A slow moving low forming in the Tasman Sea is expected to settle over the South Island from Wednesday, and MetService is warning of prolonged heavy rain on the West Coast and enough snow inland to shut alpine highways by Thursday.
The forecaster says the system will spread onto the South Island from the Tasman on Wednesday 26 August, soaking the West Coast and the Southern Alps before colder southerly air arrives late Wednesday and on Thursday and turns the rain to snow across inland Otago and South Canterbury.
“The weather becomes increasingly active across the South Island from Wednesday as a slow-moving low in the Tasman Sea directs a large amount of moisture toward the island. Significant severe weather is expected with this event,” MetService said in a statement reported by RNZ on Monday afternoon.
MetService meteorologist Devlin Lynden told RNZ the rain would become widespread and heavy on Wednesday, with the worst of it landing on the Southern Alps and the West Coast, where a heavy rain watch was already in place. He said the low was slow moving and could bring prolonged heavy rainfall to Central Otago, the Queenstown Lakes District, the Mackenzie Basin and inland Canterbury, and that further warnings were likely as the week went on. Warning amounts of snow are possible down to 400 metres for inland parts of South Canterbury and Otago.
By Monday afternoon that heavy rain watch was the only live severe weather alert anywhere in the country. MetService issued it at 11.02am on Monday for the ranges of the Westland District, covering the period from midday Wednesday to the end of Thursday. The official alert says there will be “prolonged periods of heavy rain, and amounts will exceed warning criteria” and that “this Watch will be upgraded to a Warning Tuesday morning when there will be a clearer picture of the total amount of rainfall expected”. It rates the chance of that upgrade as high, warns that streams and rivers may rise rapidly, and tells people to clear their drains and gutters, avoid low lying areas and drive cautiously. The next update is due at 10am on Tuesday.
The snow is the part that will strand people. In a traffic bulletin issued at 12.35pm on Monday, NZ Transport Agency Waka Kotahi said private forecasting supplied to the agency suggested more than 30 centimetres of snow could fall on State Highways 8 and 80 in the Mackenzie Basin and South Canterbury on Thursday 27 August, with up to 20 centimetres on State Highway 73 above 700 metres between Christchurch and the West Coast.
“Accumulations at this level are likely to impede travel and NZTA may close sections of highway or introduce restrictions to keep people safe and avoid vehicles becoming trapped,” the agency said.
Those three highway numbers matter more than they sound. State Highway 8 carries the Lindis Pass, the main inland link between Otago and the Mackenzie country. State Highway 80 is the only road into Mount Cook village, a dead end with nowhere to divert to. State Highway 73 is Arthur’s Pass, one of just three sealed crossings of the Southern Alps. Close all three at once and a large part of the South Island’s inland road network simply stops working, which is why the agency is flagging the risk three days out rather than on the morning.
NZTA said roading crews would be out around the clock clearing snow and other obstructions, and urged anyone planning to travel inland or over an alpine pass to watch both the forecast and the highway status. In winter conditions where traction is affected by snow, ice or heavy rain, it asked drivers to cut their speed, increase following distances, avoid sudden braking or turning, and carry warm clothing, food and water in case of delays or detours.
Fiordland is already feeling it. State Highway 94, the Milford Road, reopened on Monday morning after an overnight snow closure and was due to shut again at 4.30pm on Monday at Lower Hollyford Road because of avalanche risk, with reopening expected about 9am on Tuesday. The Otago Daily Times reported that all day closures are possible on Wednesday and Thursday if snow falls to road level.
Milford Road Alliance manager Kevin Thomson told the paper the route sits under some of the biggest avalanches in the world, so being proactive matters. “A typical season will see us observe between 200-400 avalanches above and around the road. We manage the risk to those going to Milford Sound by using both passive controls like no stopping areas, short closures, signage and patrolling and moving people along, and also using active controls such as full road closures, and use of explosives to reduce snow volumes above the road by creating avalanches before nature does,” he said.
The forecast lands at the end of a strange few days for the South Island. On Saturday evening a scrub fire ran about a kilometre through Albert Town and forced 22 Wanaka homes out, and on Monday afternoon Fire and Emergency crews from six brigades and three helicopters were working a vegetation fire of roughly 100 hectares on Karaka Road in the Hurunui district, as reported by the New Zealand Herald. Dry, wind scoured country on the eastern side of the island has been burning at the same time as the western side has been repeatedly drenched, and by Thursday much of both will be under either rain or snow.
The North Island gets the opposite deal. Lynden said it would stay comparatively settled through the first half of the week with light winds and dry weather, and that northwesterlies would push a warmer airmass across the island as the week goes on, making it feel noticeably warmer while the south gets hammered. That contrast is the same mechanism at work in both places, with air rising and dumping its moisture on the west of the South Island, then descending warm and dry over the north.
For anyone with travel booked through the middle of the week, the practical advice is simple enough. Watch for the watch becoming a warning on Tuesday morning, check the highway status before leaving rather than after, and treat a pass crossing on Thursday as something that may not happen. The Mackenzie and Queenstown Lakes districts carry a steady stream of visitors through late winter, many of them in rental cars, and a closed pass on a dead end road like State Highway 80 leaves no alternative route at all.
Are you in the South Island and planning to travel this week, or do you live somewhere that will cop the rain? Tell us what you are seeing in the comments below.
This article was written by AI, briefed to report the facts, hopefully without some of the bias people bring to the job π
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