NZ Weather forecast: Confronting chart reveals how weekend's severe weather event compares to previous storms

MetService has released a confronting new chart, revealing how the incoming "September Storm" compares with previous severe weather events.

Kiwis are being warned of a possible offshore weather bomb developing over the weekend. The "significant severe weather event", as MetService calls it, is expected to impact most of the country, whipping up severe gales and snow at low levels in the south.

"An active front is forecast to move eastwards across New Zealand over the weekend, preceded by a strong and moist north to northwest flow and followed by a strong and cold west to southwest flow into early next week," the forecaster said.

"This front delivers a period of heavy rain to some western areas, severe gales for many places, and snow to low levels across western and southern parts of the South Island."

It's also expected to dredge up a blast of Antarctic air to chill southern parts of the country early next week.

NIWA on Friday morning said the low is near Tasmania at the moment but will strengthen over the coming days, which is when it is expected to move east.

MetService has now released a new chart comparing the incoming storm with previous events. It says the "September Storm" will have "unusually widespread hazards".

The chart compares the pressure difference between Auckland and Invercargill during each of the events. The forecaster says this is an indicator of severe gales.

"A old rule of thumb is any value over 30hPa indicates widespread gales, and some models take it as high as 39hPa early Sunday morning," it said.

Sitting at around 34hPa, the average of forecast models for the storm is ranked fourth on the list of events in terms of pressure difference. That's just above an event from 2018 that led to flooding and road closures on the West Coast.

A 2015 event, which had the greatest pressure difference with about 36hpa, caused flights to be diverted in Wellington and power cuts in Otago. Behind that was a 2017 event which caused windows to be blown out in Wellington and roads to be flooded in Queenstown. In third, a storm in 2014 caused 4500 homes to lose power and snow in Queenstown. 

MetService said Kiwis should expect "significant transport disruptions" just as the school holidays kick off and a risk to livestock from snow early next week.

That's something Weatherwatch is also warning of.

"Wind chills of below -10C are possible in some farming communities of the lower and alpine South Island on Monday and Tuesday as an explosive storm rapidly develops over the south Tasman Sea and then the Southern Ocean this weekend."

Northern parts of the country are expected to be windy on Monday and Tuesday, but won't be "caught up in the main event" due to the Antarctic blast's southern position. 

"There is a chance up in the mountains wind chills of -10C to -15C or more are likely" says head forecaster Philip Duncan.

"Wind chill below zero can kill newborn livestock, especially with hail, sleet, snow and rain showers in the mix to make the animals wet."

But there will be a brief stint of warm weather. Weatherwatch says on Saturday temperatures will be above normal for most regions.

MetService says severe weather warnings and/or watches are likely to be issued later on Friday.