Warning of two months’ rain in one day as storm approaches

South Islanders are being urged to prepare for the storm expected to lash parts of the mainland this week.

Heavy rain, flooding, gale-force winds and snow on high-altitude roads were forecast to arrive overnight between tomorrow and Friday, just days after last week’s disruptive storm rolled away.

Forecasters have warned the blast could bring two months worth of rain to some areas in one day and heavy snow above 400 metres, including up to a metre at Mt Hutt. The fronts will be warmer, but wetter, than last week’s.

Niwa principal forecasting scientist Chris Brandolino said rain would be heaviest inland, bringing between 25 millimetres and 35mm over 24 hours in Christchurch and up to 70mm in areas such as Darfield. Gale force winds would mainly affect areas south of the city.

To manage the downfall, Christchurch City Council would deploy pumps at several flooding trouble spots, including Mairehau’s Flockton St, South New Brighton’s Ebbtide St, Wainoni’s Breezes Rd and Bromley’s Maces Rd.

Council manager of land drainage Keith Davison said staff would also monitor streams, grates and stopbanks at high tide. They would also use the new Dudley Creek bypass pipe if necessary, although it was not complete, and that would reduce the flood risk to the Flockton area, he said.

‘‘We have also recently completed up-stream storage facilities for the Heathcote [River] that will help.’’

Because street flooding was not always preventable, council might close some roads if needed, Davison said.

Out of the city, the New Zealand Transport Agency (NZTA) was preparing for highway disruptions.

MetService had issued a warning of heavy snow above 400m.

NZTA journey manager Tresca Forrester said road crews and police would be ‘‘keeping a close eye’’ on the state highways, especially on SH7, the part of the alternative Christchurch to Picton route that included the Lewis Pass.

Extra care would be needed because of the heavy traffic at the end of the school holidays, poor visibility and the possibility of slips with heavy rain and strong wind, she said.

‘‘Across the whole network, drivers need to watch for surface flooding and slips: slow down and drive to the conditions, build in rest stops and extra time to minimise stress.’’

She also urged drivers to check their vehicles and be prepared for disruptions.

Road works on SH7 had already added 45 minutes to the route. Forrester said some of this work had been delayed due to last week’s bad weather and might have to be postponed again.

Crews were repairing damage on several of the alpine passes caused by last week’s snow.

Coastal SH1 and the rail link from Christchurch to Picton were expected to remain out of action until the end of the year while crews repair damage from the November earthquake.

Between half a metre and one metre of snow has been forecast for 2190m-high Mt Hutt and mountain passes were expected to get a good covering.

Mt Hutt Ski Area manager James McKenzie said teams had been organised to clear the skifield road, de-ice lifts and set off explosives to clear snow build-ups and prevent avalanches. Winds on the field could reach 70 kilometres an hour. ‘‘We’re pulling out our snow preparations now. It’s going to be a big storm’’.