A second sub-tropical airmass in less than a week is set to bring sweltering temperatures across New Zealand, and a month’s worth of rain to the south.

Most of the South Island is today under strong wind and heavy rain alerts as forecasters warn half a metre of rainfall could saturate western regions over a 32-hour period.

MetService meteorologist Lewis Ferris said while the West Coast did collect most rain in New Zealand this event was on the “upper end of intensities”.

An incredible 550mm of rain was expected to drench Westland from 1am tomorrow and not let up until Thursday morning.

The forecaster warned of streams and rivers rising rapidly and a threat of surface flooding and slips. Driving conditions were likely to be hazardous.

Severe gales were also expected to hit parts of Canterbury and Fiordland tonight with storm-force gusts of up to 140 km/h expected to pound the regions across the day.

MetService said it had been consulting with West Coast council officials about the possibility of upgrading the current alert to a rare red warning, Ferris said.

“But the regional council is happy for it to remain as an orange warning for now. But there is still a chance that it may be upgraded later today or even tomorrow morning.”

Ferris said the current models have suggested that “over a month’s worth of rain will fall in a 24-hour period”.

“This is a pretty good proxy for weather-related impacts which may affect travel along the West Coast.

“People around the area need to be aware that Wednesday is looking incredibly wet from persistent heavy rain and should follow advice from local authorities.

“This potentially significant rain event is being fuelled by warm, humid air being dragged down from the subtropics.”

This subtropical air would bring a continuation of muggy nights and warm days especially across the North Island, Ferris said.

“Napier is picked to reach 28°C on Thursday and only drop to 17°C overnight into Friday.

“A decent portion of North Island can expect to see temperatures hitting mid-20s.”

However, Ferris said the heat looked to be short-lived.

“As we head into Friday we would begin to see things starting to cool down.”

The rain eventually moves away from the West Coast and crosses the North Island on Thursday, he said.

“Strong northwest winds are also in the mix and we recommend people keep up with the forecasts.”

Ferris said there was good news at the end of the week with high pressure building across the nation’s shores bringing a bout of settled weather.

Yesterday, MetService had issued an “orange” heavy rain warning for Westland throughout Wednesday and into early Thursday morning, when 200mm to 300mm was forecast about the ranges south of Otira, with peak rates of 20mm to 30mm per hour.

The ranges between Ross and Bruce Bay could see 300mm to 400mm, or possibly more – while about 100mm to 150mm was expected in the wider region, with the potential for swollen rivers, slips and surface flooding.