New Zealand is entering the time of year when the land can dry out fast, and the weekly soil-moisture deficit maps are now showing constant change, Farmers Weekly's weather column reports. Longer days, higher temperatures and windier westerlies are drawing the drier areas out, especially around the North Island, even though rainmakers are still arriving.

Two things have been happening in recent weeks. The highs around southern and eastern Australia have been large and powerful, bringing widespread dry weather across much of the continent, and air pressure has been higher than normal for the season, as El Niño tends to produce on this side of the Pacific. Over the Southern Ocean the opposite has occurred, with pressure below normal for the past month. Sitting between the two, New Zealand has had surge after surge of westerly-driven weather since August.

Towards the end of September even more high pressure is expected in the New Zealand area. Most highs have sat to the west or just north of the country rather than over it; by mid to late this week a high from the Tasman may push into the upper North Island while another burst of windy westerlies hits the South Island, and next week high pressure may again stretch across the subtropics to the north and over the Tasman and southern and eastern Australia.

The pattern to watch for in spring is high-pressure zones connecting up with no lows between them. When that happens a ceiling of high pressure north and west of the country keeps cold fronts coming into the South Island but limits how much rain reaches northern New Zealand and parts of the east of both islands. Southland is therefore likely to stay windier, colder and wetter this month and into October, while Northland leans drier, calmer and milder, with no immediate end in sight.

The long-range rain maps to early October put the bulk of the rain, 300mm or more, in Fiordland and possibly south Westland, with about 100mm for the top half of the West Coast, 100mm for coastal Southland and 60mm for western Otago. The western North Island can expect 20-40mm, and eastern areas from Northland to Canterbury only 5-30mm through to 2 October.

Source: Farmers Weekly NZ