A large area of low pressure is moving towards New Zealand this weekend and brings a fair bit of weather worth tracking – some kicking off as early as overnight tonight.

Generally rain looks to be very heavy on the West Coast at times, there’s a burst of gales (although they are mostly South island/Cook Strait focused) before the weekend arrives and then during the weekend we may see rain shift from the western side to the eastern side of the South Island – mainly focused on Canterbury too, where further pockets of flooding are possible.

In fact we’d say the main area of concern over the next several days looks to be that Canterbury has another potential period of rain coming. It’s too far out to lock in totals but modelling suggests 20 to 50mm possible across the region but there may be greater amounts at higher elevations, so it’s definitely what we call “one to watch”. Rain may also cause problems on the West Coast in places and some other regions yet to be locked in.

Also over the weekend we’re likely to have mild weather continue over the North Island while temperatures drop in the South Island to single digits in many places. (Due to the large area of low pressure being centered around Cook Strait/halfway mark of New Zealand – mild NWers for northern and cooler S to SE winds for southerners). Rain will turn to snow in the South Island ranges too, possibly heavy in some spots. There may also be a few thunderstorms or heavy downpours that pop up in northern New Zealand or other areas too – but some of these smaller things may be harder to lock in until the weekend itself. 

The best way to explain the incoming system is to focus on the main weather headlines/situation day by day. You’ll notice the severe weather looks to be in pockets of the country with each day having a different set up.

• Today: A mild nor’west flow builds across New Zealand, partially from the sub-tropics. Heavy rain arrives on the West Coast. Gale to Severe Gale NW winds build over the South Island, mostly in less populated areas. Temperatures may reach 20 degrees in Hawke’s Bay.

• Thursday: Rain on the West Coast slowly eases back in intensity, some rain spreads into the North Island. Gale to sometimes Severe Gale NW winds move northwards towards central NZ, perhaps a bit blustery in the upper North Island… but peak intensity eases late morning or during the afternoon. Generally speaking the winds look blustery in many areas for a time. Rain continues on the West Coast overnight. Temperatures may reach 20 degrees in some eastern areas.

• Friday: Heavy rain on the lower West Coast first thing, otherwise most places have a fairly average August day with little severe weather – but Friday is when some of the lowest air pressure moves into the South Island.

• Saturday: A deep low will lie around the western side of Cook Strait/Tasman Sea area with central air pressure around 980hPa. While it isn’t really a storm it may create some problems – as it could produce yet another area of rain in EASTERN New Zealand (like Canterbury). Pockets of flooding could again be another issue in Canterbury on Saturday or Sunday – but rainfall totals aren’t extreme at this stage, just another burst of rain that could be set in, but like we saw just this week it may be patchy and light for some, more set in for others. Details will become clearer on rainfall totals as we get closer to the weekend but this rain looks likely to start at some point on Saturday and then may become heavier on Sunday. Also, gales at sea circling around this low (circling around New Zealand) may brush some coastal areas but generally most places look blustery at worst – again, “on to watch” in case the forecast becomes more serious.

• Sunday: There is a chance of heavy rain in Canterbury.

• Monday: Low pressure falls apart over the North Island (air pressure rises), rain eases in the South Island. There may be some heavy downpours in the North Island worth monitoring.

– Saturday’s air pressure map shows the very large and deep area of low pressyre around New Zealand (purple) with high pressure over Australia (white to faint/light yellow shading) / Earth.nullschool.net

– WeatherWatch.co.nz