Nelson-Tasman braces for three-day downpour on already sodden ground

• An ‘atmospheric river’ is bringing three days of heavy rain to Nelson-Tasman
• The MetService has issued orange warnings for Nelson and Tasman, and red warnings for Buller and Westland
• The impact of the rain will be worsened by ground that hasn’t dried out from heavy July rain
• Residents should get prepared and avoid unnecessary travel

Nelson-Tasman could experience more than a month’s worth of rain this week as an “atmospheric river” flows across the top of the south.

Now, residents around Te Tauihu (the top of the south) are being warned to brace for road closures, slips, rapidly rising rivers and falling trees as a three-day rain front threatens to soak the region.

The MetService has issued an orange heavy rain warning for Tasman west of Motueka from 5am Tuesday until 6pm Thursday, with up to 700mm of rain expected to accumulate in the ranges – with potential for up to 800mm – and 180mm to 280mm in low-lying areas.

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A warning is also in place for the Nelson region, including Bryant Range, Richmond Ranges, the Marlborough Sounds and Rai Valley, with up to 500mm expected in the ranges.

“Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous,” both warnings stated.

Meanwhile, heavy rain warnings for Buller and Westland were upgraded to the red alert level on Tuesday, with the rain expected to “cause dangerous river conditions and significant flooding”.

NIWA meteorologist Tristan Meyers said atmospheric rivers were “huge plumes of moisture that move from the tropics”.

This particular river was “pretty exceptional” because of its large moisture content, he said.

“Current forecast models indicate that this would be classified as an ‘extreme’ to ‘exceptional’ atmospheric river.”

The rain was likely to come in two bursts – one from Tuesday to Thursday and a second at the weekend.

“All up, over a month of rainfall could fall for large parts of the West Coast, Canterbury High Country, Tasman, Marlborough and Nelson, with the heaviest rain about the elevated ground,” he said.

University of Otago senior lecturer in geography Dr Daniel Kingston said atmospheric rivers were “relatively rare” during winter, but the damage caused by this event would be worse because it was coming on the back of an “exceptionally wet winter”. Nelson recorded its second-highest rainfall for July on record.

“The ground is already very wet with limited capacity to absorb further rain.”

The forecast deluge has prompted warnings from local councils.

Tasman District Council warned that while rain rates may not be extreme, the amount forecast was large, and it would be falling on ground that was already saturated.

Some rivers were expected to reach five to 10 year flood levels, although people should plan for higher levels, and the expected rise in groundwater levels, particularly on the Waimea Plains and in Golden Bay, meant surface flooding would impact some roads, along with stormwater, and potentially sewer, capacity.

In preparation for this, the Motueka Wastewater Treatment Plant was “over-pumping” treated wastewater to an adjecnt “soakage” area on Tuesday. People should avoid collecting shellfish the estuary until further notice, the council warned on Facebook.

Landslips and travel disruption were also likely, and if there were regular heavy rain bursts it could become more serious and cause road closures.

“If people are planning to travel over the three days from Tuesday afternoon it may be wise to make contingencies or delay the travel until there is a better picture of what is actually happening,” local controller Rob Smith said.

Civil Defence was in “monitoring mode”, he said.

“The full effect of the event may not be felt until Wednesday evening, although now is the time to prepare.”

Nelson City Council also urged people to prepare, with slips expected around the city.

Meanwhile, a forecast storm surge meant Rocks Rd may have to close for a period, which would mean extra traffic on Waimea Rd and Rutherford St, which is already subject to a detour due to major roadworks.

People should expect longer delays and leave home earlier, the council said in a statement.

Local reserves already sodden from a wet winter were also expected to experience slips and fallen trees.

Get prepared: What you need to know

• Plan travel carefully, allow extra time and work from home if you can
• Drive to the conditions, or avoid travel
• Secure outdoor furniture
• Check drains/gutters are clear
• Consider filling and placing sandbags – sandbags are not available from councils
• If you’re near water or in a low-lying area prepare for flooding

What the MetService forecast says

• Tasman west of Motueka: an orange warning from 5am Tuesday until 6pm Thursday. Ean expect between 450mm to 700mm of rain expected to accumulate in the ranges – with potential for up to 800mm – and 180mm to 280mm in low-lying areas. Rain would peak at 10mm to15mm an hour on Tuesday, and 15mm/h to 25mm/h on Wednesday and Thursday,.
• Nelson region, including Bryant Range, Richmond Ranges, the Marlborough Sounds and Rai Valley: an orange warning 10pm Tuesaday to 9pm Thursday, with 350mm to 500mm expected in the ranges and peak rates of 10mm/h to 15mm/h. In the Sounds 150mm to 250mm is expected.
• Buller: a red warning from 9am Tuesday to 5pm Thursday. Periods of heavy rain with 350mm to 550mm accumulating about the ranges south of Little Wanganui, and 150mm to 280mm elsewhere. Peak rates of 15mm/h to 20mm/h about the ranges on Wednesday.
• Tasman district from Motueka eastwards: a heavy rain watch in place from 10am Tuesday to 6pm Thursday.
• Nelson, Tasman and Buller: Wind watch in place: northeast winds which could approach gale at times.