal commonly seen across western areas of both Islands. Nelson recorded only 29mm of rain (35% of September normal), while Tauranga saw 33mm (39%), equating to its third driest September in 77 years of Airport records.Hamilton recorded 57mm (57%), while Hokitika observed 95mm (37%), and its second driest September in 55 years of observations.

Meanwhile, it was very wet for Gisborne, Hawkes Bay and parts of the Wairarapa, which were affected by heavy rain during the first week of the month. Napier Airport received 118mm of rain in just five days from the 4th to the 8th of the month; and ended up recording its 8th wettest September (220%). It was also wet across inland Otago, due to two rain/snow events during the month. Dunedin itself recorded 46mm (107% of September normal).

Despite an extremely cold start to the month, temperatures rebounded in the middle of the month, and monthly temperatures ended up close to the September average in most areas. The exceptions were along the east coast of both Islands,as well as for Northland and Wellington. The Capital experienced its second longest southerly on record, during the prolonged southerly event 3 to 12 September.

Notably, Tropical Cyclone Liua formed on the 27th, near the Solomon Islands.This is the earliest named Cyclone since reliable satellite records began 49 years ago, and the first ever to form in September.

The Ocean

Around the New Zealand coastline, sea temperatures have varied considerably during the month. The prolonged cold southerly at the start of September significantly cooled North Island coastal waters, by over 2C in parts. Sea temperatures rebounded in the middle of the month but cooled again at the end of September. At the time of writing, waters around the lower half of the South Island remain warmer than usual. Elsewhere, seas are currently cooler than normal.

In the tropics, the climate system continues in a neutral state. Whilst there remains a good chance of an El Nino event forming in the next 4 to 6 weeks, the climate models have eased off the odds. If El Nino does form before Christmas,it is likely to be a modest event, and impacts back here in New Zealand may not run true to typical El Nino form.

The Southern Ocean has, overall, been the main driver of our weather patterns in the last 6 months. During September, the Southern Ocean became stuck in its quiet (positive) phase, enabling higher than normal pressure over the country.This is in stark contrast to the previous five months of semi regular switching between its stormy and quiet modes.

The October 2018 Outlook

The Southern Ocean is forecast to continue in its quiet (dry) mode to start October, but we likely see a change to cooler southwesterlies next week (8 to 14 October). The second half of the month should see a persistence in drier than normal conditions across the southwest of both Islands, with a positive (quiet)Southern Ocean reappearing again. In contrast, we should see periodic rain events affecting the north and east of the North Island.

Temperatures yoyo from somewhat warmer this week (except over the south of the South Island), to well below average across the country next week. The second half of the month should see average to cooler conditions continue, overall,noting that October is a time of year in which temperatures typically increase sharply.

Bottom Line

An average to cooler October, nationally. Below average rainfall across the South Island, as well as for the western and central parts of the North Island south of about Pukekohe. Along the eastern coastline of the North Island between Northland and Gisborne, near average October rainfall totals.