Most of North Island coastline at risk of flooding on Thursday and Friday

Surface flooding has hit Palmerston North and Masterton following severe thunderstorms which pelted the area this evening.

Streets filled with water as the dark clouds rolled in, bringing intense downpours that overwhelmed stormwater drains.

Just before 4.30pm the MetService issued an urgent alert for Palmerston North, Manawatu and Rangitikei as severe storms bore down on the area.

The storms were expected to be accompanied by very heavy rain, the forecaster said.

"Very heavy rain can cause surface and/or flash flooding about streams, gullies and urban areas, and make driving conditions extremely hazardous."

The alert was valid until just after 5pm today. Between 3.20pm and 4.15pm, the MetService weather station at the airport measured 42mm of rainfall.

Looking north from work in Feilding @WeatherWatchNZ @PhilipDuncan pic.twitter.com/jCtUugryyf

— Silver Slave (@Nzchookie) January 3, 2018

The floods come ahead of two days of menacing weather that is expected to cause chaos in parts of the North Island.

Campers are being urged to moved to higher ground as a subtropical storm closes in, bringing heavy rain, gales, and potential coastal flooding to most of the North Island.

Strong onshore winds mean that king tides on Thursday and Friday, coupled with waves the size of a double-decker bus, pose a threat to many low-lying communities.

All northward facing bays in Auckland, coastlines from Northland to the Bay of Plenty and Firth of Thames, and the North Island's west coast from tomorrow, are at risk.

The storm is on track to hit the top of the North Island on Thursday morning, sweeping across the island before hitting northern and eastern regions of the South Island on Friday and clearing the country on Saturday.

Niwa warned the approaching storm bore similarities to ex-Tropical Cyclone Ita, which struck at Easter three years ago. It inundated coastal communities across the upper North Island and blew buildings to pieces on the South Island's West Coast.

This map shows where the storm is predicted to be at 4pm tomorrow.

A weather map of New Zealand as Thursday's storm closes in. Photo / Earth.nullschool.net

Between 100 and 150mm of rain is forecast to fall in the area over 12 hours from 1pm on Thursday, reaching peak intensity levels of 30-40mm. Northeasterly winds are expected to become severe from Thursday evening until dawn on Friday, gusting to 120km/h in exposed places.

Waikato Regional Council regional hazards team leader Rick Liefting said it coincides with a king tide on Thursday evening which will hinder the ability of fast-flowing coastal rivers and streams to run out.

"Combined with dry catchments there is the real potential for localised flooding and coastal inundation. That's because any rainfall is likely to run straight off the land and into our waterways, rather than being absorbed."

Strong north-easterly winds and king tides may also be a recipe for flooding issues in susceptible areas such as Whiritoa, Miranda, Kaiaua and parts of East Coast Road, Hauraki District Council Civil Defence Controller Steve Fabish said.

Fabish said people should keep a close eye on weather forecasts and have supplies ready in case they are cut off.

"They should also put some prior thought into where they might self-evacuate to, such as friends and family."

Severe Thunderstorm Warning issued for Wellington Radar Area https://t.co/Q6zq5PZTrC pic.twitter.com/Or3c4zSwGZ

— Severe Weather Info (@MetServiceWARN) January 3, 2018

Niwa is warning the approaching storm is bearing similarities to ex-Tropical Cyclone Ita, which struck at Easter three years ago inundating coastal communities across the upper North Island and blowing buildings to pieces on the South Island's West Coast.

Strong onshore winds meant king tides overnight Thursday and Friday coupled with waves the size of a double-decker bus pose a threat to many low-lying communities.

They included:

• all northward facing bays in Auckland

• Northland to Bay of Plenty, including Firth of Thames

• the North Island's west coast during midday high tide on Friday

Examining Thu-Fri coastal flood risk, we see some similarities btwn the upcoming sub-tropical storm & Ex-Tropical Cyclone Ita, April 2014 -- the latter inundated Tamaki Dr.

Strong onshore NE winds mean that king tides @ 10:15pm Thu/10:49am Fri (east AKL) are ones to watch 🌊🌊🌊 pic.twitter.com/x2FMAUpsqZ

— NIWA Weather (@NiwaWeather) January 2, 2018

MetService is forecasting up to 160mm of rain to fall over Mt Taranaki and the Nelson and Bay of Plenty ranges.

Coromandel is expecting 150mm and Northland 120mm of rain. Auckland is in line for up to 90mm which is due to start falling at lunchtime with torrential downpours in the evening.

The deluge is coupled with potentially destructive winds of up to 120 km/h which will barrel through the upper north Island overnight Thursday.

Weatherwatch.co.nz head analyst Philip Duncan said the storm would rapidly deepen and intensify before it made landfall tomorrow.

Campers and trampers were warned to be aware of the deteriorating conditions.

Duncan said models showed the storm was expected to peak early Friday. A wind map put the worst of the winds becoming hurricane force - 120 km/h - for a time at the centre, although such winds would stay mainly out at sea.

Significant wave heights will be ... very significant.

On Friday, 6m+ swells are possible along the western North Island & 4-6m in the east -- thus, coastal waters are expected to be particularly dangerous. pic.twitter.com/hR3gH0r5cN

— NIWA Weather (@NiwaWeather) January 3, 2018

MetService says severe gales will buffet the upper North Island from Taranaki to Bay of Plenty north.

The heaviest rain is expected in Northland, Auckland, Coromandel Peninsula, Bay of Plenty, Mt Taranaki, Marlborough and Nelson.

Most Auckland beaches will also be unsafe to swim at until Saturday, the council's SafeSwim website warns.

Northern Coastguard operations manager Ray Burge today urged boaties to keep an ear on the marine forecast in coming days.

Given the dreadful forecast boaties were advised to consider returning home early or go to a safe harbour to sit out the coming storm.

Burge said the big tides threatened to amplify the effect the storm had on coastal communities. He said the indications were the storm would be ferocious but pass quickly.

He advised boaties to stay ashore if the weather was marginal and to not put themselves at risk securing their vessels.