Weather, at last, shows its fangs

If this year's mild winter and sunny start to spring fooled anyone into thinking summer was just around the corner, a nasty surprise is in store.

Although the worst of the weather should stay away long enough for voters to get to the polling booths today, a deepening low-pressure system is bearing down on the country from the western Tasman Sea, forecast to bring a sharp southerly change with plummeting temperatures, rain and even snow in the lower South Island.

"It's going to be a shock for the lower south because they've been getting temperatures in the 20s," said MetService forecaster Janet Syme.

"Everyone is going to wonder where their lovely spring weather has gone."

The low is expected to hit the North Island tomorrow. Rain will spread onto the northwest of the North Island tonight and could become heavy tomorrow.

In the South Island, the forecast is for mainly showers tomorrow, but it will be much colder.

By Monday, wintry southerlies and showers will spread over southern and central parts of the country, with snow to low levels. Showers are forecast in the north and west from about Taranaki northwards.

The gloomy forecast comes after New Zealand's sixth-warmest winter and a record-breaking warm and dry August, with the first two weeks of September continuing the trend.

September temperatures had been 1C to 3C above normal for the time of year, said National Institute of Water and Atmospheric Research scientist Georgina Griffiths.

The settled, warm beginning to spring followed a winter when the average temperature was 9.1C, 0.7C warmer than normal and 0.1C shy of the most recent warmest winters, in 1984 and 2000.

Last month, the national average temperature of 9.8C was 1.1C above normal. It was the fourth-highest average temperature for August since reliable records began in the mid-1860s.

Sunshine hours were also well above average throughout the North Island last month, especially in the west between Auckland and Wellington, and in coastal Otago, where many centres (including Auckland, Hamilton, Tauranga, Taumarunui, Gisborne, New Plymouth, Stratford, and Palmerston North) had record high totals.

Rainfall was below average in all main centres with Christchurch getting around a third of normal.

Two all-time August temperature records were broken, when Hanmer and Amberley near Christchurch had 25.1C and 25.4C respectively.