Is winter here or not? The weather is about to get colder before getting much warmer next week with sub-tropical airflows and rainfall possibly heavy enough to cause flooding in both main islands.

Before that milder airflow kicks in, one of the coldest weeks of the year (if not the coldest for some) is only just arriving. The South Island’s alpine high country may barely climb above freezing for the next 72 hours.

But northern NZ only has a brief stint with the cold later this week and weekend – before a sub-tropical airflow kicks in next week. As an example, Hamilton has a low of -2C this Friday night, but the overnight minimum will be +12C one week from today.

FROSTS:
Snow showers are today and tomorrow moving into the North Island, impacting the Hawke’s Bay ranges and Central Plateau in particular. At the same time high pressure is growing in the lower South Island and will expand into the North Island later this week, bringing frosts as far north as Waikato and the Bombay Hills (maybe even into some rural parts of Auckland).

Coldest weather will be tonight in the South Island with parts of Otago forecast by RuralWeather.co.nz to drop down to -10C or colder. Tekapo’s overnight low tonight is -7C, Gore’s overnight low coming up is -6C, Lumsden -5C, Alexandra & Queenstown -4C. Near these areas the mercury will likely drop to -10C or below.

The North Island has -3C for Central Plateau (Waiouru) later this week, while Hamilton will be down to -2C on Friday night.

Frosty weather may not impact the eastern North Island’s regions as much, due to more cloud cover, with perhaps Sunday being the coldest night in Hawke’s Bay.

In Tekapo and Twizel there is just a two or three hour period in the afternoon where temperatures are expected to go above zero/freezing over the next few days!

#UPDATED: 3 Day #Rainfall & #Snowfall Map.https://t.co/gb7E1quzdb / https://t.co/D35ySCkVqG pic.twitter.com/ocudKwKXyE

— WeatherWatch.co.nz (@WeatherWatchNZ) August 8, 2022

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FLOOD POTENTIAL NEXT WEEK
Data is starting to align better for next week and it paints the picture of a powerful high pressure zone near the Chatham Islands that does two things. 1) It pulls down sub-tropical air over NZ. 2) It blocks rainmakers from moving through – which can dramatically increase rainfall totals.

This wet northerly set up then looks likely to be fuelled by low pressure in the Tasman Sea near the North Island. This set up, if it does eventuate as forecast today by various models, could then see rainfall totals of 100 to 200mm for the upper North Island and upper South Island, mainly Northland and Nelson, with surrounded regions also at risk. The Nelson region may even go above 200mm, based on the longrange GFS rainfall data and our IBM data now suggests 70mm in the city later next week (and often you can double Nelson’s total to get an idea of rainfall in the Nelson ranges).

Being still a week out, this is by no means yet locked in – but the set up is now showing up in our hyper-local 10 day forecasts and in public global modelling. The powerful blocking high pressure zone to NZ’s east could hold up rain clouds and produce bigger rainfall totals. It’s one to closely monitor over the week ahead – especially after the recent heavy rain, slips and localised flooding.

HELPFUL LINKS:
– NZ FROST Forecaster – RuralWeather.co.nz
– FROST RISK Maps
– NZ Accumulated RAINFALL Maps (next 6 days) (later this week the rain event for next week will start to show up here).

• Prepared by head forecaster Philip Duncan

• WeatherWatch.co.nz – New Zealand’s weather data & alerting authority