A very large and complicated low pressure system is building in our part of the world and peaks next week as a number of lows merge to create one enormous area of low pressure.

Large lows can be quite complex to forecast because the big size also means the severe weather can be spread out so much it ‘thins’ out. This allows for large areas of calm among the areas of rough weather and usually leads to some people asking “where the wind and rain is” despite a low being over top.

To make matters more confusing for New Zealanders our large mountains and ranges can enhance rain, snow and wind for one group (making the weather ‘severe’) while for another group of people on the other side it may be sunny and calm. Many towns and cities will have “the usual” winter weather in the days ahead (some rain, some cold, but nothing to write home about) but the large nature of this low means severe weather will be here and there across both main islands, as MetService has already said in their latest severe weather outlook.

As we said in our initial press release, the centre of this eventual big low will be well south of New Zealand – it’s more about how it’s altering our weather pattern afterwards and dredging up a cold southerly while many farmers are in the middle of calving and some are now starting lambing already too.

So let’s break it down as simply as we can, to highlight the main areas of concern, mostly for farmers with newborn livestock, that way you can work out if this big Southern Ocean storm next week will be an issue for where you live, or nothing really at all. Many places will not have severe weather due to the blocking action of our mountains and ranges – but others are exposed to heavy rain, snow or strong bitterly cold winds. Keep up to date with MetService and WeatherWatch over the days ahead as large lows sometimes see large changes to forecasts too.

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TODAY THURSDAY:
A couple of low pressure systems merge around SE Australia/Tasmania. NZ unaffected for now.

FRIDAY:
These two Aussie lows merge and grow over the southern Tasman Sea. Some rain after midnight on the West Coast of NZ otherwise most of our regions unaffected.

SATURDAY:
The newly formed larger low starts to A) Deepen, and B) Grow in size, covering much of the Tasman Sea by the end of the day. It drives rain in to the West Coast with Heavy Falls possible. Rain or showers move in to the western North Island too, with perhaps a few isolated heavy falls further south in the ranges. NW winds pick up in the North Island. Colder for the lower South Island with snow flurries possible around Queenstown and Northern Southland but drier towards the coast.

SUNDAY:
The low fills up the entire Tasman Sea area and the air pressure continues to drop. Heavy Rain on the West Coast while rain spreads into the western and northern North Island with heavy falls too, maybe a few isolated northern thunderstorms. NW winds picking up, may become blustery in some exposed areas (mainly below gale force although gales in marine areas are likely). Heaviest downpours look to be at night across various parts of the North Island. Rain eases by night on the West Coast and much of the eastern South Island is dry but rain will clip the upper South Island with possible Heavy falls.

MONDAY:
Low pressure transfers from west of NZ to east of us and merges with a big low in the Southern Ocean to produce an enormous area of low pressure by the end of the day. This southern placement of the low encourages W to NW winds over the upper North Island which may produce another burst of heavy rain. Maybe some isolated thunderstorms again too. Cold southerlies push into Southland with showers and the odd low level snow flurry. Cold wind chills dropping to -4 or -5 in exposed areas. Alpine passes like

TUESDAY:
Peak cold comes into the South Island with wind chills potentially down to -10 in exposed areas. Snow flurries in Southland and Otago down to low levels (maybe 100m). However snow amounts may not be heavy enough for snow warnings. It will be very cold in exposed areas, a danger to newborn livestock. Heavy snow in the mountains for a time with rain on the West Coast mixed in with sleet higher up. Main centres most at risk of snow: Queenstown, Wanaka, Lumsden, Te Anau and possibly even to sea level in Milford Sound and parts of Fiordland. The North Island is mostly dry with a few showers and cold SW winds. Wind chills will tough higher up (Central Plateau and hills and ranges) for newborn livestock. Snow flurries around Central Plateau and the Desert Road. Generally speaking the South Island shoulders most of this southerly punch for the North Island though.

WEDNESDAY:
The low moves away slowy and SW winds ease.

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NOON FRIDAY

NOON SATURDAY:

NOON SUNDAY:

NOON MONDAY:

NOON TUESDAY (ECWMF – European model)

NOON TUESDAY (GFS – American model)

– WeatherWatch.co.nz