Another Wintry Storm

A low pressure system which formed in the tropics last week has brought steady rain to much of New Zealand since the weekend.

Heavy rainfall was reported in Northland, parts of the Bay of Plenty and from the East Coast north of Gisborne. Steady rain fell in south Taranaki and the lower North Island causing surface flooding in areas where the soil was still sodden from the three July storms. Surface flooding was reported in Christchurch as some areas experienced steady rain while snow fell on higher areas.

The low pressure system formed in the tropics last week and drifted past the Solomon Islands, Vanuatu and New Caledonia collecting moist air as it deepened and moved into the Tasman Sea.

Early projections had the low drifting across Northland or across the central North Island or even as far south as Cook Strait. However, by late Friday its track was becoming more certain and forecasters were able to prepare weather forecasts based on the low passing across the central North Island.

Auckland city woke to cooler temperatures of 10 °C on the morning of Saturday August 5th, and rain began falling in Northland. By evening, surface flooding was affecting several locations in Northland, and 40 mm of rain had fallen at Brynderwyn, near Waipu, by 9 p.m.

By Sunday morning, parts of Auckland were experiencing windy conditions with steady rain and it was still raining steadily in Northland, where surface flooding closed roads. Heavy rain had started to affect the Coromandel Peninsula where some rain gauges topped 100 mm by 10 a.m.

Heavy rain began falling in Hamilton on Sunday evening, and the rain gauge at The Pinnacles on Coromandel Peninsula reported 245 mm of rain. By midnight 96 mm of rain had been recorded at Te Puke and 55 mm at Rotorua. Most parts of the North Island had experienced rain during the day, and 17 mm was recorded at Tawa, near Wellington. Christchurch reported steady rain falling late in the evening.

On Monday morning, Te Araroa on the East Coast was isolated by slips on State Highway 35 caused by heavy overnight rain; and reports were coming in of surface flooding and road closures in the Bay of Plenty. Rain fell steadily in the lower North Island (Tawa recorded 21 mm for the day), and road closures and surface flooding caused problems in the Wanganui district.

In the South Island, snow was reported to be falling in Hanmer Springs by evening and temperatures in Christchurch dropped to 4 °C accompanied by sleet and intermittent heavy showers, while snow closed the Lindis Pass.

Wellington woke to a southerly gale on Tuesday morning as rain continued to fall, but at a lower rate. The Tawa weather station reported 8 mm of rain for the day, with the thermometer hovering around 7 or 8 °C. In the Hutt Valley, a slip which had developed overnight caused the evacuation of three houses in the suburb of Kelson. As the day progressed, one house began breaking up with parts sliding down the slope, and the main sewer line for the suburb was severed, pouring more fluid into the unstable slope.

In the Wairarapa, still recovering from flooding last month, council staff announced a threat to the town of Martinborough as a local river rose abruptly, but conditions eased before their worst fears were realised.

In Christchurch, cold driving rain caused localised surface flooding and the Heathcote River burst its banks mid-morning, flooding low-lying areas in the east of the city. A local weather station at Hillmorton reported 35 mm of rain for Monday and 39 mm to about 9 p.m. on Tuesday. The Lindis Pass remained closed by snow, but the Desert Road in the North Island, which had also been closed by snow, re-opened later in the day.

This latest event brought 50 mm of rain to Tawa, and the total for August now sits at 87 mm – four times the amount that fell during August 2005.

Many parts of the country are now carrying a “water burden” with rain-soaked soils unable to take up further rainfall. Authorities are now warning residents to be aware of the signs of land slippage and take appropriate action. The Insurance Council, which is already receiving claims related to the storm, have warned that people need to take more responsibility for limiting their risk by planting trees and avoiding flood-prone areas. Residents of Northland, Coromandel, Bay of Plenty, South Taranaki, Wanganui, Wairarapa and Wellington should be particularly vigilant if another heavy rain event occurs before the ground dries out.

Weather models are already tracking the next low destined for the Tasman Sea…

[Compiled from items posted on the Weather New Zealand weather forum, emails, phone calls from local observers, private weather stations, and other sources.]

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