Tasman ranges face up to 400mm of rain as twin storm systems sweep the South Island

The South Island’s western flank is preparing for one of the wettest weeks of the autumn after MetService issued orange heavy rain warnings stretching from northwest of Motueka down to Fiordland, with the highest ranges in the Tasman District tipped to receive up to 400mm of rain before the worst of the weather clears.

The first of two low pressure systems began moving onto the West Coast on Wednesday, dragging a long band of moisture across the alpine fault and dumping rain across the high country at peak rates of 25 to 30 millimetres an hour. Westland is forecast to receive up to 300mm in the ranges, climbing toward 350mm south of the Franz Josef and Fox glaciers, while Buller’s ranges are on a similar trajectory of up to 300mm over a 33 hour stretch.

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MetService warned that “streams and rivers might rise rapidly, and there could be surface flooding, slips and difficult driving conditions” across the orange warning zone. The forecaster has also placed heavy rain watches over Nelson Lakes National Park, the Richmond and Bryant ranges, the Rai Valley, the Grey District, and the lake headwaters of Canterbury and Otago south of Arthur’s Pass, with a high chance several of those watches will be upgraded to full warnings as the system tightens.

Thunderstorms are on the cards for both the West Coast and the Canterbury high country, where moist northwesterly air will be forced over the Southern Alps and squeezed into towering convective cells. Localised downpours in those storms could push hourly totals well past the headline 30mm rate, raising the risk of flash flooding in narrow catchments that drain quickly.

After a brief reprieve heading into Friday morning, a second system is queued up to roll in on Friday afternoon. That one is expected to swap the steady rain for stronger winds, with gales possible across exposed ridgelines and large southwesterly swells building along the western coastline through Saturday. Households along the Tasman, Westland and Fiordland coasts have been told to expect powerful surf and possible coastal erosion at the next high tides.

The New Zealand Transport Agency has urged motorists who use State Highway 6 between Westport and Haast, and State Highway 60 over Takaka Hill, to plan for challenging conditions and to delay non essential travel until the warnings expire. Slip prone sections of the road network, particularly the Buller Gorge and the cliffs above Fox River, often close at short notice when river levels rise. Drivers crossing the Lewis Pass or Arthur’s Pass have been told to watch for surface water, debris, and reduced visibility in heavier showers.

Civil defence groups in Tasman, Buller and on the West Coast have asked residents to clear drains and gutters before the heaviest falls arrive, to move stock away from low lying paddocks, and to keep an eye on rivers that have been running high after the rain that fell over the long weekend. The advice from MetService is blunt and consistent across all of its warnings, telling people to avoid low lying areas, drive cautiously, and keep up to date with the latest information.

The forecast is also testing the catchments above small West Coast settlements that have been the focus of recent climate adaptation work. Towns such as Karamea and Westport have invested in flood walls, river mouth dredging and improved gauging on the Mokihinui and the Buller, and local hydrologists will be watching catchment response curves closely once the rain peaks. Soils across the West Coast remain near saturation after a wet April, which means most of the rainfall heading down the hills will end up in the rivers rather than soaking into the ground.

Inland, the lake headwaters watches stretching from Aoraki Mount Cook down through the Hawea catchment matter for the lower South Island in a different way. Inflows into Lakes Pukaki, Tekapo and Hawea drive the Waitaki and Clutha hydro chains, and a substantial rain band over the headwaters tends to lift national hydro storage just as winter electricity demand starts to climb. Transpower and Meridian Energy will be tracking the inflow data closely.

For trampers already in Aoraki Mount Cook National Park or on the Routeburn, Milford and Kepler tracks, the Department of Conservation has flagged that river crossings could become impassable and that hut wardens will be steering walkers toward shelter rather than onward travel during the peak of the rain. Anyone with bookings later this week is being asked to check the DOC alerts page before leaving the trailhead, and to carry a personal locator beacon.

The rain is, in the longer view, far from unprecedented. The West Coast routinely chalks up well over 5,000mm a year, and individual storm cells over the Tasman and Westland ranges have produced more than half a metre of rain in 24 hours during past atmospheric river events. What makes this week notable is the back to back delivery of two systems on top of an already wet catchment, which is the combination that turns a soggy week into a state of emergency for somewhere in the affected zone.

Have you made any preparations for the heavy rain in your part of the South Island, and are you watching a particular river or road as the warning unfolds? Let us know in the comments below and tell us what your community is doing to get ready.

This article was written by AI, briefed to report the facts, hopefully without some of the bias people bring to the job 🙂